What is dew? How does dew form, measurement of dew, what is dew point, information on dew.
DEW; is a condensation of atmospheric moisture on objects that are colder than the dew-point temperature of the surrounding air. (Dew point is the temperature at which air of a given water vapor content and at a given pressure becomes saturated—that is, when its relative humidity is 100%.) The term “dew” is generally restricted to meteorological conditions, but the “sweating” of pipes, the deposit of moisture on the outside of drinking glass«: containing ice, and the condensation of water from steam, are all caused by the same physical process.
Formation of Dew:
Meteorologists recognize several kinds of weather conditions that lead to condensation of moisture from the air onto exposed surfaces. The most common form of dew is the deposit that forms during calm clear nights. The droplets are often so numerous and tighdy packed that they may cover exposed surfaces with an extended film of water. The essential process in this kind of dew formation is the cooling of a surface at night by heat radiation to a clear sky. The dew forms only on those surfaces whose temperature falls below the dew-point temperature of the adjacent air.
A second, less common kind of dew formation sometimes occurs when a brisk, warm, moist wind blows over a cold ground, usually under a cloudy sky. The ground becomes visibly damp or even wet, but the moisture does not stand out in tiny droplets (the usual concept of dew).
Certain objects acquire dew while others do not. For example, the dew often covers only the upper portions of blades of grass, dry leaves, small plants, boards, fences, or roofs; on the bare ground, op sidewalks and stones, and at the base of the grass it is absent. The explanation lies in the differing heat conductivity of the objects. Thus, objects in extensive contact with the ground can draw at night upon the heat stored up in the ground during the day. If they are good conductors, their temperature will not fall so fast during the night. Isolated small objects and poor conductors cool more rapidly than others and thus will sooner reach the dew-point temperature of the adjacent air.
If the moisture content of the air is too low, its saturation point will not be reached, and dew will not be formed, despite cooling. Although the maximum water vapor content of the air for a given temperature determines the quantity of dew deposit possible, it is not usually a critical factor in determining whether dew will or will not form. The probability of dew formation is best indicated by the spread between actual air temperatures in the preceding afternoon and the dew-point temperature of the air, provided the sky remains clear and the wind light or calm. The larger the spread between these temperatures, the more cooling is required for dew to form.
Evaporation from the ground and transpiration from vegetation are not likely to change the outlook for dew once night has begun, because the amounts of moisture thus added to the air are negligible at night. Wind during the night tends to delay or prevent the formation of dew by mixing warm air from above into the lower atmospheric layers, so that the surfaces cool less rapidly. The wind also evaporates any dew that forms.
The right combination of conditions for dew formations occurs frequently in middle latitudes and in most parts of the tropics, but is rare or impossible in polar regions. In middle latitudes, the favorable weather situation is usually found in the central regions of transient high-pressure areas, or anticyclones, where winds are light and skies are clear. However, in winter the air may be too dry; and, when the temperature of the surfaces falls below freezing, hoarfrost forms instead of dew. In tropical latitudes, dew forms at night during the dry season and in the deserts. In some deserts, dew may form only on the scattered plants, whose growth depends to a considerable degree on this source of moisture.
Measurement of Dew:
It is possible to measure the quantity of dew deposited by means of special gauges. The Duvdevani dew gauge, which is the most widely used, consists of specially prepared sticks of wood exposed from sunset to sunrise at standard heights over standard plots of ground. The dew accumulated on the stick by sunrise is photographed and, from a previous calibration, the photographs are interpreted in terms of millimeters of water per stick per night. Other gauges, called drosometers, weigh the amount of dew deposited on a suitably exposed plate.