Why Does Dew Fall Overnight?
Dew is the result of condensation on the surface of the earth. At night the earth sometimes becomes colder than the air, and when water vapour in the air touches leaves and other objects on the earth’s cold surface in the early hours of the morning, it condenses to form “dewfall”. If the earth is very cold the vapour freezes and hoar frost, instead of dew, is formed. If, on the other hand, water vapour rising from the earth meets a leaf colder than itself, it will condense to form a different kind of dewfall.
Typical dew nights are classically considered calm, because the wind transports (nocturnally) warmer air from higher levels to the cold surface. However, 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 vapor, however (this type of dew formation is called distillation), wind always seems 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.