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 vapor 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.
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.
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.