Science Puzzle
Which Cloud Brings the Rain?
You are planning an afternoon outdoors. The sky has three kinds of cloud in it: thin wispy streaks very high up, small puffy cotton-wool clouds, and one enormous dark tower with a strangely flat top.
Which one should worry you, and how can you tell just by looking?
The Answer
The tower with the flat top. That is a cumulonimbus, and it is the only cloud in the sky that can soak you, deafen you and hurl hailstones at you.
The wispy streaks are cirrus, so high and cold that they are made entirely of ice crystals, far too thin to produce rain. They can hint at weather arriving in a day or two, but they will not rain on you today. The puffy cotton-wool clouds are fair weather cumulus, the classic child's drawing of a cloud, formed by gentle rising air on a warm day. Harmless.
The cumulonimbus is a cumulus that kept growing. Powerful updraughts drive it upward for kilometres, packing it with water and ice until it can hold no more. That flat top, called the anvil, is the giveaway: the cloud has risen so high that it has hit the ceiling of the lower atmosphere and can go no further, so it spreads sideways instead. A cloud tall enough to hit the ceiling has enough energy to produce torrential rain and lightning.
The principle: Cloud classification and weather prediction. A cloud's height, shape and vertical development reveal the air movements inside it, and therefore what weather it can produce.