Geography · Ch 15 — Atmospheric Circulation and Weather Systems
Thunderstorms and Tornadoes
Thunderstorms and Tornadoes
Thunderstorms and tornadoes belong to a family of severe local storms — short-lived, confined to small areas, yet intensely violent. They are the atmosphere's way of releasing built-up energy: when warm, moist air rises rapidly, the potential and heat energy stored in the air get converted into kinetic energy, and once the storm spends itself, the restless atmosphere settles back into a stable state.
Thunderstorms
A thunderstorm is essentially a well-grown cumulonimbus cloud that produces thunder and lightning. It forms on hot, moist days when intense convection sets in — the ground heats the air above it, and this warm, humid air rises vigorously.
The key feature of a thunderstorm is a strong updraft of rising warm air. This updraft feeds the cloud, making it grow bigger and climb to greater heights. As the air rises, it cools, moisture condenses, and precipitation begins. Eventually, a downdraft develops, bringing cool air and rain back down to the earth's surface.
What happens at the top of the cloud depends on temperature. If the cloud extends to heights where the temperature is below zero, hails form and fall to the ground as a hailstorm. If, on the other hand, there is insufficient moisture in the air, the thunderstorm cannot produce much rain — instead, it generates a dust-storm.
Tornadoes
Sometimes, from a severe thunderstorm, a spiralling wind descends with great force, looking like an elephant's trunk. At its centre, the pressure is very low, and as it moves across the land it causes massive destruction along its path. This phenomenon is called a tornado.
Tornadoes generally occur in middle latitudes. When the same phenomenon happens over the sea, it is called a water spout.
The difference is simply the surface: a tornado over land, a water spout over the sea. The mechanism — a violently rotating column of air with very low central pressure — is the same.
The Bigger Picture …
Other severe local storms are thunderstorms and tornadoes -- they are of short duration, occurring over a small area, but are violent. Thunderstorms are caused by intense convection on moist hot days. A thunderstorm is a well-grown cumulonimbus cloud producing thunder and lightning. When the clouds extend to heights where sub-zero temperature prevails, hails are formed and they come down as hailstorm. If there is insufficient moisture, a thunderstorm can generate dust-storms. A thunderstorm is characterised by an intense updraft of rising warm air, which causes the clouds to grow bigger and rise to greater height -- this causes precipitation; later, downdraft brings down to earth the cool air and the rain. …