The updraughts of warm, moist air that form clouds as they cool also serve to keep them in the sky. The total amount of water in a typical cloud weighs as much as 200 bull elephants, but it doesn’t crash down to the ground because the water is broken up into tiny droplets and ice crystals. Even the largest dropletsonly have a radius of 0.1mm. A droplet of this size falling freely would experience so much air resistance that its maximum speed would be a mere 30cm/s, but in a cloud the downward speed is balanced by the upward speed of the rising air. Only when many such droplets coalesce can they become big enough to fall as rain The air speed necessary to keep clouds ‘floating’ depends on the type of cloud. Flat, spread-out stratiform clouds are formed and supported by weak air currents rising at only a few centimeters per second. Cumulus (or convective) clouds, which are the ones responsible for heavy rain and storms, contain bigger droplets and need updraughts of a few metres per second to support them.