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Surface weather analysis

2025-10-16T19:03:31Z

2405:6E00:655:B199:99E7:2891:DECD:B143:

{{short description|Type of weather map}}
{{Redirect|Surface analysis|other uses|surface science|and|etymology}}

[[Image:Surface analysis.gif|thumb|400px|A surface weather analysis for the United States on October 21, 2006. By that time, [[Hurricane Paul (2006)|Tropical Storm Paul]] was active (Paul later became a hurricane).]]
'''Surface weather analysis''' is a special type of [[weather map]] that provides a view of [[weather]] elements over a geographical area at a specified time based on information from ground-based weather stations.<ref>''Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather.'' University of Chicago PressChicago: 1999.</ref>

Weather maps are created by plotting or tracing the values of relevant quantities such as [[sea level pressure]], [[temperature]], and [[cloud cover]] onto a [[geographical map]] to help find [[synoptic scale]] features such as [[weather fronts]].

The first weather maps in the 19th century were drawn well after the fact to help devise a theory on storm systems.<ref name="MILLER">Eric R. Miller. [http://docs.lib.noaa.gov/rescue/mwr/061/mwr-061-07-0189.pdf American Pioneers in Meteorology.] Retrieved on 2007-04-18.</ref> After the advent of the [[telegraph]], simultaneous [[surface weather observation]]s became possible for the first time, and beginning in the late 1840s, the [[Smithsonian Institution]] became the first organization to draw real-time surface analyses. Use of surface analyses began first in the United States, spreading worldwide during the 1870s. Use of the [[Norwegian cyclone model]] for frontal analysis began in the late 1910s across Europe, with its use finally spreading to the United States during [[World War II]].

Surface weather analyses have special symbols that show frontal systems, cloud cover, [[Precipitation (meteorology)|precipitation]], or other important information. For example, an ''H'' may represent [[high-pressure area|high pressure]], implying clear skies and relatively warm weather. An ''L'', on the other hand, may represent [[low pressure]], which frequently accompanies precipitation. Various symbols are used not just for frontal zones and other surface boundaries on weather maps, but also to depict the present weather at various locations on the weather map. Areas of precipitation help determine the frontal type and location.

== History of surface analysis ==
{{see also|History of surface weather analysis}}
[[Image:10 PM March 12 surface analysis of Great Blizzard of 1888.png|thumb|upright|Surface analysis of [[Great Blizzard of 1888]] on March 12 at 10 pm]]

The use of weather charts in a modern sense began in the middle portion of the 19th century in order to devise a theory on storm systems.<ref>Human Intelligence.[http://www.indiana.edu/~intell/galton.shtml Francis Galton.] Retrieved on 2007-04-18.</ref> The development of a [[telegraph]] network by 1845 made it possible to gather weather information from multiple distant locations quickly enough to preserve its value for real-time applications. The Smithsonian Institution developed its network of observers over much of the central and eastern United States between the 1840s and 1860s. The [[U.S. Army Signal Corps]] inherited this network between 1870 and 1874 by an act of Congress, and expanded it to the west coast soon afterwards.<ref>Frank Rives Millikan. Smithsonian Institution. [http://www.si.edu/archives/ihd/jhp/joseph03.htm Joseph Henry: Father of the Weather Service.] Retrieved on 2006-10-22. {{webarchive |url=https://web.archive.org/web/20061020020548/http://www.si.edu/archives/ihd/jhp/joseph03.htm |date=October 20, 2006 }}</ref>

The weather data was at first less useful as a result of the different times at which weather observations were made. The first attempts at time standardization took hold in Great Britain by 1855. The entire United States did not finally come under the influence of time zones until 1905, when [[Detroit]] finally established standard time.<ref>WebExhibits. [http://webexhibits.org/daylightsaving/d.html Daylight Saving Time]. Retrieved on 2007-06-24.</ref> Other countries followed the lead of the United States in taking simultaneous weather observations, starting in 1873.<ref>NOAA. [http://celebrating200years.noaa.gov/foundations/weather/#expand An Expanding Presence.] Retrieved on 2007-05-05.</ref> Other countries then began preparing surface analyses. The use of frontal zones on weather maps did not appear until the introduction of the [[Norwegian cyclone model]] in the late 1910s, despite Loomis' earlier attempt at a similar notion in 1841.<ref>David M. Schultz. ''[http://www.cimms.ou.edu/~schultz/sanders/sanders.pdf Perspectives on Fred Sanders's Research on Cold Fronts]'', 2003, revised, 2004, 2006, p. 5. Retrieved on 2006-07-14.</ref> Since the leading edge of air mass changes bore resemblance to the [[military front]]s of [[World War I]], the term "front" came into use to represent these lines.<ref>Bureau of Meteorology. [http://www.bom.gov.au/info/ftweather/page_6.shtml Air Masses and Weather Maps.] Retrieved on 2006-10-22.</ref>

[[Image:Weather symbolsNEW2.png|thumb|Present weather symbols used on weather maps]]

Despite the introduction of the Norwegian [[cyclone]] model just after World War I, the United States did not formally analyze fronts on surface analyses until late 1942, when the WBAN Analysis Center opened in downtown [[Washington, D.C.]]<ref>[[Hydrometeorological Prediction Center]]. [https://www.wpc.ncep.noaa.gov/html/WPC_history.pdf A Brief History of the Hydrometeorological Prediction Center.] Retrieved on 2007-05-05.</ref> The effort to automate map plotting began in the United States in 1969,<ref>ESSA. [http://www.ncep.noaa.gov/officenotes/NOAA-NPM-NCEPON-0001/013FD50A.pdf Prospectus for an NMC Digital Facsimile Incoder Mapping Program.] Retrieved on 2007-05-05.</ref> with the process complete in the 1970s. [[Hong Kong]] completed their process of automated surface plotting by 1987.<ref>Hong Kong Observatory. [http://www.weather.gov.hk/wservice/tsheet/computer.htm The Hong Kong Observatory Computer System and Its Applications.] {{Webarchive|url=https://web.archive.org/web/20061231001802/http://www.weather.gov.hk/wservice/tsheet/computer.htm |date=2006-12-31 }} Retrieved on 2007-05-05.</ref> By 1999, computer systems and software had finally become sophisticated enough to allow for the ability to underlay on the same workstation satellite imagery, radar imagery, and model-derived fields such as atmospheric thickness and [[frontogenesis]] in combination with surface observations to make for the best possible surface analysis. In the United States, this development was achieved when [[Intergraph]] workstations were replaced by n-[[Advanced Weather Interactive Processing System|AWIPS]] workstations.<ref>[[Hydrometeorological Prediction Center]]. [http://www.wpc.ncep.noaa.gov/html/Accomplish99/Accomplish99.html Hydrometeorological Prediction Center 1999 Accomplishment Report.] Retrieved on 2007-05-05.</ref> By 2001, the various surface analyses done within the National Weather Service were combined into the Unified Surface Analysis, which is issued every six hours and combines the analyses of four different centers.<ref name="DR">David Roth. Hydrometeorological Prediction Center. [https://www.wpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf Unified Surface Analysis Manual.] Retrieved on 2006-10-22.</ref> Recent advances in both the fields of [[meteorology]] and [[geographic information system]]s have made it possible to devise finely tailored weather maps. Weather information can quickly be matched to relevant geographical detail. For instance, icing conditions can be mapped onto the road network. This will likely continue to lead to changes in the way surface analyses are created and displayed over the next several years.<ref>Saseendran S. A., Harenduprakash L., Rathore L. S. and Singh S. V. [http://www.gisdevelopment.net/application/environment/conservation/envm0004.htm A GIS application for weather analysis and forecasting.] Retrieved on 2007-05-05.</ref>

== Station model used on weather maps ==
{{see also|Station model}}
[[Image:Station model.gif|thumb|[[Station model]] plotted on surface weather analyses]]

When analyzing a weather map, a station model is plotted at each point of observation. Within the station model, the temperature, dewpoint, [[wind speed]] and [[Wind direction|direction]], atmospheric pressure, pressure tendency, and ongoing weather are plotted.<ref>National Weather Service. [http://www.srh.noaa.gov/ohx/educate/station_model.gif Station Model Example.] Retrieved on 2007-04-29. {{webarchive |url=https://web.archive.org/web/20071025135958/http://www.srh.noaa.gov/ohx/educate/station_model.gif |date=October 25, 2007 }}</ref> The circle in the middle represents cloud cover; fraction it is filled in represents the degree of [[overcast]].<ref>Dr Elizabeth R. Tuttle. [http://www.du.edu/~etuttle/weather/weather.htm Weather Maps.] {{webarchive|url=https://web.archive.org/web/20080709005941/http://www.du.edu/~etuttle/weather/weather.htm |date=2008-07-09 }} Retrieved on 2007-05-10.</ref> Outside the United States, temperature and dewpoint are plotted in degrees [[Celsius]]. The [[wind barb]] points in the direction from which the wind is coming. Each full flag on the wind barb represents {{convert|10|kn|km/h|0}} of wind, each half flag represents {{convert|5|kn|km/h|0}}. When winds reach {{convert|50|kn|km/h|0}}, a filled in triangle is used for each {{convert|50|kn|km/h|0}} of wind.<ref>American Meteorological Society. [http://www.ametsoc.org/amsedu/dstreme/extras/wxsym2.html Selected DataStreme Atmosphere Weather Map Symbols.] Retrieved on 2007-05-10.</ref> In the United States, rainfall plotted in the corner of the station model are in [[inch]]es. The international standard rainfall measurement unit is the [[millimeter]]. Once a map has a field of station models plotted, the analyzing [[isobar (meteorology)|isobars]] (lines of equal pressure), isallobars (lines of equal pressure change), isotherms (lines of equal temperature), and [[isotach]]s (lines of equal wind speed) are drawn.<ref>CoCoRAHS. [http://ccc.atmos.colostate.edu/~hail/teachers/lessons/isopleths.htm INTRODUCTION TO DRAWING ISOPLETHS.] Retrieved on 2007-04-29. {{webarchive |url=https://web.archive.org/web/20070428172620/http://ccc.atmos.colostate.edu/~hail/teachers/lessons/isopleths.htm |date=April 28, 2007 }}</ref> The abstract weather symbols were devised to take up the least room possible on weather maps.{{Citation needed|date=December 2021}}

== Synoptic scale features ==
{{see also|Synoptic scale meteorology}}
A synoptic scale feature is one whose dimensions are large in scale, more than several hundred kilometers in length.<ref>Glossary of meteorology. [http://amsglossary.allenpress.com/glossary/search?id=synoptic-scale1 Synoptic scale.] {{webarchive|url=https://web.archive.org/web/20070811102318/http://amsglossary.allenpress.com/glossary/search?id=synoptic-scale1 |date=2007-08-11 }} Retrieved on 2007-05-10.</ref> Migratory pressure systems and frontal zones exist on this scale.{{Citation needed|date=December 2021}}

=== Pressure centers ===
[[Image:Wind barbs.gif|thumb|upright|Wind barb interpretation]]

Centers of surface high- and low-pressure areas that are found within closed isobars on a surface weather analysis are the absolute maxima and minima in the pressure field, and can tell a user in a glance what the general weather is in their vicinity. Weather maps in English-speaking countries will depict their highs as Hs and lows as Ls,<ref>Weather Doctor. [http://www.islandnet.com/~see/weather/elements/high.htm Weather's Highs and Lows: Part 1 The High.]</ref> while Spanish-speaking countries will depict their highs as As and lows as Bs.<ref>Agencia Estatal de Meteorología. [http://www.aemet.es/en/divulgacion/aeronautica/detalles/Meteorologia_del_aeropuerto_de_La_Palma Meteorología del aeropuerto de La Palma.].</ref>

==== Low pressure ====
Low-pressure systems, also known as [[cyclone]]s, are located in minima in the pressure field. Rotation is inward at the surface and counterclockwise in the [[Northern Hemisphere]] as opposed to inward and clockwise in the [[Southern Hemisphere]] due to the [[Coriolis force]]. Weather is normally unsettled in the vicinity of a cyclone, with increased cloudiness, increased winds, increased temperatures, and upward motion in the atmosphere, which leads to an increased chance of precipitation. [[Polar low]]s can form over relatively mild ocean waters when cold air sweeps in from the ice cap. The relatively warmer water leads to upward convection, causing a low to form, and precipitation usually in the form of snow. Tropical cyclones and winter storms are intense varieties of low pressure. Over land, [[thermal low]]s are indicative of hot weather during the summer.<ref>BBC Weather. [https://web.archive.org/web/20030220020730/http://www.bbc.co.uk/weather/features/basics_lowpressure.shtml Weather Basics - Low Pressure.] Retrieved on 2007-05-05.</ref>

==== High pressure ====
High-pressure systems, also known as [[anticyclone]]s, rotate outward at the surface and clockwise in the northern hemisphere as opposed to outward and counterclockwise in the southern hemisphere. Under surface highs, sinking of the atmosphere slightly warms the air by compression, leading to clearer skies, winds that are lighter, and a reduced chance of precipitation.<ref>BBC Weather. [https://web.archive.org/web/20040907014518/http://www.bbc.co.uk/weather/features/understanding/highpressure.shtml High Pressure.] Retrieved on 2007-05-05.</ref> The descending air is dry, hence less energy is required to raise its temperature. If high pressure persists, air pollution will build up due to pollutants trapped near the surface caused by the subsiding motion associated with the high.<ref>United Kingdom School System. [http://atschool.eduweb.co.uk/radgeog/metlink/ppt/highs/Press-System.ppt Pressure, Wind and Weather Systems.] {{webarchive|url=https://web.archive.org/web/20070927190000/http://atschool.eduweb.co.uk/radgeog/metlink/ppt/highs/Press-System.ppt |date=2007-09-27 }} Retrieved on 2007-05-05.</ref>

=== Fronts ===
{{Main|Weather front}}
[[Image:Occluded cyclone.svg|thumb|Occluded cyclone example. The triple point is the intersection of the cold, warm, and occluded [[weather front|fronts]].]]

Fronts in meteorology are boundaries between [[air mass]]es that have different density, air temperature, and [[humidity]]. Strictly speaking, the front is marked at the warmer edge of a ''frontal zone'' where the [[gradient]] is very large. When a front passes over a point, it is marked by changes in temperature, moisture, wind speed and direction, a minimum of atmospheric pressure, and a change in the cloud pattern, sometimes with precipitation. [[Cold front]]s develop where the cold air mass is advancing, [[warm front]]s where the warm air is advancing, and a [[stationary front]] is not moving. Fronts classically wrap around low pressure centers as indicated in the [[:File:Occluded cyclone.svg|image]] here depicted for the Northern Hemisphere. On a larger scale, the Earth's [[polar front]] is a sharpening of the general equator-to-pole temperature gradient, underlying a high-altitude [[jet stream]] for reasons of [[Thermal wind|thermal wind balance]]. Fronts usually travel from west to east, although they can move in a north-south direction or even east to west (a [[backdoor cold front|"backdoor" front]]) as airflow wraps around a low pressure center. Frontal zones can be distorted by such geographic features as mountains and large bodies of water.<ref name="DR"/>

==== Cold front ====
{{Main|Cold front}}
A cold front is located at the leading edge of a sharp temperature gradient on an [[Isotherm (contour line)|isotherm]] analysis, often marked by a sharp surface pressure [[Trough (meteorology)|trough]]. Cold fronts can move up to twice as quickly as warm fronts and produce sharper changes in [[weather]] since cold air is denser than warm air and rapidly lifts as well as pushes the warmer air. Cold fronts are typically accompanied by a narrow band of clouds, showers and thunderstorms. On a weather map, the surface position of the cold front is marked with a blue line of triangles (pips) pointing in the direction of travel, at the leading edge of the cooler air mass.<ref name="DR"/>

==== Warm front ====

{{Main|Warm front}}

[[Warm front]]s mark the position on the Earth's surface where a relatively warm body of air is advancing into colder air. The front is marked on the warm edge of the gradient in isotherms, and lies within a low-pressure trough that tends to be broader and weaker than that of a cold front. Warm fronts move more slowly than cold fronts because cold air is denser, and is only pushed along (not lifted from) the Earth's surface. The warm air mass overrides the cold air mass, so temperature and cloud changes occur at higher altitudes before those at the surface. Clouds ahead of the warm front are mostly [[Stratus cloud|stratiform]] with precipitation that increases gradually as the front approaches. Ahead of a warm front, descending cloud bases will often begin with [[Cirrus cloud|cirrus]] and [[Cirrostratus cloud|cirrostratus]] (high-level), then [[Altostratus cloud|altostratus]] (mid-level) clouds, and eventually lower in the atmosphere as the front passes through. [[Fog]] can precede a warm front when precipitation falls into areas of colder air, but increasing surface temperatures and wind tend to dissipate it after a warm front passes through. Cases with environmental [[Convective instability|instability]] can be conducive to thunderstorm development. On weather maps, the surface location of a warm front is marked with a red line of half circles pointing in the direction of travel.

[[Image:Warmfrontai.svg|thumb|Illustration clouds overriding a [[warm front]]]]

==== Occluded front ====
{{Main|Occluded front}}The classical view of an [[occluded front]] is that they are formed when a cold front overtakes a warm front.<ref>University of Illinois. [http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/af/frnts/ofdef.rxml Occluded Front.] Retrieved on 2006-10-22.</ref> A more modern view suggests that they form directly during the wrap-up of the [[Baroclinity|baroclinic zone]] during [[cyclogenesis]], and lengthen due to flow [[Deformation (meteorology)|deformation]] and rotation around the cyclone.<ref>{{Cite journal|last1=Schultz|first1=David M.|last2=Vaughan|first2=Geraint|date=2011-04-01|title=Occluded Fronts and the Occlusion Process: A Fresh Look at Conventional Wisdom|journal=Bulletin of the American Meteorological Society|language=en|volume=92|issue=4|pages=443–466|doi=10.1175/2010BAMS3057.1|bibcode=2011BAMS...92..443S|issn=0003-0007|doi-access=free}}</ref>

Occluded fronts are indicated on a weather map by a purple line with alternating half-circles and triangles pointing in direction of travel: that is, with a mixture of warm and cold frontal colors and symbols. Occlusions can be divided into warm vs. cold types.<ref>{{Cite journal|last1=Stoelinga|first1=Mark T.|last2=Locatelli|first2=John D.|last3=Hobbs|first3=Peter V.|title=Warm Occlusions, Cold Occlusions, and Forward-Tilting Cold Fronts |date=2002-05-01|journal=Bulletin of the American Meteorological Society|language=en|volume=83|issue=5|pages=709–722|doi=10.1175/1520-0477(2002)083<0709:WOCOAF>2.3.CO;2|bibcode=2002BAMS...83..709S |issn=0003-0007|doi-access=free}}</ref> In a cold occlusion, the air mass overtaking the warm front is cooler than the cool air ahead of the warm front, and plows under both air masses. In a warm occlusion, the air mass overtaking the warm front is not as cool as the cold air ahead of the warm front, and rides over the colder air mass while lifting the warm air. Occluded fronts are indicated on a weather map by a purple line with alternating half-circles and triangles pointing in direction of travel.<ref name="DR"/>

Occluded fronts usually form around low-pressure systems in the mature or late stages of their life cycle, but some continue to deepen after occlusion, and some do not form occluded fronts at all. The weather associated with an occluded front includes a variety of cloud and precipitation patterns, including dry slots and banded precipitation. Cold, warm and occluded fronts often meet at the point of occlusion or triple point.<ref>National Weather Service Office, Norman, Oklahoma. [http://www.srh.noaa.gov/oun/severewx/glossary4.php#t Triple Point.] Retrieved on 2006-10-22. {{webarchive|url=https://web.archive.org/web/20061009165650/http://www.srh.noaa.gov/oun/severewx/glossary4.php#t|date=October 9, 2006}}</ref> [[Image:NWS weather fronts.svg|thumb|A guide to the symbols for [[weather fronts]] that may be found on a weather map: 
1. cold front 
2. warm front 
3. stationary front 
4. occluded front 
5. surface trough 
6. squall line 
7. dry line 
8. tropical wave 
9. Trowal]]

==== Stationary fronts and shearlines ====
{{Main|Stationary front}}
A stationary front is a non-moving boundary between two different air masses. They tend to remain in the same area for long periods of time, sometimes undulating in waves.<ref>University of Illinois. [http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/af/frnts/sfdef.rxml Stationary Front.] Retrieved on 2006-10-22.</ref> Often a less-steep temperature gradient continues behind (on the cool side of) the sharp frontal zone with more widely spaced isotherms. A wide variety of weather can be found along a stationary front, characterized more by its prolonged presence than by a specific type. Stationary fronts may dissipate after several days, but can change into a cold or warm front if conditions aloft change, driving one air mass toward the other. Stationary fronts are marked on weather maps with alternating red half-circles and blue spikes pointing in opposite directions, indicating no significant movement.{{Citation needed|date=December 2021}}

As airmass temperatures equalize, stationary fronts may become smaller in scale, degenerating to a narrow zone where wind direction changes over a short distance, known as a shear line,<ref>Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/search?p=1&query=shear+line Shear Line.] {{webarchive|url=https://web.archive.org/web/20070314081220/http://amsglossary.allenpress.com/glossary/search?p=1&query=shear+line |date=2007-03-14 }} Retrieved on 2006-10-22.</ref> depicted as a blue line of single alternating dots and dashes.<ref name="DR"/><ref>Aviation Weather. [https://www.aviationweather.ws/093_Transitory_Systems.php] Retrieved on 2021-03-13.</ref>

== Mesoscale features ==
{{see also|Mesoscale meteorology}}
[[Mesoscale meteorology|Mesoscale]] features are smaller than [[synoptic scale]] systems like fronts, but larger than [[storm-scale]] systems like thunderstorms. Horizontal dimensions generally range from over ten kilometres to several hundred kilometres.<ref>Fujita, T. T., 1986. ''Mesoscale classifications: their history and their application to forecasting.'' '''Mesoscale Meteorology and Forecasting.''' American Meteorological Society, Boston, p. 18–35.</ref>

=== Dry line ===
The [[dry line]] is the boundary between dry and moist air masses east of mountain ranges with similar orientation to the [[Rocky Mountains|Rockies]], depicted at the leading edge of the [[dew point]], or moisture, gradient. Near the surface, warm moist air that is denser than warmer, dryer air wedges under the drier air in a manner similar to that of a cold front wedging under warmer air.<ref>Huaqing Cai. [http://www.atmos.ucla.edu/~caihq/pic/fig23.html Dryline cross section.] {{webarchive|url=https://web.archive.org/web/20080120180130/http://www.atmos.ucla.edu/~caihq/pic/fig23.html |date=2008-01-20 }} Retrieved on 2006-12-05.</ref> When the warm moist air wedged under the drier mass heats up, it becomes less dense and rises and sometimes forms thunderstorms.<ref>{{cite web|url=http://www.accd.edu/sac/earthsci/sgirhard/1370.090/chap3.htm|archive-url=https://web.archive.org/web/20070927000112/http://www.accd.edu/sac/earthsci/sgirhard/1370.090/chap3.htm|title=Lecture 3|archive-date=27 September 2007}}</ref> At higher altitudes, the warm moist air is less dense than the cooler, drier air and the boundary slope reverses. In the vicinity of the reversal aloft, severe weather is possible, especially when a triple point is formed with a cold front.{{Citation needed|date=December 2021}}

During daylight hours, drier air from aloft drifts down to the surface, causing an apparent movement of the dryline eastward. At night, the boundary reverts to the west as there is no longer any solar heating to help mix the lower atmosphere.<ref>Lewis D. Grasso. [https://archive.today/20130116214601/http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175/1520-0493(2000)128%3C2816:ANSODS%3E2.0.CO;2 A Numerical Simulation of Dryline Sensitivity to Soil Moisture.] Retrieved on 2007-05-10.</ref> If enough moisture converges upon the dryline, it can be the focus of afternoon and evening thunderstorms.<ref>Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/search?id=lee-trough1 Lee Trough.] {{webarchive|url=https://web.archive.org/web/20110919042152/http://amsglossary.allenpress.com/glossary/search?id=lee-trough1 |date=2011-09-19 }} Retrieved on 2006-10-22.</ref> A dry line is depicted on United States surface analyses as a brown line with scallops, or bumps, facing into the moist sector. Dry lines are one of the few surface fronts where the special shapes along the drawn boundary do not necessarily reflect the boundary's direction of motion.<ref>University of Illinois. [http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/af/frnts/dfdef.rxml Dry Line: A Moisture Boundary.] Retrieved on 2006-10-22.</ref>

=== Outflow boundaries and squall lines ===
[[File:DangerousShelfCloud.jpg|thumb|A [[shelf cloud]] such as this one can be a sign that a [[squall]] is imminent]]

Organized areas of thunderstorm activity not only reinforce pre-existing frontal zones, but they can outrun cold fronts. This outrunning occurs in a pattern where the upper level jet splits into two streams. The resultant [[mesoscale convective system]] (MCS) forms at the point of the upper level split in the wind pattern at the area of the best low-level [[Inflow (meteorology)|inflow]]. The convection then moves east and equatorward into the warm sector, parallel to low-level thickness lines. When the convection is strong and linear or curved, the MCS is called a squall line, with the feature placed at the leading edge where the significant wind shifts and pressure rises.<ref>Office of the Federal Coordinator for Meteorology.[http://www.ofcm.gov/slso/pdf/slsochp2.pdf Chapter 2: Definitions.] {{webarchive|url=https://web.archive.org/web/20090506002006/http://www.ofcm.gov/slso/pdf/slsochp2.pdf |date=2009-05-06 }} Retrieved on 2006-10-22.</ref> Even weaker and less organized areas of thunderstorms will lead to locally cooler air and higher pressures, and [[Outflow boundary|outflow boundaries]] exist ahead of this type of activity, "SQLN" or "SQUALL LINE", while outflow boundaries are depicted as troughs with a label of "OUTFLOW BOUNDARY" or "OUTFLOW BNDRY".{{Citation needed|date=December 2021}}

=== Sea and land breeze fronts ===
[[File:LAKE BREEZE-en.gif|thumb|Idealized circulation pattern associated with a [[sea breeze]]]]

[[Sea breeze]] fronts occur on sunny days when the landmass warms the air above it to a temperature above the water temperature. Similar boundaries form downwind on lakes and rivers during the day, as well as offshore landmasses at night. Since the [[specific heat]] of water is so high, there is little diurnal temperature change in bodies of water, even on the sunniest days. The water temperature varies less than {{convert|1|C-change}}. By contrast, the land, with a lower specific heat, can vary several degrees in a matter of hours.<ref name=seabreeze>Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/search?p=1&query=sea+breeze Sea Breeze.] {{webarchive|url=https://web.archive.org/web/20070314081147/http://amsglossary.allenpress.com/glossary/search?p=1&query=sea+breeze |date=2007-03-14 }} Retrieved on 2006-10-22.</ref>

During the afternoon, air pressure decreases over the land as the warmer air rises. The relatively cooler air over the sea rushes in to replace it. The result is a relatively cool onshore wind. This process usually reverses at night where the water temperature is higher relative to the landmass, leading to an offshore land breeze. However, if water temperatures are colder than the land at night, the sea breeze may continue, only somewhat abated. This is typically the case along the [[California]] coast, for example.{{Citation needed|date=December 2021}}

If enough moisture exists, thunderstorms can form along sea breeze fronts that then can send out outflow boundaries. This causes chaotic wind/pressure regimes if the steering flow is weak. Like all other surface features, sea breeze fronts lie inside troughs of low pressure.{{Citation needed|date=December 2021}}

== Microscale features ==
{{See also|Microscale meteorology}}

=== Descending reflectivity core ===
{{Full article|Descending reflectivity core}}
A descending reflectivity core (DRC) is a [[Meteorology|meteorological]] phenomenon observed in [[supercell thunderstorms]], characterized by a localized, small-scale area of enhanced [[Weather radar|radar reflectivity]] that [[Subsidence (atmosphere)|descends]] from the [[echo overhang]] into the [[Supercell#Structure|lower levels]] of the storm.

== See also ==
* ''[[Bowditch's American Practical Navigator]]''
* [[Extratropical cyclone]]
* [[Frontolysis]]
* [[Outline of meteorology]]
* [[Ridge (meteorology)]]

== References ==
{{Reflist|30em}}

== External links ==
{{Commons category|Atmospheric fronts}}
* [http://www.physicalgeography.net/fundamentals/7s.html "The Mid-Latitude Cyclone"]
* [https://web.archive.org/web/20160104214105/http://www.srh.weather.gov/srh/jetstream/synoptic/cyclone.htm Norwegian Cyclone Model — NWS]
* [http://www.wpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf Unified Surface Analysis Manual — NWS]
* [https://web.archive.org/web/20171016122221/http://www.opc.ncep.noaa.gov/unified_analysis.php Unified Surface Analysis — NWS]
* [http://amsglossary.allenpress.com/glossary/ Glossary of Meteorology]
* [http://ww2010.atmos.uiuc.edu/(Gl)/guides/mtr/af/frnts/cfrnt/def.rxml Cold Front Page]

{{Meteorological variables|state=autocollapse}}
{{climate change}}
{{authority control}}

{{DEFAULTSORT:Surface Weather Analysis}}
[[Category:Synoptic meteorology and weather]]
[[Category:Weather prediction]]

Sunlight

2025-10-16T19:01:59Z

2405:6E00:655:B199:99E7:2891:DECD:B143:

{{Short description|Light emitted by the Sun}}
{{other uses}}
{{Redirect|Sunshine}}
[[File:STS-134 EVA4 view to the Russian Orbital Segment.jpg|thumb|right|The Sun, as seen from low Earth orbit overlooking the [[International Space Station]]. This sunlight is not filtered by the lower atmosphere, which blocks much of the solar spectrum.]]
{{multiple image
| align = right
| direction = vertical
| image1 = Spring Lake, New Jersey Beach at Sunrise.jpg|
| width1 = 225
| caption1 =
| alt1 =
| image2 = Cape may.jpg
| width2 = 225
| caption2 = Sunlight shining upon two different sides of the [[U.S. state]] of [[New Jersey]]. [[Sunrise]] on the [[Jersey Shore]] at [[Spring Lake, New Jersey|Spring Lake]], [[Monmouth County, New Jersey|Monmouth County]] (above), and [[sunset]] on the Shore at [[Sunset Beach (New Jersey)|Sunset Beach]], [[Cape May County, New Jersey|Cape May County]] (below). Both are filtered through high [[stratus clouds]].
| alt2 =
}}
[[File:Apollo 7 Florida.jpg|thumb|right|Sunrise over the [[Gulf of Mexico]] and [[Florida]]. Taken on 20 October 1968 from [[Apollo 7]].]]

'''Sunlight''' is the portion of the [[electromagnetic radiation]] which is emitted by the [[Sun]] (i.e. solar radiation) and received by the [[Earth]], in particular the [[visible spectrum|visible]] light perceptible to the [[human eye]] as well as invisible [[infrared]] (typically perceived by humans as warmth) and [[ultraviolet]] (which can have physiological effects such as [[sunburn]]) lights. However, according to the [[American Meteorological Society]], there are "conflicting conventions as to whether all three [...] are referred to as light, or whether that term should only be applied to the visible portion of the spectrum".<ref>{{cite web |title=Sunlight |website=Glossary of Meteorology |publisher=American Meteorological Society |access-date=2025-03-23 |url=https://glossary.ametsoc.org/wiki/Sunlight#:~:text=Light%20from%20the%20sun%3B,greater%20than%20about%200.7%20%CE%BCm.}}</ref> Upon reaching the Earth, sunlight is [[light scattering by particles|scattered]] and [[attenuation|filtered]] through the [[atmosphere of Earth|Earth's atmosphere]] as [[daylight]] when the Sun is above the [[horizon]]. When direct [[solar radiation]] is not blocked by [[cloud]]s, it is experienced as '''sunshine''', a combination of bright [[light]] and [[radiant heat]] (atmospheric). When [[cloud cover|blocked by clouds]] or [[diffuse reflection|reflected off other objects]], sunlight is [[diffuser (optics)|diffused]]. Sources estimate a global average of between 164 watts to 340 watts<ref>{{cite web|access-date=2022-01-27|title=Climate and Earth's Energy Budget|url=https://earthobservatory.nasa.gov/features/EnergyBalance/page2.php|date=14 January 2009|website=earthobservatory.nasa.gov}}</ref> per square meter over a 24-hour day;<ref>{{cite web | url=http://zebu.uoregon.edu/disted/ph162/l4.html | title=Basics of Solar Energy | access-date=2016-12-06 | url-status=live | archive-url=https://web.archive.org/web/20161128021408/http://zebu.uoregon.edu/disted/ph162/l4.html | archive-date=2016-11-28 }}</ref> this figure is estimated by NASA to be about a quarter of Earth's average [[total solar irradiance]].

The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a requisite for [[vitamin D3|vitamin D3]] synthesis and a [[mutagen]].

Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun.<ref>{{cite book |title=An Introduction to the Sun and Stars |edition=illustrated |first1=S. Jocelyn |last1=Bell Burnell |publisher=Cambridge University Press |year=2004 |isbn=978-0-521-54622-5 |page=56 |url=https://books.google.com/books?id=lb5owLGIQGsC}} [https://books.google.com/books?id=lb5owLGIQGsC&pg=PA56 Extract of page 56]</ref> A photon starting at the center of the Sun and changing direction every time it encounters a [[charged particle]] would take between 10,000 and 170,000 years to get to the surface.<ref>{{cite web |url= http://sunearthday.nasa.gov/2007/locations/ttt_sunlight.php |title= The 8-minute travel time to Earth by sunlight hides a thousand-year journey that actually began in the core |publisher= [[NASA]] |website= SunEarthDay.NASA.gov |access-date= 2012-02-12 |archive-url= https://web.archive.org/web/20120122162340/http://sunearthday.nasa.gov/2007/locations/ttt_sunlight.php |archive-date= 2012-01-22 }}</ref>

Sunlight is a key factor in [[photosynthesis]], the process used by plants and other [[autotroph]]ic organisms to convert [[light energy]], normally from the Sun, into [[chemical energy]] that can be used to synthesize carbohydrates and fuel the organisms' activities.

''[[Daylighting (architecture)|Daylighting]]'' is the natural lighting of interior spaces by admitting sunlight.
''[[Solar irradiance]]'' is the rate of solar energy received by a unit area from sunlight.

==Measurement==
Researchers can measure the intensity of sunlight using a [[sunshine recorder]], [[pyranometer]], or [[pyrheliometer]]. To calculate the amount of sunlight reaching the ground, both the [[orbital eccentricity|eccentricity]] of Earth's [[elliptic orbit]] and the [[Extinction (astronomy)#Atmospheric extinction|attenuation]] by [[Earth's atmosphere]] have to be taken into account. The extraterrestrial solar illuminance ({{math|''E''ext}}), corrected for the elliptic orbit by using the day number of the year (dn), is given to a good approximation by<ref>{{cite journal|author=C. KANDILLI|author2=K. ULGEN|name-list-style=amp|title=Solar Illumination and Estimating Daylight Availability of Global Solar Irradiance|journal=Energy Sources}}</ref>
:<math>E_{\rm ext}= E_{\rm sc} \cdot \left(1+0.033412 \cdot \cos\left(2\pi\frac{{\rm dn}-3}{365}\right)\right),</math>
where dn=1 on January 1; dn=32 on February 1; dn=59 on March 1 (except on leap years, where dn=60), etc. In this formula dn–3 is used, because in modern times [[Apsis#Earth perihelion and aphelion|Earth's perihelion]], the closest approach to the Sun and, therefore, the maximum {{math|''E''ext}} occurs around January 3 each year. The value of 0.033412 is determined knowing that the ratio between the perihelion (0.98328989 AU) squared and the aphelion (1.01671033 AU) squared should be approximately 0.935338.

The solar illuminance constant ({{math|''E''sc}}), is equal to 128×103 [[lux]]. The direct normal illuminance ({{math|''E''dn}}), corrected for the attenuating effects of the atmosphere is given by:
:<math>E_{\rm dn}=E_{\rm ext}\,e^{-cm},</math>
where {{mvar|c}} is the [[atmospheric extinction]] and {{mvar|m}} is the relative optical [[airmass]]. The atmospheric extinction brings the number of lux down to around 100,000 lux.

The total amount of energy received at ground level from the Sun at the zenith depends on the distance to the Sun and thus on the time of year. It is about 3.3% higher than average in January and 3.3% lower in July (see below). If the extraterrestrial solar radiation is 1,367 watts per square meter (the value when the Earth–Sun distance is 1 [[astronomical unit]]), then the direct sunlight at Earth's surface when the Sun is at the [[zenith]] is about 1,050 W/m2, but the total amount (direct and indirect from the atmosphere) hitting the ground is around 1,120 W/m2.<ref name="Solar constant at ground level">{{cite web|title=Introduction to Solar Radiation|url= http://www.newport.com/Introduction-to-Solar-Radiation/411919/1033/content.aspx|publisher= Newport Corporation|url-status= live|archive-url= https://web.archive.org/web/20131029234117/http://www.newport.com/Introduction-to-Solar-Radiation/411919/1033/content.aspx|archive-date=October 29, 2013}}</ref> In terms of energy, sunlight at Earth's surface is around 52 to 55 percent infrared (above 700 [[nanometre|nm]]), 42 to 43 percent visible (400 to 700 nm), and 3 to 5 percent ultraviolet (below 400 nm).<ref>Calculated from data in {{cite web|url=https://www.nrel.gov/grid/solar-resource/spectra.html |title=Reference Solar Spectral Irradiance: Air Mass 1.5|access-date=2009-11-12|url-status=live|archive-url=https://web.archive.org/web/20130928011257/http://rredc.nrel.gov/solar/spectra/am1.5/ASTMG173/ASTMG173.xls|archive-date=September 28, 2013|publisher=National Renewable Energy Laboratory}} The first of each set of two figures is for total solar radiation reaching a panel aimed at the Sun (which is 42° above the horizon), whereas the second figure of each pair is the "direct plus circumsolar" radiation (circumsolar meaning coming from the part of the sky within a couple degrees of the Sun). The totals, from 280 to 4000 nm, are 1000.4 and 900.1 W/m2 respectively. It would be good to have more direct figures from a good source, rather than summing thousands of numbers in a database.</ref> At the top of the atmosphere, sunlight is about 30% more intense, having about 8% [[ultraviolet]] (UV),<ref>Calculated from the ASTM spectrum cited above.</ref> with most of the extra UV consisting of biologically damaging short-wave ultraviolet.<ref name="Solar radiation">
{{cite book |last=Qiang |first=Fu |chapter=Radiation (Solar) |chapter-url=http://curry.eas.gatech.edu/Courses/6140/ency/Chapter3/Ency_Atmos/Radiation_Solar.pdf |editor1-last=Holton |editor1-first=James R. |title=Encyclopedia of atmospheric sciences |volume=5 |publisher=Academic Press |location=Amsterdam |date=2003 |pages=1859–1863 |oclc=249246073 |isbn=978-0-12-227095-6 |url-status=live |archive-url=https://web.archive.org/web/20121101070344/http://curry.eas.gatech.edu/Courses/6140/ency/Chapter3/Ency_Atmos/Radiation_Solar.pdf |archive-date=2012-11-01 }}
</ref>

{{vanchor|Direct sunlight}} has a [[luminous efficacy]] of about 93 [[lumen (unit)|lumens]] per watt of [[radiant flux]]. This is higher than the efficacy (of source) of [[artificial lighting]] other than [[Light-emitting diode|LED]]s, which means using sunlight for illumination heats up a room less than fluorescent or incandescent lighting. Multiplying the figure of 1,050 watts per square meter by 93 lumens per watt indicates that bright sunlight provides an [[illuminance]] of approximately 98,000 [[lux]] ([[lumen (unit)|lumens]] per square meter) on a perpendicular surface at sea level. The illumination of a horizontal surface will be considerably less than this if the Sun is not very high in the sky. Averaged over a day, the highest amount of sunlight on a horizontal surface occurs in January at the [[South Pole]] (see [[insolation]]).

Dividing the [[irradiance]] of 1,050 W/m2 by the size of the Sun's disk in [[steradian]]s gives an average [[radiance]] of 15.4 MW per square metre per steradian. (However, the radiance at the center of the Sun's disk is somewhat higher than the average over the whole disk due to [[limb darkening]].) Multiplying this by π gives an upper limit to the irradiance which can be focused on a surface using mirrors: 48.5 MW/m2.<ref>{{cite book | title=Introduction to Optics | publisher=[[Prentice Hall]] | last=Pedrotti & Pedrotti | date=1993 | isbn=0-13-501545-6 | url-access=registration | url=https://archive.org/details/introductiontoop00pedr }}</ref>

==Composition and power==
[[File:Solar spectrum compared to black-body.gif|thumb|350px|Solar spectrum compared to black-body at 5775 K]]

{{See also|Ultraviolet|Infrared|Light}}

The [[frequency spectrum|spectrum]] of the Sun's solar radiation can be compared to [[black-body radiation|that of a black body]]<ref>{{cite journal | doi = 10.1038/156534b0 | volume=156 | title=Departure of Long-Wave Solar Radiation from Black-Body Intensity | year=1945 | journal=Nature | pages=534–535 | last1 = Appleton | first1 = Edward V.| issue=3966 | bibcode=1945Natur.156..534A }}</ref><ref>Iqbal, M., "An Introduction to Solar Radiation", Academic Press (1983), Chap. 3</ref> with a temperature of about 5,800 [[Kelvin|K]]<ref>[http://solarsystem.nasa.gov/planets/profile.cfm?Display=Facts&Object=Sun NASA Solar System Exploration – Sun: Facts & Figures] {{webarchive|url=https://web.archive.org/web/20150703111716/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts |date=2015-07-03 }} retrieved 27 April 2011 "Effective Temperature ... 5777 K"</ref> (see graph). The Sun emits EM radiation across most of the [[electromagnetic spectrum]]. Although the radiation created in the solar core consists mostly of [[x ray]]s, internal absorption and thermalization convert these super-high-energy [[photon]]s to lower-energy photons before they reach the Sun's surface and are emitted out into space. As a result, the [[photosphere]] of the Sun does not emit much X radiation ([[solar X-rays]]), although it does emit such "hard radiations" as X-rays and even [[gamma rays]] during [[solar flare]]s.<ref>{{cite web|url=http://www.nasa.gov/mission_pages/GLAST/news/highest-energy.html|title=Fermi Detects Solar Flare's Highest-Energy Light|first=Rob|last=Garner|date=24 January 2017|access-date=25 January 2018|url-status=live|archive-url=https://web.archive.org/web/20170517060027/https://www.nasa.gov/mission_pages/GLAST/news/highest-energy.html|archive-date=17 May 2017}}</ref> The quiet (non-flaring) Sun, including its [[Stellar corona|corona]], emits a broad range
of wavelengths: [[X-ray]]s, [[ultraviolet]], [[visible light]], [[infrared]], and [[radio wave]]s.<ref>{{cite web |url= http://www.windows2universe.org/sun/spectrum/multispectral_sun_overview.html |title= The Multispectral Sun, from the National Earth Science Teachers Association |publisher= Windows2universe.org |date= 2007-04-18 |access-date= 2012-02-12 |url-status= live |archive-url= https://web.archive.org/web/20120229041535/http://www.windows2universe.org/sun/spectrum/multispectral_sun_overview.html |archive-date= 2012-02-29 }}</ref> Different depths in the photosphere have different temperatures, and this partially explains the deviations from a black-body spectrum.<ref>See video referenced in the sentence "For more details about the comparison of the black body with the AM0 spectrum, see this video" at {{cite web |last1=Pietro Altermatt |title=The Extraterrestrial Spectrum |url=https://pvlighthouse.com.au/cms/lectures/altermatt/solar_spectrum/blackbody-radiation |website=PV Lighthouse |publisher=PV Lighthouse Pty. Ltd.}}</ref>

There is also a flux of gamma rays from the quiescent Sun, obeying a [[power law]] between 0.5 and 2.6 [[TeV]]. Some gamma rays are caused by [[cosmic rays]] interacting with the solar atmosphere, but this does not explain these findings.<ref>{{cite journal |last1=Wilkinson |first1=Ryan |title=Record-Breaking Detection of Solar Photons |journal=Physics |date=3 August 2023 |volume=16 |article-number=s107 |doi=10.1103/Physics.16.s107 |bibcode=2023PhyOJ..16.s107W |doi-access=free }}</ref><ref>{{cite journal |last1=Leah Crane |title=Astronomers have spotted inexplicably bright light coming from the sun |journal=New Scientist |date=Aug 3, 2023 |url=https://www.newscientist.com/article/2386042-astronomers-have-spotted-inexplicably-bright-light-coming-from-the-sun/}}</ref><ref>{{cite journal |last1=A. Albert |title=Discovery of Gamma Rays from the Quiescent Sun with HAWC |journal=Phys. Rev. Lett. |date=Aug 3, 2023 |volume=131 |issue=5 |article-number=051201 |doi=10.1103/PhysRevLett.131.051201|pmid=37595214 |arxiv=2212.00815 |bibcode=2023PhRvL.131e1201A }}</ref>

The only direct signature of the nuclear processes in the core of the Sun is via the very weakly interacting [[neutrino]]s.

[[File:Solar spectrum en.svg|thumb|upright=1.25|Solar [[spectral irradiance]] (watts per square metre per nanometre) above atmosphere (yellow) and at surface (red). Extreme UV and X-rays are produced (left of wavelength range) but comprise very small amounts of the Sun's total output power (area under the curve).]]
[[File:Spectral Distribution of Sunlight.svg|thumb|upright=1.25|Spectral distribution of sunlight. The different curves reflect 3 different equally valid ways of characterizing the same sunlight. These curves have peaks at different wavelengths, which demonstrates that the notion of a location where the "peak" amount of sunlight is emitted is not meaningful, and is not a characteristic of the light itself (but is merely an artifact of how the spectrum is represented). Percentiles offer a way of thinking about the distribution of energy which is independent of the representation. 50 percent of solar irradiance is associated with wavelengths less than about 711 nm (based on approximating sunlight by the emissions of a 5775 K blackbody).]]

Although the [[Stellar corona|solar corona]] is a source of [[extreme ultraviolet]] and X-ray radiation, these rays make up only a very small amount of the power output of the Sun (see spectrum at right). The spectrum of nearly all (roughly 98.7%) of the solar [[electromagnetic radiation]] striking the [[Atmosphere of Earth|Earth's atmosphere]] spans a range of 200 [[Nanometre|nm]] to about 4000 nm.<ref>{{Cite journal |last=Gueymard |first=Christian A. |date=April 2004 |title=The sun's total and spectral irradiance for solar energy applications and solar radiation models |journal=Solar Energy |language=en |volume=76 |issue=4 |page=432 |doi=10.1016/j.solener.2003.08.039 |bibcode=2004SoEn...76..423G }}</ref> This band of significant radiation power can be divided into five regions in increasing order of [[wavelength]]s:<ref>{{cite web
|last = Naylor
|first = Mark
|author2 = Kevin C. Farmer
|title = Sun damage and prevention
|work = Electronic Textbook of Dermatology
|publisher = The Internet Dermatology Society
|date = 1995
|url = http://www.telemedicine.org/sundam/sundam2.4.1.html
|access-date = 2008-06-02
|archive-url = https://web.archive.org/web/20080705111726/http://telemedicine.org/sundam/sundam2.4.1.html
|archive-date = 2008-07-05
}}</ref>
* '''Ultraviolet C''' or (UVC) range, which spans a range of 100 to 280 nm. The term ''ultraviolet'' refers to the fact that the radiation is at higher frequency than violet light (and, hence, also invisible to the [[human eye]]). Due to absorption by the atmosphere very little reaches Earth's surface. This spectrum of radiation [[Ultraviolet germicidal irradiation|has germicidal properties]], as used in [[germicidal lamp]]s.
* '''Ultraviolet B''' or (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the Earth's atmosphere, and along with UVC causes the [[photochemical reaction]] leading to the production of the [[ozone layer]]. It directly damages DNA and causes [[sunburn]].<ref name="Sunlight and Vitamin D: A global pe">{{cite journal | title = Sunlight and Vitamin D: A global perspective for health. | vauthors = Wacker M, Holick, MF | date = 2013 | journal = Dermato-Endocrinology | volume = 5 | issue = 1 | pages = 51–108 | doi = 10.4161/derm.24494 | pmid = 24494042 | pmc=3897598}}</ref> In addition to this short-term effect it enhances skin ageing and significantly promotes the development of skin cancer,<ref>{{Cite web|title=Radiation: Ultraviolet (UV) radiation
| date = 9 March 2016
| author= World Health Organization
|url=https://www.who.int/news-room/questions-and-answers/item/radiation-ultraviolet-(uv)|access-date=2023-02-08|language=en}}</ref> but is also required for [[vitamin D]] synthesis in the skin of mammals.<ref name="Sunlight and Vitamin D: A global pe"/>
* '''Ultraviolet A''' or (UVA) spans 315 to 400 nm. This band was once{{when|date=December 2016}} held to be less damaging to [[DNA]], and hence is used in cosmetic artificial [[sun tanning]] ([[tanning booth]]s and [[tanning bed]]s) and [[PUVA]] therapy for [[psoriasis]]. However, UVA is now known to cause significant damage to DNA via indirect routes (formation of [[free radicals]] and [[reactive oxygen species]]), and can cause cancer.<ref>{{cite journal|title=Ultraviolet Radiation Exposure and Its Impact on Skin Cancer Risk|date=1 August 2017|pmc = 5036351|last1 = Watson|first1 = M.|last2 = Holman|first2 = D. M.|last3 = Maguire-Eisen|first3 = M.|journal = Seminars in Oncology Nursing|volume = 32|issue = 3|pages = 241–254|doi = 10.1016/j.soncn.2016.05.005|pmid = 27539279}}</ref>
* '''[[Visible light|Visible]] range''' or '''light''' spans 380 to 700 nm.<ref>{{Cite web | url=https://science.nasa.gov/ems/09_visiblelight |title = Visible Light {{pipe}} Science Mission Directorate| date=10 August 2016 }}</ref> As the name suggests, this range is visible to the naked eye.
* '''[[Infrared]]''' range that spans 700 nm to 1,000,000 nm (1 [[Millimeter|mm]]). It comprises an important part of the electromagnetic radiation that reaches Earth. Scientists divide the infrared range into three types on the basis of wavelength:
** Infrared-A: 700 nm to 1,400 nm
** Infrared-B: 1,400 nm to 3,000 nm
** Infrared-C: 3,000 nm to 1 mm.

The sunlight reaching Earth's surface is 49.4% infrared, 42.3% visible, and 8% ultraviolet.<ref name="fondriest">{{cite web |title=Solar Radiation and Photosynethically Active Radiation |url=https://www.fondriest.com/environmental-measurements/parameters/weather/solar-radiation/ |publisher=Fondriest Environmental |access-date=7 March 2025 |date=March 21, 2014}}</ref>

It is sometimes asserted that the Sun's maximum output is in the visible range. However, this statement is a misconception based on only seeing the solar spectral irradiance plotted on a per-wavelength basis. When plotted that way, the power spectral density of sunlight peaks at a wavelength of about 501 nm, which is in the visible range. However, the solar spectral irradiance can with equal validity be calculated on a per-frequency basis, in which case the maximum is at {{val|3.40e14|u=Hz}}, corresponding to a wavelength of about 882 nm, which is in the near infrared (Infrared-A) range. Counterintuitively, it is not meaningful to assert that the solar output is greatest at some precise location in the spectrum.<ref name="oomisconcept">{{cite web |last1=Mobley |first1=Curtis |title=A Common Misconception |url=https://www.oceanopticsbook.info/view/light-and-radiometry/level-2/common-misconception |website=Ocean Optics |access-date=6 March 2025}}</ref>

===Published tables===
Tables of direct solar radiation on various slopes from 0 to 60 degrees north latitude, in calories per square centimetre, issued in 1972 and published by Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Portland, Oregon, USA, appear on the web.<ref>
{{cite web
|url = https://www.fs.usda.gov/pnw/pubs/pnw_rp142.pdf
|title = Direct Solar Radiation On Various Slopes From 0 To 60 Degrees North Latitude
|author = John Buffo
|author2 = Leo J. Fritschen
|author3 = James L. Murphy
|publisher = Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Portland, Oregon, USA
|date = 1972
|access-date = 15 Jan 2014
|url-status = live
|archive-url = https://web.archive.org/web/20131127124158/http://www.fs.fed.us/pnw/pubs/pnw_rp142.pdf
|archive-date = 2013-11-27
}}
</ref>

==Intensity in the Solar System==
[[File:Mars sunset PIA00920.jpg|thumb|right|Sunlight on Mars is dimmer than on Earth. This photo of a Martian sunset was imaged by ''[[Mars Pathfinder]]''. ]]
Different bodies of the [[Solar System]] receive light of an intensity inversely proportional to the square of their distance from the Sun.

A table comparing the amount of solar radiation received by each planet in the Solar System at the top of its atmosphere:<ref>{{cite web|archive-url=https://web.archive.org/web/20091122194548/http://starhop.com/library/pdf/studyguide/high/SolInt-19.pdf|archive-date=2009-11-22|url=http://starhop.com/library/pdf/studyguide/high/SolInt-19.pdf|title=Solar Intensity|publisher=McAuliffe-Shepard Discovery Center}}</ref>
{| class="wikitable"
|-
! rowspan=2 |[[Planet]] or [[dwarf planet]]
! colspan=2 |distance ([[Astronomical unit|AU]])
! colspan=2 |Solar radiation (W/m2)
|-
|[[Perihelion]]||[[Aphelion]]
|maximum||minimum
|-
| [[Mercury (planet)|Mercury]]
| 0.3075 || 0.4667
| 14,446 || 6,272
|-
| [[Venus]]
| 0.7184 || 0.7282
| 2,647 || 2,576
|-
| [[Earth]]
| 0.9833 || 1.017
| 1,413 || 1,321
|-
| [[Mars]]
| 1.382 || 1.666
| 715 || 492
|-
| [[Jupiter]]
| 4.950 || 5.458
| 55.8 || 45.9
|-
| [[Saturn]]
| 9.048 || 10.12
| 16.7 || 13.4
|-
| [[Uranus]]
| 18.38 || 20.08
| 4.04 || 3.39
|-
| [[Neptune]]
| 29.77 || 30.44
| 1.54 || 1.47
|-
| [[Pluto]]||29.66||48.87|| 1.55|| 0.57
|}

The actual brightness of sunlight that would be observed at the surface also depends on the presence and composition of an [[atmosphere]]. For example, [[Atmosphere of Venus|Venus's thick atmosphere]] reflects more than 60% of the solar light it receives. The actual illumination of the surface is about 14,000 lux, comparable to that on Earth "in the daytime with overcast clouds".<ref>{{cite journal |title=The Unveiling of Venus: Hot and Stifling |journal=Science News |volume=109 |issue=25 |pages=388–389 |date=1976-06-19 |quote=100 watts per square meter ... 14,000 lux ... corresponds to ... daytime with overcast clouds |jstor=3960800 |doi=10.2307/3960800}}</ref>

Sunlight on Mars would be more or less like daylight on Earth during a slightly overcast day, and, as can be seen in the pictures taken by the rovers, there is enough [[diffuse sky radiation]] that shadows would not seem particularly dark. Thus, it would give perceptions and "feel" very much like Earth daylight. The spectrum on the surface is slightly redder than that on Earth, due to scattering by reddish dust in the Martian atmosphere.

For comparison, sunlight on Saturn is slightly brighter than Earth sunlight at the average sunset or sunrise. Even on Pluto, the sunlight would still be bright enough to almost match the average living room. To see sunlight as dim as full [[moonlight]] on Earth, a distance of about 500 AU (~69 [[light-hour]]s) is needed; only a handful of objects in the Solar System have been discovered that are known to orbit farther than such a distance, among them [[90377 Sedna]] and {{mpl|(87269) 2000 OO|67}}.

==Variations in solar irradiance==

===Seasonal and orbital variation===
{{Further|Insolation|Sunshine duration}}
On Earth, the solar radiation varies with the angle of the Sun above the [[horizon]], with longer sunlight duration at high latitudes during summer, varying to no sunlight at all in winter near the pertinent pole. When the direct radiation is not blocked by clouds, it is experienced as ''sunshine''. The warming of the ground (and other objects) depends on the [[absorption (electromagnetic radiation)|absorption of the electromagnetic radiation]] in the form of [[heat]].

The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. Earth's [[orbit]] and [[obliquity]] change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an [[orbital eccentricity]] of 5% (currently 1.67%). As the orbital eccentricity changes, the average distance from the Sun (the [[semimajor axis]] does not significantly vary, and so the total [[insolation]] over a year remains almost constant due to [[Kepler's second law]],
:<math>\tfrac{2A}{r^2}dt = d\theta,</math>

where <math>A</math> is the "areal velocity" invariant. That is, the integration over the orbital period (also invariant) is a constant.

:<math>\int_{0}^{T} \tfrac{2A}{r^2}dt = \int_{0}^{2\pi} d\theta = \mathrm{constant}.</math>

If we assume the solar radiation power {{mvar|P}} as a constant over time and the [[solar irradiation]] given by the [[inverse-square law]], we obtain also the average insolation as a constant. However, the [[season]]al and latitudinal distribution and intensity of solar radiation received at Earth's surface does vary.<ref>{{cite web |url=http://www.museum.state.il.us/exhibits/ice_ages/insolation_graph.html |title=Graph of variation of seasonal and latitudinal distribution of solar radiation |publisher=Museum.state.il.us |date=2007-08-30 |access-date=2012-02-12 |url-status=live |archive-url=https://web.archive.org/web/20120112043906/http://www.museum.state.il.us/exhibits/ice_ages/insolation_graph.html |archive-date=2012-01-12 }}</ref> The [[effect of Sun angle on climate]] results in the change in solar energy in summer and winter. For example, at [[latitude]]s of 65 degrees, this can vary by more than 25% as a result of Earth's orbital variation. Because changes in winter and summer tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent [[ice age]]s (see: [[Milankovitch cycles]]).

===Solar intensity variation===
{{Further|Solar variation}}
Space-based observations of solar irradiance started in 1978. These measurements show that the solar constant is not constant. It varies on many time scales, including the 11-year sunspot solar cycle.<ref name="acrim"/> When going further back in time, one has to rely on irradiance reconstructions, using sunspots for the past 400 years or cosmogenic radionuclides for going back 10,000 years.
Such reconstructions have been done.<ref>{{cite journal | title = Modeling the Sun's Magnetic Field and Irradiance since 1713 | last1 = Wang | display-authors = etal | date = 2005 | journal = The Astrophysical Journal | volume = 625 | issue = 1| pages = 522–538 | doi = 10.1086/429689 | bibcode=2005ApJ...625..522W| doi-access = free }}</ref><ref>{{cite journal | title = Total solar irradiance since 1996: is there a long-term variation unrelated to solar surface magnetic phenomena? | last1 = Steinhilber | display-authors = etal | date = 2009 | url =https://www.dora.lib4ri.ch/eawag/islandora/object/eawag%3A6539/datastream/PDF/view | journal = Geophysical Research Letters | volume = 36 | page = L19704 | doi = 10.1051/0004-6361/200811446 | bibcode=2010A&A...523A..39S| doi-access = free }}</ref><ref>{{cite journal | title = Evolution of the solar irradiance during the Holocene | last1 = Vieira | display-authors = etal | date = 2011 | journal = Astronomy & Astrophysics | volume = 531 | page = A6 | doi = 10.1051/0004-6361/201015843 | bibcode=2011A&A...531A...6V |arxiv = 1103.4958 }}</ref><ref>{{cite journal | title = 9,400 years of cosmic radiation and solar activity from ice cores and tree rings | last1 = Steinhilber | display-authors = etal | date = 2012 | journal = Proceedings of the National Academy of Sciences | volume = 109| issue = 16| pages = 5967–5971| doi = 10.1073/pnas.1118965109 |bibcode = 2012PNAS..109.5967S | pmid=22474348 | pmc=3341045 | doi-access = free }}</ref> These studies show that in addition to the solar irradiance variation with the solar cycle (the (Schwabe) cycle), the solar activity varies with longer cycles, such as the proposed 88 year (Gleisberg cycle), 208 year (DeVries cycle) and 1,000 year (Eddy cycle).

==Solar irradiance==
{{Main|Solar irradiance}}
===Solar constant===

[[File:Solar irradiance spectrum 1992.gif|thumb|upright=1.3|Solar irradiance spectrum at top of atmosphere, on a linear scale and plotted against [[wavenumber]]]]
{{Main|Solar constant}}

The '''solar constant''' is a measure of [[flux density]], is the amount of incoming solar [[electromagnetic radiation]] per unit area that would be incident on a plane perpendicular to the rays, at a distance of one [[astronomical unit]] (AU) (roughly the mean distance from the Sun to Earth). The "solar constant" includes all types of solar radiation, not just the [[visible light]]. Its average value was thought to be approximately 1,366 W/m2,<ref name=TSI20>{{cite web |url=http://acrim.com/TSI%20Monitoring.htm |title=Satellite observations of total solar irradiance |publisher=Acrim.com |access-date=2012-02-12 |url-status=live |archive-url=https://web.archive.org/web/20030204191816/http://acrim.com/TSI%20Monitoring.htm |archive-date=2003-02-04 }}</ref> varying slightly with [[solar cycle|solar activity]], but recent recalibrations of the relevant satellite observations indicate a value closer to 1,361 W/m2 is more realistic.<ref name=KoppLean11>{{cite journal|last1=G. Kopp|author2=J. Lean |title=A new, lower value of total solar irradiance: Evidence and climate significance|journal=Geophys. Res. Lett.|date=2011|pages=L01706|doi=10.1029/2010GL045777|bibcode = 2011GeoRL..38.1706K|first1=Greg|volume=38 |issue=1 |doi-access=free}}</ref>

===Total solar irradiance (TSI) and spectral solar irradiance (SSI) upon Earth===
{{Anchor|Total Solar Irradiance|TSI}}
Since 1978, a series of overlapping NASA and ESA satellite experiments have measured '''[[total solar irradiance]]''' (TSI) – the amount of solar radiation received at the top of Earth's atmosphere – as 1.365 kilo⁠watts per square meter (kW/m2).<ref name=TSI20 /><ref>{{cite journal | last1 = Willson | first1 = R. C. | last2 = Mordvinov | first2 = A. V. | year = 2003 | title = Secular total solar irradiance trend during solar cycles 21–23 | journal = Geophys. Res. Lett. | volume = 30 | issue = 5 | page = 1199 | doi = 10.1029/2002GL016038 | bibcode = 2003GeoRL..30.1199W | doi-access = free }}</ref><ref name=TSI>{{cite web |title= Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present |url= http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant |access-date= 2005-10-05 |archive-url= https://web.archive.org/web/20110830221302/http://www.pmodwrc.ch/pmod.php?topic=tsi%2Fcomposite%2FSolarConstant |archive-date= 2011-08-30 }}</ref><ref>{{cite web|url= http://www.acrim.com/index.htm|title= Current Projects|website= www.acrim.com|access-date= 25 January 2018|url-status= live|archive-url= https://web.archive.org/web/20171016112445/http://www.acrim.com/Index.htm|archive-date= 16 October 2017}}</ref> TSI observations continue with the [[ACRIMSAT]]/ACRIM3, [[Solar and Heliospheric Observatory|SOHO]]/VIRGO and [[Solar Radiation and Climate Experiment|SORCE]]/TIM satellite experiments.<ref>{{cite web|url= http://www.acrim.com/Comparison%20of%20TSI%20Results.htm|website= ACRIM.com|title= Comparison: ACRIMSAT/ACRIM3, SOHO/VIRGO and SORCE/TIM results|access-date= 25 January 2018|url-status= live|archive-url= https://web.archive.org/web/20171016112535/http://www.acrim.com/Comparison%20of%20TSI%20Results.htm|archive-date= 16 October 2017}}</ref> Observations have revealed variation of TSI on many timescales, including the solar magnetic cycle<ref name="acrim">{{cite web |url= http://www.acrim.com/TSI%20Monitoring.htm |title= Graphics Gallery |publisher= Acrim.com |access-date= 2014-04-21 |url-status= live |archive-url= https://web.archive.org/web/20140502013019/http://acrim.com/TSI%20Monitoring.htm |archive-date= 2014-05-02 }}</ref> and many shorter periodic cycles.<ref>{{cite web |url= http://www.acrim.com/Comparison%20of%20TSI%20Results.htm |title= Comparison: ACRIMSAT/ACRIM3, SOHO/VIRGO and SORCE/TIM results |website= ACRIM.com |access-date= 2013-03-14 |url-status= live |archive-url= https://web.archive.org/web/20130530223744/http://www.acrim.com/Comparison%20of%20TSI%20Results.htm |archive-date= 2013-05-30 }}</ref> TSI provides the energy that drives Earth's climate, so continuation of the TSI time-series database is critical to understanding the role of solar variability in climate change.

Since 2003, the SORCE Spectral Irradiance Monitor (SIM) has monitored '''Spectral solar irradiance''' (SSI) – the spectral distribution of the TSI. Data indicate that SSI at UV (ultraviolet) wavelength corresponds in a less clear, and probably more complicated fashion, with Earth's climate responses than earlier assumed, fueling broad avenues of new research in "the connection of the Sun and stratosphere, troposphere, biosphere, ocean, and Earth's climate".<ref name=goddard>
{{cite web
|url= http://atmospheres.gsfc.nasa.gov/climate/index.php?section=136
|title= NASA Goddard Space Flight Center: Solar Radiation
|publisher= Atmospheres.gsfc.nasa.gov |date= 2012-02-08
|access-date= 2012-02-12 |archive-url= http://archive.wikiwix.com/cache/20110920123747/http://atmospheres.gsfc.nasa.gov/climate/index.php?section=136
|archive-date= 2011-09-20
}}
</ref>

== Surface illumination and spectrum==
{{see also|Diffuse sky radiation}}
[[File:Sun_over_Lake_Hawea,_New_Zealand.jpg|thumb|upright=1.25|Sunlight shining through [[cloud]]s, giving rise to [[crepuscular rays]]]]
The spectrum of surface illumination depends upon solar elevation due to atmospheric effects, with the blue spectral component dominating during twilight before and after sunrise and sunset, respectively, and red dominating during sunrise and sunset. These effects are apparent in natural light [[photography]] where the principal source of illumination is sunlight as mediated by the atmosphere.

While the color of the sky is usually determined by [[Rayleigh scattering]], an exception occurs at sunset and twilight. "Preferential absorption of sunlight by ozone over long horizon paths gives the zenith sky its blueness when the sun is near the horizon".<ref>{{cite web|url=http://homepages.wmich.edu/%7Ekorista/atmospheric_optics.pdf|title=Atmospheric Optics|author=Craig Bohren|author-link=Craig Bohren|url-status=live|archive-url=https://web.archive.org/web/20131206175627/http://homepages.wmich.edu/%7Ekorista/atmospheric_optics.pdf|archive-date=2013-12-06}}</ref>

=== Spectral composition of sunlight at Earth's surface ===
The Sun may be said to [[Lighting|illuminate]], which is a measure of the light within a specific sensitivity range. Many animals (including humans) have a sensitivity range of approximately 400–700 nm,<ref name="BuserImbert1992">{{cite book|last1=Buser|first1=Pierre A.|last2=Imbert|first2=Michel|title=Vision|url=https://archive.org/details/vision0000buse|url-access=registration|access-date=11 October 2013|date=1992|publisher=MIT Press|isbn=978-0-262-02336-8|page=[https://archive.org/details/vision0000buse/page/50 50]|quote=Light is a special class of radiant energy embracing wavelengths between 400 and 700 nm (or mμ), or 4000 to 7000 Å.}}</ref> and given optimal conditions the absorption and scattering by Earth's atmosphere produces illumination that approximates an [[Standard illuminant|equal-energy illuminant]] for most of this range.<ref name="MacEvoy">{{cite book|last1=MacEvoy|first1=Bruce|title=color vision|date=2008|url=http://www.handprint.com/HP/WCL/color1.html|access-date=27 August 2015|quote=Noon sunlight (D55) has a nearly flat distribution...|url-status=live|archive-url=https://web.archive.org/web/20150924024814/http://www.handprint.com/HP/WCL/color1.html|archive-date=24 September 2015}}</ref> The useful range for color vision in humans, for example, is approximately 450–650 nm. Aside from effects that arise at sunset and sunrise, the spectral composition changes primarily in respect to how directly sunlight is able to illuminate. When illumination is indirect, [[Rayleigh scattering]] in the upper atmosphere will lead blue wavelengths to dominate. Water vapour in the lower atmosphere produces further scattering and ozone, dust and water particles will also absorb particular wavelengths.<ref name="Wyszecki_and_Stiles">{{cite book|last1=Wyszecki|first1=Günter|last2=Stiles|first2=W. S.|title=Color Science: Concepts and Methods, Quantitative Data and Formulas|date=1967|publisher=John Wiley & Sons|page=8}}</ref><ref name="MacAdam">{{cite book|last1=MacAdam|first1=David L.|title=Color Measurement: Theme and Variations|url=https://archive.org/details/colormeasurement00ddav|url-access=limited|edition=Second Revised|date=1985|isbn=0-387-15573-2|publisher=Springer|pages=[https://archive.org/details/colormeasurement00ddav/page/n46 33]–35}}</ref>

[[File:Spectrum of Sunlight en.svg|thumb|center|upright=3.5|Spectrum of the visible wavelengths at approximately sea level; illumination by direct sunlight compared with direct sunlight scattered by cloud cover and with indirect sunlight by varying degrees of cloud cover. The yellow line shows the power spectrum of direct sunlight under optimal conditions. To aid comparison, the other illumination conditions are scaled by the factor shown in the key so they match at about 470 nm (blue light).]]

==Life on Earth==
[[File:Dülmen, Göversheide -- 2015 -- 7718-22.jpg|thumb|upright|Sunlight penetrating through a [[forest canopy]] in Germany]]
The existence of nearly all [[life]] on Earth is fueled by light from the Sun. Most [[autotroph]]s, such as plants, use the energy of sunlight, combined with carbon dioxide and water, to produce simple sugars—a process known as [[photosynthesis]]. These sugars are then used as building-blocks and in other synthetic pathways that allow the organism to grow.

[[Heterotroph]]s, such as animals, use light from the Sun indirectly by consuming the products of autotrophs, either by consuming autotrophs, by consuming their products, or by consuming other heterotrophs. The sugars and other molecular components produced by the autotrophs are then broken down, releasing stored solar energy, and giving the heterotroph the energy required for survival. This process is known as [[cellular respiration]].

In [[prehistory]], humans began to further extend this process by putting plant and animal materials to other uses. They used animal skins for warmth, for example, or wooden weapons to hunt. These skills allowed humans to harvest more of the sunlight than was possible through glycolysis alone, and human population began to grow.

During the [[Neolithic Revolution]], the domestication of plants and animals further increased human access to solar energy. Fields devoted to crops were enriched by inedible plant matter, providing sugars and [[nutrients]] for future harvests. Animals that had previously provided humans with only meat and tools once they were killed were now used for labour throughout their lives, fueled by [[grasses]] inedible to humans. [[Fossil fuel]]s are the remnants of ancient plant and animal matter, formed using energy from sunlight and then trapped within Earth for millions of years.

==Cultural aspects==
[[File:Edouard Manet - Luncheon on the Grass - Google Art Project.jpg|thumb|left|[[Édouard Manet]]: ''[[Le déjeuner sur l'herbe]]'' (1862–63)]]
The effect of sunlight is relevant to [[painting]], evidenced for instance in works of [[Édouard Manet]] and [[Claude Monet]] on outdoor scenes and landscapes.
[[File:Winter Sunshine.jpg|thumb|''Téli verőfény'' ("Winter Sunshine") by [[László Mednyánszky]], early 20th century]]
Many people find direct sunlight to be too [[brightness|bright]] for comfort; indeed, looking directly at the Sun can cause long-term vision damage.<ref>{{Cite journal |last1=Chawda |first1=Dishita |last2=Shinde |first2=Pranaykumar |date=2022-10-29 |title=Effects of Solar Radiation on the Eyes |journal=Cureus |volume=14 |issue=10 |article-number=e30857 |language=en |doi=10.7759/cureus.30857 |doi-access=free |pmc=9709587 |pmid=36465785}}</ref> To compensate for the brightness of sunlight, many people wear [[sunglasses]]. [[Automobile|Cars]], many [[helmet]]s and [[cap]]s are equipped with [[visor]]s to block the Sun from direct vision when the Sun is at a low angle. Sunshine is often blocked from entering buildings through the use of [[wall]]s, [[window blind]]s, [[awning]]s, [[Window shutter|shutters]], [[curtain]]s, or nearby [[shade tree]]s. Sunshine exposure is [[Health effects of sunlight exposure|needed biologically]] for the production of [[Vitamin D]] in the skin, a vital compound needed to make strong bone and muscle in the body.

In many world religions, such as [[Hinduism]], the Sun [[Surya|is considered to be a god]], as it is the source of life and energy on Earth. The Sun was also [[Ra|considered to be a god]] in [[Ancient Egypt]].

===Sunbathing===
{{Main|Sun tanning}}
{{Unreferenced section|date=January 2015}}
[[File:YBF 2010 ja Bikini Bar.jpg|thumb|upright|Sun bathers in Finland]]
Sunbathing is a popular [[leisure]] activity in which a person sits or lies in direct sunshine. People often sunbathe in comfortable places where there is ample sunlight. Some common places for sunbathing include [[beach]]es, open air [[swimming pool]]s, [[park]]s, [[garden]]s, and [[sidewalk cafe]]s. Sunbathers typically wear limited amounts of clothing or some simply go [[Nudity|nude]]. For some, an alternative to sunbathing is the use of a [[Tanning bed|sunbed]] that generates [[ultraviolet]] light and can be used indoors regardless of weather conditions. Tanning beds have been banned in a number of states in the world.

For many people with light skin, one purpose for sunbathing is to darken one's [[Human skin color|skin color]] (get a sun tan), as this is considered in some cultures to be attractive, associated with outdoor activity, [[Vacation|vacations/holidays]], and health. Some people prefer [[Naturism|naked]] sunbathing so that an "all-over" or "even" tan can be obtained, sometimes as part of a specific lifestyle.

Controlled [[heliotherapy]], or sunbathing, has been used as a treatment for [[psoriasis]]<ref>{{cite book |last1=Nguyen |first1=Tien V. |last2=Koo |first2=John Y. M. |title=Advances in Psoriasis |chapter=Ultraviolet Therapy for Psoriasis |date=2014 |pages=91–110 |doi=10.1007/978-1-4471-4432-8_8 |isbn=978-1-4471-4431-1 }}</ref> and other maladies.<ref>{{Cite journal |last1=Alora |first1=M. B. T. |last2=Fitzpatrick |first2=T. B. |last3=Taylor |first3=C. R. |date=1997-10-12 |title=Total body heliotherapy |journal=Photodermatology, Photoimmunology & Photomedicine |volume=13 |issue=5–6 |pages=178–180 |doi=10.1111/j.1600-0781.1997.tb00225.x |pmid=9542753 }}</ref>

Skin tanning is achieved by an increase in the dark pigment inside skin cells called [[melanocyte]]s, and is an automatic response mechanism of the body to sufficient exposure to ultraviolet radiation from the Sun or from artificial sunlamps.<ref>{{Cite book |last=Ahmad |first=Shamim I. |title=Ultraviolet Light in Human Health, Diseases and Environment |date=2017 |publisher=Springer International Publishing AG |isbn=978-3-319-56017-5 |series=Advances in Experimental Medicine and Biology |location=Cham}}</ref> Thus, the tan gradually disappears with time, when one is no longer exposed to these sources.

==Effects on human health==
{{Main|Health effects of sunlight exposure}}
The [[ultraviolet radiation]] in sunlight has both positive and negative health effects, as it is both a principal source of [[vitamin D3|vitamin D3]] and a [[mutagen]].<ref>{{cite journal | pmid = 12174089 | volume=147 | issue=2 | title=Vitamin D and systemic cancer: is this relevant to malignant melanoma? |date=August 2002 | journal=Br. J. Dermatol. | pages=197–213 | author=Osborne JE | author2=Hutchinson PE | doi = 10.1046/j.1365-2133.2002.04960.x }}</ref> A dietary supplement can supply [[vitamin D]] without this mutagenic effect,<ref name="ODS-vitamin-D-sheet">{{cite web|url=http://dietary-supplements.info.nih.gov/factsheets/vitamind.asp|title=Dietary Supplement Fact Sheet: Vitamin D|publisher=Office of Dietary Supplements, National Institutes of Health|archive-url=https://web.archive.org/web/20070716065832/http://dietary-supplements.info.nih.gov/factsheets/vitamind.asp|archive-date=2007-07-16}}</ref> but bypasses natural mechanisms that would prevent overdoses of vitamin D generated internally from sunlight. Vitamin D has a wide range of positive health effects, which include strengthening bones<ref>{{cite journal | display-authors = 4| author = Cranney A| author2 = Horsley T| author3 = O'Donnell S| author4 = Weiler H| author5 = Puil L| author6 = Ooi D| author7 = Atkinson S| author8 = Ward L| author9 = Moher D| author10 = Hanley D| author11 = Fang M| author12 = Yazdi F| author13 = Garritty C| author14 = Sampson M| author15 = Barrowman N| author16 = Tsertsvadze A| author17 = Mamaladze V | title = Effectiveness and safety of vitamin D in relation to bone health | journal = Evidence Report/Technology Assessment | issue = 158 | pages = 1–235 | date = August 2007 | pmid = 18088161 | pmc=4781354}}</ref> and possibly inhibiting the growth of some cancers.<ref>{{cite journal | display-authors = 4| author = John E| author2 = Schwartz G| author3 = Koo J| author4 = Van Den Berg D| author5 = Ingles S | title = Sun Exposure, Vitamin D Receptor Gene Polymorphisms, and Risk of Advanced Prostate Cancer | journal = Cancer Research | volume = 65 | issue = 12 | pages = 5470–5479 | date = June 15, 2005 | doi=10.1158/0008-5472.can-04-3134 | pmid=15958597| doi-access = free }}</ref><ref>{{cite journal | author = Egan K | author2 = Sosman J | author3 = Blot W | title = Sunlight and Reduced Risk of Cancer: Is The Real Story Vitamin D? | journal = J Natl Cancer Inst | volume = 97 | issue = 3 | pages = 161–163 | date = February 2, 2005 | doi = 10.1093/jnci/dji047 | pmid = 15687354 | doi-access = free }}</ref> Sun exposure has also been associated with the timing of [[melatonin]] synthesis, maintenance of normal [[circadian rhythm]]s, and reduced risk of [[seasonal affective disorder]].<ref>{{cite journal |author=Mead MN |title=Benefits of sunlight: a bright spot for human health |journal=Environmental Health Perspectives |volume=116 |issue=4 |pages=A160–A167 |date=April 2008 |pmid=18414615 |pmc=2290997 |doi=10.1289/ehp.116-a160}}</ref>

Long-term sunlight exposure is known to be associated with the development of [[skin cancer]], [[photoaging|skin aging]], [[immune suppression]], and eye diseases such as [[cataracts]] and [[macular degeneration]].<ref>{{cite journal |author=Lucas RM|author2=Repacholi MH|author3=McMichael AJ |title=Is the current public health message on UV exposure correct? |journal=Bulletin of the World Health Organization |volume=84 |issue=6 |pages=485–491 |date=June 2006 |pmid=16799733 |pmc=2627377 |doi=10.2471/BLT.05.026559}}</ref> Short-term overexposure is the cause of [[sunburn]], [[snow blindness]], and [[solar retinopathy]].

UV rays, and therefore sunlight and sunlamps, are the only listed [[carcinogen]]s that are known to have health benefits,<ref name="13th Report on Carcinogens">{{cite web |url=https://ntp.niehs.nih.gov/sites/default/files/ntp/roc/content/profiles/ultravioletradiationrelatedexposures.pdf |title=13th Report on Carcinogens: Ultraviolet-Radiation-Related Exposures |publisher=National Toxicology Program |date=October 2014 |access-date=2014-12-22 |url-status=live |archive-url=https://web.archive.org/web/20141222172504/http://ntp.niehs.nih.gov/ntp/roc/content/profiles/ultravioletradiationrelatedexposures.pdf |archive-date=2014-12-22 }}</ref> and a number of public health organizations state that there needs to be a balance between the risks of having too much sunlight or too little.<ref>{{cite web |url=http://www.cancer.org.au//File/PolicyPublications/PSRisksBenefitsSunExposure03May07.pdf |title=Risks and Benefits |access-date=2010-05-13 |url-status=live |archive-url=https://web.archive.org/web/20101120224439/http://cancer.org.au//File/PolicyPublications/PSRisksBenefitsSunExposure03May07.pdf |archive-date=2010-11-20 }}</ref> There is a general consensus that sunburn should always be avoided.

Epidemiological data shows that people who have more exposure to sunlight have less high blood pressure and cardiovascular-related mortality. While sunlight (and its UV rays) are a risk factor for skin cancer, "sun avoidance may carry more of a cost than benefit for over-all good health".<ref name="wellersunlight">{{cite journal|last1=Weller|first1=RB|title=Sunlight Has Cardiovascular Benefits Independently of Vitamin D.|journal=Blood Purification|date=2016|volume=41|issue=1–3|pages=130–4|doi=10.1159/000441266|pmid=26766556|hdl=20.500.11820/8f7d93d4-db22-418d-a1cc-3dbf9ddad8c3 |hdl-access=free}}</ref> A study found that there is no evidence that UV reduces lifespan in contrast to other risk factors like smoking, alcohol and high blood pressure.<ref name="wellersunlight" />

==Effect on plant genomes==

Elevated solar [[ultraviolet|UV]]-B doses increase the frequency of [[DNA]] [[homologous recombination|recombination]] in ''[[Arabidopsis thaliana]]'' and tobacco (''[[Nicotiana tabacum]]'') plants.<ref name="pmid10894550">{{cite journal |vauthors=Ries G, Heller W, Puchta H, Sandermann H, Seidlitz HK, Hohn B |title=Elevated UV-B radiation reduces genome stability in plants |journal=Nature |volume=406 |issue=6791 |pages=98–101 |year=2000 |pmid=10894550 |doi=10.1038/35017595 |bibcode=2000Natur.406...98R |url=https://publikationen.bibliothek.kit.edu/1000016569 }}</ref> These increases are accompanied by strong induction of an enzyme with a key role in recombinational repair of DNA damage. Thus the level of terrestrial solar UV-B radiation likely affects [[genome instability|genome stability]] in plants.

==See also==
* [[Color temperature]]
* [[Coronal radiative losses]]
* [[Diathermancy]]
* [[Fraunhofer lines]]
* [[List of cities by sunshine duration]]
* [[Moonlight]]
* [[Light pollution]]
* [[Photic sneeze reflex]]
* [[Photosynthesis]]
* [[Solar energy]]
* [[Starlight]]
* {{annotated link|Sunbeam}}

==References==
{{Reflist|30em}}

==Further reading==
* [[Thom Hartmann|Hartmann, Thom]] (1998). ''The Last Hours of Ancient Sunlight''. London: Hodder and Stoughton. {{ISBN|0-340-82243-0}}.

== External links ==
* {{Commons category-inline|Sunlight}}
* [http://www.eoearth.org/article/Solar_radiation Solar radiation – Encyclopedia of Earth]

* [https://www.ncei.noaa.gov/products/space-weather/legacy-data/total-solar-irradiance Total Solar Irradiance (TSI) Daily mean data] at the website of the [[National Geophysical Data Center]]
* [https://web.archive.org/web/20110830221302/http://www.pmodwrc.ch/pmod.php?topic=tsi%2Fcomposite%2FSolarConstant Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present] by World Radiation Center, Physikalisch-Meteorologisches Observatorium Davos (pmod wrc)
* [http://www.macaulay.ac.uk/LADSS/papers.html?2002 A Comparison of Methods for Providing Solar Radiation Data to Crop Models and Decision Support Systems], Rivington et al.
* [http://www.macaulay.ac.uk/LADSS/papers.html?2005 Evaluation of three model estimations of solar radiation at 24 UK stations], Rivington et al.
* [http://bass2000.obspm.fr/solar_spect.php High resolution spectrum of solar radiation] from [[Paris Observatory|Observatoire de Paris]]
* [https://web.archive.org/web/20040213233926/http://avc.comm.nsdlib.org/cgi-bin/wiki_grade_interface.pl?Measuring_Solar_Radiation Measuring Solar Radiation] : A lesson plan from the National Science Digital Library.
* [https://web.archive.org/web/20060423090912/http://websurf.nao.rl.ac.uk/surfbin/first.cgi Websurf astronomical information]: Online tools for calculating Rising and setting times of Sun, Moon or planet, Azimuth of Sun, Moon or planet at rising and setting, Altitude and azimuth of Sun, Moon or planet for a given date or range of dates, and more.
* [https://web.archive.org/web/20070305050844/http://www.ecy.wa.gov/programs/eap/models/solrad.zip An Excel workbook] with a solar position and solar radiation time-series calculator; by [https://web.archive.org/web/20070525051436/http://www.ecy.wa.gov/programs/eap/models.html Greg Pelletier]
* [http://www.astm.org/Standards/G173.htm ASTM Standard] for solar spectrum at ground level in the US (latitude ~37 degrees).
* [http://apod.nasa.gov/apod/ap100627.html Detailed spectrum of the Sun] at [http://apod.nasa.gov/apod/archivepix.html Astronomy Picture of the Day].

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