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 aroundcondensation 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. 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.