Thunderstorms form when an air mass becomes unstable (when air in the lowest layers is very warm and humid, or air in the upper layers is unusually cold, or if both occur). Rising near-surface air in an unstable air mass expands and cools, making it warmer than its environment, which causes it to rise even farther.
Weather satellites have been used by the National Weather Service since the sixties to map clouds, sea surface temperatures, and vertical temperature and moisture distributions. These data are supplemented by crucial ground observations, and are subsequently input to numerical models which try to predict the short-term evolution of the weather. The results are then made available to the public, most directly in the form of composite weather maps. The rain shown on these maps comes mostly from ground weather radars.
Depending on their type, clouds can consist of dry air mixed with liquid water drops, ice particles, or both. Low, shallow clouds are mostly made of water droplets of various sizes. Thin, upper level clouds (cirrus) are made of tiny ice particles. Deep thunderstorm clouds which can reach up to 20 km in height contain both liquid and ice in the form of cloud and raindrops, cloud ice, snow, graupel and hail.
This important question is still under investigation. Much of the rain is produced by clouds whose tops do not extend to temperatures colder than 0° C. The mechanism responsible for rain formation in these "warm" clouds is merging or "coalescence" among cloud droplets, which are first formed by vapor condensation. Coalescence is probably the dominant rain-forming mechanism in the tropics. It is also effective in some mid-latitude clouds whose tops may extend to subfreezing temperatures.
Hail stones vary in size. Most commonly they are 1 cm in diameter but have been observed to be as large as 10 to 15 cm. Hailstones are formed when either aggregated ice ("graupel") or large frozen raindrops grow by collecting cloud droplets with below-freezing temperatures. An important aspect of hail growth is the latent heat of fusion which is released when the collected cloud water freezes.
