View source for Dew

{{Short description|Droplets of water that form in the morning or evening}}
{{redirect|Morning dew|the song by Bonnie Dobson|Morning Dew||Morning Dew (disambiguation)|and|Dew (disambiguation)}}
{{Not to be confused with|Guttation|text=[[Guttation]], the process by which plants release excess water from the tips of their leaves}}
{{refimprove|date=October 2015}}
[[File:A drop of dew on a clover.jpg|thumb|A drop of dew in the middle of a clover|alt=A dewdrop on a clover]]
[[File:Dew Formed on the Surface of Strawberry Leafs.jpg|thumb|Dew formed on the surface of [[strawberry]] leaves]]
[[File:Dew drop.jpg|thumb|A drop of dew on a ''[[Colocasia]]'' leaf at the [[Garden Society of Gothenburg]]]]
[[File:Chrysantheme rot tautropfen -20191024-RM-102058.jpg|thumb|Dew drops on a flower]]
[[File:Dense dew on grass.jpg|thumb|Dense dew on grass]]
[[File:Slovenia (12248224706).jpg|thumb|Under cold temperatures, dew may freeze and form a layer of ice over plants and objects.]]
'''Dew''' is [[water]] in the form of [[droplets]] that appears on thin, exposed objects in the [[morning]] or [[evening]] due to [[condensation]].<ref>{{cite Merriam-Webster|dew}}</ref> 
As the exposed surface cools by [[thermal radiation|radiating]] its heat, atmospheric moisture condenses at a rate greater than that at which it can [[evaporation|evaporate]], resulting in the formation of water droplets.<ref>{{cite encyclopedia |title=dew |url=https://www.encyclopedia.com/topic/dew.aspx |encyclopedia=[[The Columbia Encyclopedia]] |edition=6th |access-date=18 May 2013}}</ref>

When temperatures are low enough, dew takes the form of ice, called [[frost]].

Because dew is related to the temperature of surfaces, in late summer it forms most easily on surfaces that are not warmed by conducted heat from deep ground, such as grass, leaves, railings, car roofs, and bridges.

== Formation ==
Water vapor will condense into droplets depending on the temperature. The [[temperature]] at which droplets form is called the ''[[dew point]]''. When surface temperature drops, eventually reaching the dew point, atmospheric water vapor condenses to form small droplets on the surface.<ref name=":0">{{Cite web |title=Dew and Frost Development |url=https://www.weather.gov/source/zhu/ZHU_Training_Page/fog_stuff/Dew_Frost/Dew_Frost.htm |access-date=2021-10-21 |work=ZHU Training Page |publisher=National Weather Service, NOAA}}</ref> This process distinguishes dew from those [[Precipitation (meteorology)|hydrometeors]] (meteorological occurrences of water), which form directly in air that has cooled to its dew point (typically around [[condensation nuclei]]), such as [[fog]] or [[clouds]]. The [[Thermodynamics|thermodynamic]] principles of formation, however, are the same. Dew is commonly formed during select times of the day. Nights, early mornings, and early evenings are all times during which dew is likely to be found.<ref>{{cite journal |first=L.B. |last=Leopold |title=Dew as a source of plant moisture |journal=Pac Sci |volume=6 |issue=3 |pages=259–261 |date=1952 |hdl=10125/8818/ |url=https://scholarspace.manoa.hawaii.edu/bitstream/10125/8818/1/vol6n3-259-261.pdf}}</ref>

== Occurrence ==
Adequate cooling of the surface typically takes place when it loses more energy by [[infrared]] [[radiation]] than it receives as [[solar radiation]] from the [[Sun]], which is especially the case on clear nights. Poor [[thermal conductivity]] restricts the replacement of such losses from deeper ground layers, which are typically warmer at night.<ref name=":0" /> Preferred objects of dew formation are thus poor conducting or well isolated from the ground, and non-metallic, while shiny metal coated surfaces are poor infrared radiators. Preferred weather conditions include the absence of clouds and little water [[vapor]] in the higher atmosphere to minimize greenhouse effects and sufficient humidity of the air near the ground. Typical dew nights are classically considered calm, because the wind transports (nocturnally) warmer air from higher levels to the cold surface. However, if the atmosphere is the major source of moisture (this type is called dewfall), a certain amount of ventilation is needed to replace the vapor that is already condensed. The highest optimum wind speeds could be found on arid islands. Wind always seems adverse, however, if the wet soil beneath is the major source of vapor (in which case dew is said to form by [[distillation]]).

The processes of dew formation do not restrict its occurrence to the night and the outdoors. They are also working when eyeglasses get steamy in a warm, wet room or in industrial processes. However, the term [[condensation]] is preferred in these cases.

== Measurement ==
A classical device for dew measurement is the [[drosometer]]. A small (artificial) condenser surface is suspended from an arm attached to a pointer or a pen that records the weight changes of the condenser on a drum. Besides being very wind sensitive, however, this, like all artificial surface devices, only provides a measure of the [[Meteorology|meteorological]] potential for dew formation. The actual amount of dew in a specific place is strongly dependent on surface properties. For its measurement, plants, leaves, or whole soil columns are placed on a balance with their surface at the same height and in the same surroundings as would occur naturally, thus providing a small [[lysimeter]]. Further methods include estimation by means of comparing the droplets to standardized photographs or volumetric measurement of the amount of water wiped from the surface. Some of these methods include [[guttation]], while others only measure dewfall and/or [[distillation]].

== Significance ==
Due to its dependence on [[radiation balance]], dew amounts can reach a theoretical maximum of about 0.8&nbsp;mm per night; measured values, however, rarely exceed 0.5&nbsp;mm. In most climates of the world, the annual average is too small to compete with rain. In regions with considerable dry seasons, adapted plants like lichen or pine seedlings benefit from dew. Large-scale, natural irrigation without rainfall, such as in the [[Atacama]] and [[Namib]] deserts, however, is mostly attributed to [[fog]] water. In the Negev Desert in Israel, dew has been found to account for almost half of the water found in three dominant desert species: ''[[Salsola inermis]], [[Artemisia sieberi]] and [[Haloxylon scoparium]]''.<ref>{{cite journal |last1=Hill |first1=Amber |title=The Role of Dew in Negev Desert plants |journal=Oecologia |volume=178 |issue=2 |pages=317–327 |doi=10.1007/s00442-015-3287-5 |pmid=25783489 |year=2015 |bibcode=2015Oecol.178..317H |s2cid=18134058}}</ref>

Another effect of dew is its hydration of fungal substrates and the mycelia of species such as [[Parasola plicatilis|pleated inkcaps]], often found on lawns, and ''[[Phytophthora infestans]]'' which causes blight on potato plants.<ref>{{Cite web |title=Late Blight in Potato — Publications |url=https://www.ag.ndsu.edu/publications/crops/late-blight-in-potato |access-date=2021-10-21 |website=www.ag.ndsu.edu |date=5 May 2017 |language=en}}</ref>

=== Historic ===
The book ''[[On the Universe]]'' (''De Mundo'') (composed before 250 BC or between 350 and 200 BC) stated: "Dew is moisture minute in composition falling from a clear sky; ice is water congealed in a condensed form from a clear sky; [[Hoar frost|hoar-frost]] is congealed dew, and 'dew-frost' is dew which is half congealed".<ref name=1908DeMundo>{{cite book |url=https://archive.org/details/demundoarisrich |title=De Mundo |year=1914 |author=Aristotle |author2=Forster, E. S. |author3=Dobson, J. F. |page=End of chapter 3}}</ref>

In Greek mythology, [[Ersa]] is the goddess and personification of dew.
Also, according to the myth, the dew in the morning was created when [[Eos]] (Ersa's aunt), goddess of the dawn, cried for [[Memnon (mythology)|her son]]'s death, although later he received immortality.

Dew, known in [[Hebrew language|Hebrew]] as טל (''tal''), is significant in the [[Judaism|Jewish religion]] for agricultural and [[theological]] purposes. On the first day of [[Passover]], the ''[[Chazan]]'', dressed in a white ''[[kittel]]'', leads a service in which he prays for dew between that point and [[Sukkot]]. During the rainy season between December and Passover there are also additions in the [[Amidah]] for blessed dew to come together with rain. There are many [[midrash]]im that refer to dew as being the tool for ultimate resurrection.<ref>{{cite encyclopedia |title=Resurrection |encyclopedia=[[Jewish Encyclopedia]] |url=http://jewishencyclopedia.com/view.jsp?artid=233&letter=R |access-date=21 Dec 2008}}</ref> "Dewy" or "my father is the morning dew" are approximate etymologies of the Hebrew given name, [[Avital (given name)|Avital]].<ref name="Hanks Hardcastle Hodges 2006">{{cite book |last1=Hanks |first1=Patrick |url=https://books.google.com/books?id=9nd05X_awIgC&pg=PT42 |title=A Dictionary of First Names |last2=Hardcastle |first2=Kate |last3=Hodges |first3=Flavia |publisher=[[Oxford University Press]] |year=2006 |isbn=978-0-19-861060-1 |series=Oxford paperback reference |page=42 |access-date=2018-11-17 |authorlink=Patrick Hanks}}</ref><ref>{{cite web |title=Abital |url=http://biblesuite.com/hebrew/37.htm |accessdate=2013-06-27}}</ref>

In the Biblical [[Torah]] or [[Old Testament]], dew is used symbolically in {{bibleverse||Deuteronomy|32:2|KJV}}: "My doctrine shall drop as the rain, my speech shall distill as the dew, as the small rain upon the tender herb, and as the showers upon the grass."<ref>[https://www.kingjamesbibleonline.org/Deuteronomy-Chapter-32/ Deuteronomy 32: King James Version], accessed 22 September 2019</ref>

In the Catholic Mass in the Western Rite, whenever the Second Eucharistic Prayer is used, the priest prays over bread and wine, to God the Father;  ‘Make holy, therefore, these gifts, we pray, by sending down your Spirit upon them like the dewfall, so that they may become for us the Body and  Blood of our Lord Jesus Christ.’ The idea that the Holy Spirit enters the world and our lives in a quiet, undramatic way, ‘like the dewfall’,  has great appeal for many Christians.{{cn|date=July 2024}}

== Artificial harvesting ==
{{Main|Air well (condenser)}}
The [[v:Rainwater harvesting/Dew harvesting/Dew collection and storage|harvesting of dew]] potentially allows water availability in areas where supporting weather conditions, such as rain, are lacking. Several man-made devices such as antique big stone piles in [[Ukraine]], medieval [[dew pond]]s in [[Southern England]], and volcanic stone covers on the fields of [[Lanzarote]] have been thought to be dew-catching devices, but could be shown to work on other principles. At present, the [[International Organisation for Dew Utilization|International Organization for Dew Utilization]] (OPUR) is working on effective, foil-based condensers for regions where rain or fog cannot cover water needs throughout the year.

Large-scale dew harvesting systems have been made by the [[Indian Institute of Management Ahmedabad]] (IIMA) with the participation of OPUR in the coastal, semiarid region of [[Kutch]].<ref>{{Cite journal |last1=Sharan |first1=G. |last2=Clus |first2=O. |last3=Singh |first3=S. |last4=Muselli |first4=M. |last5=Beysens |first5=D. |date=2011-07-01 |title=A very large dew and rain ridge collector in the Kutch area (Gujarat, India) |bibcode=2011JHyd..405..171S |journal=Journal of Hydrology |volume=405 |issue=1–2 |pages=171–181 |doi=10.1016/j.jhydrol.2011.05.019 |issn=0022-1694}}</ref> These condensers can harvest more than 200 liters (on average) of dew water per night for about 90 nights in the October-to-May dew season. The IIMA research laboratory has shown that dew can serve as a supplementary source of water in coastal arid areas.

A large-scale dew harvesting scheme envisages circulating cold sea water in EPDM collectors near the seashore. These condense dew and fog to supply clean drinking water.<ref>{{cite journal |author-link=Anil K. Rajvanshi |first1=Anil K. |last1=Rajvanshi |title=Large Scale Dew Collection as a Source of Fresh Water Supply |journal=Desalination |volume=36 |issue=3 |pages=299–306 |date=March 1981 |doi=10.1016/S0011-9164(00)88647-6 |bibcode=1981Desal..36..299R |citeseerx=10.1.1.730.5055}}</ref> Other, more recent, studies display possible roof integration for dew harvesting devices.<ref>{{Cite journal |last1=Carvajal |first1=Danilo |last2=Minonzio |first2=Jean-Gabriel |last3=Casanga |first3=Elvira |last4=Muñoz |first4=Jorge |last5=Aracena |first5=Alvaro |last6=Montecinos |first6=Sonia |last7=Beysens |first7=Daniel |date=2018-05-15 |title=Roof-integrated dew water harvesting in Combarbalá, Chile |journal=Journal of Water Supply: Research and Technology-Aqua |volume=67 |issue=4 |pages=357–374 |doi=10.2166/aqua.2018.174|doi-access=free }}</ref>

== References ==
{{Reflist}}

== External links ==
{{Commons}}
{{Wikiquote}}
* [http://www.opur.fr/ OPUR (International Organisation for Dew Utilization)]
* [https://web.archive.org/web/20070129192343/http://www.geo.puc.cl/fogconference/html_en/index.html International Conference on Fog, Fog Collection and Dew]
* [https://web.archive.org/web/20090203145152/http://faujsa.fau.edu/jsa/search_music_LP.php?jsa_num=401295&queryWhere=jsa_num&queryValue=401295&artisttext=&artist=contains&titletext=tal&title=contains&selectgenre=&selectlanguage=&musiconly=&id=&select=title&side=A&track=04&fetch=25&pagenum=2 Audio Recording of Cantor Yossele Rosenblatt's Dew Service] from the Florida Atlantic University Judaica Sound Archives ''(Hebrew audio)''

[[Category:Psychrometrics]]
[[Category:Precipitation]]