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.
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.
Water 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
. The thermodynamic principles of formation, however, are virtually the same.
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 part of dew 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 vapor, 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 eyeglasses get steamy in a warm, wet room or in industrial processes. However, the term condensation is preferred in these cases.
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
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
desert, however, is mostly attributed to fog
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.
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 work on other principles. At present, the International Organization for Dew Utilization is working on effective, foil-based condensers for regions where rain or fog cannot cover water needs throughout the year. An interesting strategy for harvesting dew on large scale for drinking water was proposed in 1980.
Large scale dew harvesting systems have been made by Indian Institute of Management Ahmedabad (IIMA) 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.