1 Passing clouds One of the pleasures of flying is seeing clouds close up. Even though they seem insubstantial they carry a considerable weight of water – around 500 tonnes in a small cumulus cloud. And water is denser than air. So why don’t clouds fall out of the sky like rain? They do. But the droplets take a long time to sink. An average cloud would take a year to fall one metre. 2 On cloud nine Most of us are happy to label clouds “fluffy ones” or “nasty black ones ”, but meteorologists identify more than 50 cloud types based on shape and altitude. These fit into categories given numbers from one to nine. Cloud nine is the vast, towering cumulonimbus, so to be “on cloud nine” implies being on top of the world. 3 Around the rainbow There’s no better place to see a rainbow than from a plane. Rainbows are produced when sunlight hits raindrops. We see a bow because the Earth gets in the way, but, from a plane, a rainbow is a complete circle. When passing over clouds, the plane’s shadow appears neatly in the centre of the effect. 4 Mr blue sky Sunlight is white, containing all the colours of the spectrum but, as it passes through air, some of the light is scattered when it interacts with the gas molecules. Blue light scatters more than the lower-energy colours, so the blue appears to come from the sky. 5 There’s life out there Apart from clouds