Dew
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- This article is about condensation of water vapour. For the missile detection system, see DEW.
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 radiating its heat to the sky, atmospheric moisture condenses at a rate greater than that of which it can evaporate, resulting in the formation of water droplets.
When temperatures are low enough dew takes the form of ice; this form is called frost.
As dew is related to the temperature of surface, 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.
Culturally dew is often associated with purity and freshness. This is evidenced by the names of certain popular beverages such as Mountain Dew and Sierra Mist.
Dew should not be confused with Guttation, which is the process by which plants release excess water from the tips of their leaves.
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[edit] Formation
The amount of water vapour air can hold depends on temperature. The temperature at which air is saturated with water vapour is called the Dew point. When surface temperature drops, eventually reaching the dew point, atmospheric water vapour condenses to form small droplets on the surface. This process distinguishes dew from those 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.
[edit] 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 in clear nights. As another important factor, 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 few water vapour in the higher atmosphere to minimize glasshouse effects and sufficient humidity of the air near the ground. Typical dew-nights are classically considered to be calm as 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 vapour that already is condensed. The highest optimum wind speeds could be found on arid islands. If the wet soil beneath is the major source of vapour, 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 glasses get steamy in a warm wet room or in industrial processes. However, the term condensation is preferred in these cases.
[edit] 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 recording 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 in the same height and in the same surrounding as would be natural, 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.
[edit] 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 rain falling, such as in the Atacama and Namib desert, however, is mostly attributed to fog water. Several man-made devices such as antique big stone piles in the Ukraine, medieval "dew ponds" 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 either work on other principles largely. Nowadays the International Organization for Dew Utilization organization is working on foil-based effective condensers for regions where rain or fog cannot cover water needs throughout the year.
Another effect of dew on plants is its role as a habitat for pathogens such as the fungus Phytophthora infestans infecting potato plants.
In Greek mythology, Ersa is the goddess of dew.
[edit] See also
