Dew
54911
223399886
2008-07-03T22:29:39Z
Numbo3-bot
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robot Adding: [[zh-yue:露]]
{{otheruses}}
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[[Image:Dew_on_grass.jpg|right|200px|thumb|Heavy dew on grass amid sunlight and a bit of frost]]
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|[[Image:Dew 004.JPG|right|200px|thumb|Dew on some leaves]]
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|[[Image:Dew 11 bg 060103.jpg|right|200px|thumb|Some dew on a wildflower in Sequoia National Park]]
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|[[Image:Spider web with dew drops.jpg|right|200px|thumb|Dew on a spider web]]
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|[[Image:Dew_on_grass_closeup.jpg|right|200px|thumb|Dew on grass]]
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|[[Image:DirkvdM spider dew.jpg|thumb|200px|A spider in a dew-covered web near [[Orosí]], Costa Rica.]]
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|[[Image:Dew on grass Luc Viatour.jpg|thumb|200px|Morning dew on a blade of grass]]
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'''Dew''' is water in the form of droplets that appears on thin, exposed objects in the morning or evening. As the exposed surface cools by [[thermal radiation|radiating]] its heat, atmospheric moisture [[condensation|condenses]] at a rate greater than that of which it can [[evaporation|evaporate]], resulting in the formation of water droplets.
When temperatures are low enough, dew takes the form of ice; this form is called [[frost]].
Because dew is related to the temperature of surfaces, in late summer it is formed most easily on surfaces which are not warmed by conducted heat from deep ground, such as: grass, leaves, railings, car roofs, and bridges.
Dew should not be confused with [[guttation]], which is the process by which plants release excess water from the tips of their leaves.
==Formation==
Water will condense into droplets depending on the temperature. The [[temperature]] at which droplets can 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. This process distinguishes dew from those [[Precipitation (meteorology)|hydrometeors]] (meteorological occurrences of water) which are formed directly in air cooling to its dew point (typically around [[condensation nuclei]]) such as [[fog]] or [[clouds]]. The thermodynamic principles of formation, however, are virtually the same.
==Occurrence==
Sufficient 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. As another important point, poor [[thermal conductivity]] restricts the replacement of such losses from deeper ground layers which are typically warmer at night.
Preferred objects of dew formation are thus poor conducting or well isolated from the ground, and non-metallic or coated as shiny metal 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 to be calm because the wind transports (nocturnally) warmer air from higher levels to the cold surface. But, if the atmosphere is the major source of moisture (this part of dew 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. If the wet soil beneath is the major source of vapor, however (this part of dew is called [[distillation]]), wind always seems to be adverse.
The principles of dew formation do not strictly constrict 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 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. It has to be kept in mind that 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 mm per night, measured values, however, rarely exceeding 0.5 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 Desert]] and [[Namib]] desert, however, is mostly attributed to [[fog]] water.
Another effect of dew on plants is its role as a [[Habitat (ecology)|habitat]] for [[pathogen]]s such as the fungus [[Phytophthora infestans]] which infects potato plants.
In Greek mythology, [[Ersa]] is the goddess of dew.
==Artificial harvesting==
{{Main|Fog collection}}
Several man-made devices such as antique, big stone piles in the [[Ukraine]], medieval "[[dew pond]]s" in southern [[England]], or 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 Organization for Dew Utilization]] is working on effective, foil-based condensers for regions where rain or fog cannot cover water needs throughout the year. An interesting strategy for harvesting dew on large scale for drinking water was proposed in 1980.<ref>[http://nariphaltan.virtualave.net/dew.pdf Large scale dew collection as a source for water supply]</ref>
Large scale dew harvesting systems have been made by [[Indian Institute of Management Ahmedabad]] (IIMA) at coastal semi arid region [[Kutch]]. These condensers can harvest more than 200 litres (on average) of dew water per night for about 90 nights in the dew season October-May. The research lab of IIMA has shown that dew can serve as a supplementary source of water in coastal arid areas.
==References==
{{reflist}}
==External links==
* [http://www.opur.u-bordeaux.fr/ OPUR (International Organisation for Dew Utilization)]
* [http://www.geo.puc.cl/fogconference/html_en/index.html International Conference on Fog, Fog Collection and Dew]
[[Category:Psychrometrics]]
[[Category:Atmospheric thermodynamics]]
[[Category:Precipitation]]
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