A small fair-weather cloud, the cotton kind about a kilometre across, holds something like five hundred tonnes of water. That is the meteorologist Peggy LeMone’s back-of-the-envelope figure: about half a gram of water in every cubic metre, times a billion cubic metres, which comes to roughly the weight of a hundred elephants hanging over a picnic. It stays up because it isn’t really floating. Each droplet is about a fiftieth of a millimetre across and falls through the air at around a centimetre a second, but the warm air rising underneath is climbing faster than that, so the whole cloud is like someone walking down an escalator that is going up. In a warm cloud, rain begins when droplets bump into each other and merge until they are heavy enough to outwalk the escalator.