From: Steve (Primus) (Avatar) 30/10/2001 5:53:54 Subject: re: STEVE'S WEATHER FAQ post id: 476429 Just how do clouds stay aloft? I thought water vapour was heavier than air. Water vapour is lighter than air. Clouds are composed of liquid water droplets and ice crystals, not water vapour. The droplets and crystals are very small and very light and will be held up by the slightest movement in the air. Clouds themselves are formed by rising air which is enough to keep the cloud particles up. In some clouds, such as cumulonimbus, the updraughts are very strong indeed - strong enough to hold large hailstones. Also there is warm air in clouds, it comes from the latent heat released by condensation and freezing within the cloud, it is the latent heat that provides the energy for the whole system - including the updrafts which keep the water up there. "We know that hot air rises" Heard that have you? Wrong We know nothing of the sort. Less dense air rises. And when the rain does fall, what is the shape of the falling raindrop, spherical, oval, teardrop? Raindrops are basically spherical but flattened a little on the lower side due to drag. Do all raindrops fall at a uniform velocity? No. Smaller drops fall slower than large ones due to air resistance. When the rain hits the ground, are the raindrops of uniform size? Basically, no. There will be some large drops and some smaller ones. If not, what factors influence the weight of raindrops, and the standard deviation of their weight? Raindrops have a maximum size of 5-6mm diameter. Any larger than that and they are unstable and break into smaller drops. The size of raindrops is determined to an extent by the type and depth of the cloud. Bigger or deeper clouds with stronger updraughts produce larger raindrops. Raindrops are continually spitting and combining but the net result is that in any shower, the drops are of different sizes, particularly if there are large drops.