Forms of condensation

After condensation, moisture takes the form of dew, frost, fog (or mist) or clouds, depending on temperature and location. Condensation can occur with the dew point either above or below freezing.

• Dew: water droplets on cool solid surfaces such as stones, grass and leaves (not around nuclei in the air). Ideal conditions: clear sky, calm air, high relative humidity, and cold, long nights. The dew point must be above freezing.
• Frost: forms on cold surfaces when the dew point is at or below freezing (0°C): the moisture is deposited as tiny ice crystals. Conditions are as for dew, except that the air is at or below freezing.
• Fog and mist: when air full of water vapour cools suddenly, condensation happens within the air, on fine dust particles. Fog is a cloud with its base at or near the ground; it cuts visibility to less than 1 km. Mist cuts it to 1-2 km. In towns and industrial areas, smoke gives plenty of nuclei; fog mixed with smoke is smog. Mists are common over mountains, where warm air rising up the slopes meets cold surfaces; fogs are common where warm and cold air currents meet.

Clouds are masses of tiny water droplets or ice crystals formed by condensation high in the free air. By height, extent, density and transparency there are four basic types:

• Cirrus: high (8,000-12,000 m), thin, detached, feathery, always white.
• Cumulus: like cotton wool, in scattered patches with a flat base (the book gives 4,000-7,000 m).
• Stratus: layered clouds covering large parts of the sky, formed by loss of heat or the mixing of air masses of different temperatures.
• Nimbus: black or dark grey, at middle levels or very near the ground, very dense, opaque to sunlight, shapeless; they bring rain.
• Combinations give: high clouds (cirrus, cirrostratus, cirrocumulus); middle (altostratus, altocumulus); low (stratocumulus, nimbostratus); and tall vertical clouds (cumulus, cumulonimbus).

Example (new): on winter mornings in a big city, smoke from traffic and burning give the moist air far more nuclei than in the countryside, so the fog becomes thicker, greyer and longer-lasting: smog.