Dew is any water that is condensed onto the ground, rocks, grass, and so on. It generally forms at night due to radiational cooling (most effective on clear, still, cool nights), which causes the temperature of the air to fall below its dewpoint.

Dewpoint is the temperature at which saturation of the air occurs, i.e. the temperature at which the observed vapor pressure is equal to the saturation vapor pressure. If a parcel of air is hypothetically held at constant pressure and vapor content, the temperature at which it must be cooled to reach saturation is called its dewpoint or, if below 0°C, its frost point. Provided that any change in temperature of a parcel of air occurs without change in pressure or vapor content, the dewpoint will always be constant, i.e. conservative. On the other hand, if the parcel rises adiabatically, the dewpoint is quasi-conservative, because the dewpoint of moist air drops only at about one-fifth the rate of the dry adiabatic lapse rate.

A dewpoint hygrometer may be used to determine surface dewpoint. A refrigerated metal surface is brought in contact with air, causing condensation when it is slightly below the temperature of the thermodynamic dewpoint. More usually dewpoint is simply determined with a psychrometer (wet and dry bulb thermometers), used with appropriate tables (Table D3).

Table D3 Dewpoint temperature (1000 mb)
Full size table

If the temperature is below freezing, hoar frost will form, but if the temperature drops below freezing after the dew has formed the result is white dew. Dew is especially effective when the ground layer of air has a high relative humidity, as along river valleys, near swamps, etc. Dew is often the only form of moisture available to plants and animals in extreme deserts.

An optical effect of dew is known as heiligenschein or Cellini’s halo, brought about by an early-morning sun producing a shadow over the observer’s head. Reflection from the dew-covered surface produces a “saintly” halo, said to have been described first by Cellini, who found it a divine sign.