Dew and Frost

Dew

If you have a glass of ice cold water taken out of the refrigerator, water droplets soon begin to condense on the outside of the glass. This happens because the glass is at a lower temperature than the dew point of the air. If the air is very dry and the temperature of the outside of the container does not fall below its dew point, then no condensation forms.

The 'Dew point' is defined as the temperature at which the air, when cooled, will just become saturated. For example: A day in which the air temperature reaches 18°C with a dew point of 8°C. Late in the afternoon, the air temperature begins to fall, but the dew point will still be around 8°C. However, the air temperature is measured at 1 metre above the ground and, under a clear sky, the temperature of some objects may be significantly lower, due to loss of heat by radiation. Once the temperature of the object has fallen below the dew point, water vapour begins to condense on to it in the form of dew. This is particularly noticeable on the surfaces of cars and conservatories.

Dew also forms readily on grass because (a) the temperature falls more rapidly nearer to the grass and (b) the grass leaves produce water vapour, which raises the dew point of the air immediately in contact. Dew doesn't form as readily on other surfaces, such as soil, brick or stone. This is because these materials absorb heat from the sun which is then slowly emitted during the evening, causing the temperature of air immediately in contact to stay above the dew point for much longer than over grass.

In the morning, as the incoming solar radiation gathers strength, the dew will evaporate. Metal surfaces, such as car bodies, will dry relatively quickly whereas grass stays damp for considerably longer. In fact, from late autumn to early spring, in some places shaded from the sun, grass may remain damp all day after a heavy dew.

Frost

Hoar frost (composed of tiny ice crystals) is formed by the same process as dew, but when the temperature of the surface falls below freezing point. The 'feathery' variety forms when the surface temperature reaches freezing point before dew begins to form on it.

A 'white' frost, which is composed of more globular ice, occurs when the dew forms first, which then subsequently freezes. A ground frost may occur when the air temperature does not get down to freezing point. Consequently, when the grass is covered in a white hoar frost at dawn it cannot be assumed that there is or has necessarily been an air frost.