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Annie Robinson, of Ilam School, asks:

"Why do clouds float?"

Neil Cherry, a meteorologist at Lincoln University's Climate Research Unit, responds:-

Clouds float in rising air. When air falls the clouds evaporate. Clouds are made of small droplets of water and water is a lot heavier than air so we might expect the droplets, and hence clouds, to fall and not float. However, by observation clouds do float.

As air rises it expands because the air pressure decreases as you go higher up. The expanding air cools and the humidity rises. When the humidity exceeds 100% the cloud forms as tiny water droplets try to fall because of gravity. Small cloud droplets fall relative to the air at about one centimetre per second, while larger cloud drops fall at about 25cm/second. If the air is rising faster than 25cm/second then even the large cloud drops are held up and rise higher above the ground.

Air rises when it is hotter than surrounding air, when it is being pushed over hills or mountains, if it is pushed over other air (for example over a cold front) or when it is a low pressure weather system (see the big lows marked L on the weather map in the newspaper or on the TV).

Hot rising air makes puffy clouds and sometimes forms thunderstorms. Low pressure weather systems make air rise slowly over very large areas, often covering the whole sky with a flat layer of cloud. Clouds float gently when the air supporting them is rising at speeds of up to a slow walking pace. In thunderstorms the air can rise faster than 100 km/hour, rising high into the sky and producing large hail stones.

Send your questions to aas@csc.canterbury.ac.nz