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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 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.
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Formation [link]
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 [link]
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 [link]
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 [link]
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 32:2: "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 [link]
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 [link]
- ^ "Resurrection". Jewish Encyclopedia. https://www.jewishencyclopedia.com/view.jsp?artid=233&letter=R. Retrieved 21 Dec. 2008.
External links [link]
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Wikiquote has a collection of quotations related to: Dew |
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Wikimedia Commons has media related to: Dew |
- OPUR (International Organisation for Dew Utilization)
- International Conference on Fog, Fog Collection and Dew
- Audio Recording of Cantor Yossele Rosenblatt's Dew Service from the Florida Atlantic University Judaica Sound Archives (Hebrew audio)
https://wn.com/Dew
Dew (disambiguation)
Dew is the condensation of atmospheric water vapor into droplets.
Dew, Dewe, Dews or Dewes can also refer to:
Music
People
By surname Dew
Degu
The degu (Octodon degus, /ˈdeɪɡuː/) is a small caviomorph rodent that is endemic to the Chilean matorral ecoregion of central Chile.
It is sometimes referred to as the brush-tailed rat, and is also called the common degu, to distinguish it from the other members of the genus Octodon. Other members are also called degus, but they are distinguished by additional names. The name "degu" on its own, however, indicates either the genus Octodon or, more usually, O. degus. Degus are in the parvorder Caviomorpha of the infraorder Hystricognathi, along with the chinchilla and guinea pig. The word degu comes from the Mapudungun dewü (mouse, rat).
Description
The degu is a small animal with a body length of 25.0 to 31.0 centimetres (9.8–12.2 in) and a weight of 170 to 400 grams (6.0 to 14.1 oz). It has yellow-brown fur above and creamy-yellow below, with yellow around the eyes and a paler band around the neck. It has a long, thin tail with a tufted black tip, dark sparsely-furred ears, and pale grey toes. Its fifth toe is small with a nail, rather than a claw, on the forefeet. Its hindfeet are bristled. Its cheek teeth are shaped like figures-of-eight, hence the degu's genus name "Octodon".
Distant Early Warning Line
The Distant Early Warning Line, also known as the DEW Line or Early Warning Line, was a system of radar stations in the far northern Arctic region of Canada, with additional stations along the North Coast and Aleutian Islands of Alaska, in addition to the Faroe Islands, Greenland, and Iceland. It was set up to detect incoming Soviet bombers during the Cold War, and provide early warning of any sea-and-land invasion.
The DEW Line was operational from 1957 to the late 1980s and it was the northernmost and most capable of three radar lines in Canada and Alaska; the joint Canadian-US Pinetree Line ran from Newfoundland to Vancouver Island, and the Mid-Canada Line ran somewhat north of this. Between 1988 and 1993, most stations were deactivated. Those that remained were upgraded as part of the new North Warning System.
Introduction
The shortest (great circle) route for a Russian air attack on North America is through the Arctic, across the North Pole. The DEW Line was built during the Cold War to give early warning of a Soviet nuclear strike, to allow time for US bombers to get off the ground and land-based ICBMs to be launched, to reduce the chances that a preemptive strike could destroy US strategic nuclear forces. The original DEW line was designed to detect bombers and was unable to detect intercontinental ballistic missiles (ICBMs). To give warning of this threat, in 1958 a more sophisticated radar system was constructed, the Ballistic Missile Early Warning System (BMEWS).
