Dew


Dew
Some dew on an iris in Sequoia National Park
Dew on a cobweb
Dew on a leaf
Dew on a snipe fly (Rhagio scolopaceus)

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, atmospheric moisture condenses at a rate greater than that at 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 (frost is, however, not frozen dew).

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.

Contents

Formation

Water vapour 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 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 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. If the wet soil beneath is the major source of vapour, however (this type of dew formation is called distillation), wind always seems to be adverse.

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 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 for pathogens such as the fungus Phytophthora infestans which infects potato plants.

In Greek mythology, Ersa is the goddess of dew.

Dew, known in Hebrew as טל (tal), is very important in the 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 midrashim that refer to dew as being the tool for ultimate resurrection.[1]

In the Biblical Old Testament dew is used symbolically in Deuteronomy 32:3: "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."

Artificial harvesting

OPUR Dew Recovering Roof in Croatia
Dew recovering roof of OPUR in Croatia

Several man-made devices such as antique, big stone piles in 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 work on other principles. At present, the International Organisation for Dew Utilization 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 Indian Institute of Management Ahmedabad (IIMA) with the participation of the International Organisation for Dew Utilization (OPUR) 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

External links


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Look at other dictionaries:

  • DEW — (Heb. טַל), condensation of water vapor on an object near the ground, whose temperature has fallen below the dew point of the surrounding air because of radiational cooling during the night. The conditions favoring the formation of dew are clear… …   Encyclopedia of Judaism

  • Dew — (d[=u]), n. [AS. de[ a]w; akin to D. dauw, G. thau, tau, Icel. d[ o]gg, Sw. dagg, Dan. dug; cf. Skr. dhav, dh[=a]v, to flow. [root]72. Cf. {Dag} dew.] 1. Moisture from the atmosphere condensed by cool bodies upon their surfaces, particularly at… …   The Collaborative International Dictionary of English

  • DEW — steht als Abkürzung für Directed Energy Weapon, Waffensysteme, die mit gebündelter Energie militärische Ziele vertreiben, paralysieren, schädigen oder vernichten Distant Early Warning Line Dortmunder Energie und Wasserversorgung, ein deutsches… …   Deutsch Wikipedia

  • Dew — ist der Familienname mehrerer Personen: Eddie Dew (1909−1972), US amerikanischer Schauspieler und Filmregisseur John Dew (* 1944), britischer Regisseur und Intendant John Anthony Dew (* 1950), britischer Diplomat John Atcherley Dew (* 1948),… …   Deutsch Wikipedia

  • dew|y — «DOO ee, DYOO », adjective. dew|i|er, dew|i|est. 1. wet with dew: »from morn to dewy eve (Milton). 2. Figurative …   Useful english dictionary

  • Dew — Dew, v. t. [imp. & p. p. {Dewed}; p. pr. & vb. n. {Dewing}.] To wet with dew or as with dew; to bedew; to moisten; as with dew. [1913 Webster] The grasses grew A little ranker since they dewed them so. A. B. Saxton. [1913 Webster] …   The Collaborative International Dictionary of English

  • Dew — Dew, a. & n. Same as {Due}, or {Duty}. [Obs.] Spenser. [1913 Webster] …   The Collaborative International Dictionary of English

  • Dew —   [djuː], John Roland, britischer Opernregisseur, * Santiago de Cuba 1. 6. 1944; studierte bei W. Felsenstein und Wieland Wagner. 1982 95 Oberspielleiter in Bielefeld, daneben auch als Gastregisseur tätig; 1995 2001 Generalintendant in Dortmund.… …   Universal-Lexikon

  • dew — O.E. deaw, from P.Gmc. *dawwaz (Cf. O.S. dau, O.Fris. daw, M.Du. dau, O.H.G. tau, Ger. Tau, O.N. dögg dew ), from PIE root *dheu (2) to flow (Cf. Skt. dhavate flows, runs ) …   Etymology dictionary

  • dew — [do͞o, dyo͞o] n. [ME < OE deaw, akin to Ger tau < IE base * dheu , to run > Sans dhāvati, a spring, brook] 1. the condensation formed, usually during the night, on lawns, cars, etc. as a result of relatively warm air contacting a cool… …   English World dictionary

  • dew|i|ly — «DOO uh lee, DYOO », adverb. in a dewy manner; like dew …   Useful english dictionary


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