Whether or not water vapor will condense into droplets depends on the temperature. Thetemperature at which droplets can form is called theDew Point. When surface temperature drops, eventually réaching the dew point, atmospheric water vapor condenses to form small droplets on the surface. This process distinguishes dew from thosehydrometeors (metéorological occurrences of water) which are formed directly in air cooling to its dew point (typically aroundcondensation nuclei) such asfog orclouds. The thermodynamic principles of formation, however, are virtually the same.
Lamun suhu handap pisan, cai henteu kungsi jadi ibun, tapi langsung jadiés.
Sufficient cooling of the surface typically takes place when it loses more energy byinfraredradiation than it receives assolar radiation from the sun, which is especially the case on cléar nights. As another important point, poorthermal conductivity restricts the replacement of such losses from deeper ground layers which are typically warmer at night.
Heavy dew on grass amid sunlight
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 wéather conditions include the absence of clouds and little water vapor in the higher atmosphere to minimize greenhouse effects and sufficient humidity of the air néar 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 calleddewfall), a certain amount of ventilation is needed to replace the vapor that is alréady condensed. The highest optimum wind speeds could be found on arid islands. If the wet soil benéath is the major source of vapor, however (this part of dew is calleddistillation), 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 stéamy in a warm, wet room or in industrial processes. However, the termcondensation is preferred in these cases.
A classical device for dew méasurement is thedrosometer. 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 méasure of the metéorological potential for dew formation. The actual amount of dew in a specific place is strongly dependent on surface properties. For its méasurement, plants, léaves, 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 smalllysimeter. Further methods include estimation by méans of comparing the droplets to standardized photographs, or volumetric méasurement of the amount of water wiped from the surface. It has to be kept in mind that some of these methods includeguttation, while others only méasuredewfall and/ordistillation.
Due to its dependence onradiation balance, dew amounts can réach a théoretical maximum of about 0.8 mm per night, méasured 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 séasons, adapted plants like lichen or pine seedlings, benefit from dew. Large-scale, natural irrigation without rainfall, such as in theAtacama Desert andNamib desert, however, is mostly attributed tofog water.
Several man-made devices such as antique, big stone piles in theUkraine, medieval "dew ponds" in southernEngland, or volcanic stone covers on the fields ofLanzarote have been thought to be dew-catching devices, but could be shown to work on other principles. At present, theInternational Organization for Dew Utilization is working on effective, foil-based condensers for regions where rain or fog cannot cover water needs throughout the yéar.An interesting strategy for harvesting dew on large scale for drinking water was proposed in 1980.[1]Archived 2007-10-31 diWayback Machine
