Storms

Aristotle theorized about weather about 330 B.C.E., and his guesses, some close and others far wrong, persisted for the next 2,000 years. He thought  that the Sun draws both water and air upward to form clouds. Clouds return water as rain and air as wind, causing storms. When clouds bump into each other, the sound is thunder, and air pushed out in the collision burns as lightning.

Understanding improved after devices that measure temperature, air pressure, wind speed, and humidity were invented in the 17th century. But the key observation came in 1821, from William Redfield  [American: 1789-1857], who observed that a storm called a hurricane-now defined as a tropical storm with steady winds over 74 mph (119 kph)-formed a giant counterclockwise spiral. Redfield collected data from many storms after that and, in 1831, published his conclusion that all major storms on the coast of North America are similar whirlwinds. Such storms are characterized by very low pressure at their center, which causes air to move inward. The Earth’s rotation turns that inward movement into a spiral.

The nature of storms was explained further in 1920 by Vihelm Bjerknes [Norwegian: 1862-1961] who discovered that storms begin where warm and cool air masses meet at a line he called a “front”. During World War II, high-flying American pilots noticed a stream of air, now called the jet stream, that flows from west to east. This is also the front between warm and cool air, so movement of the jet stream determines much weather and the location of storms.

Not all storms, however, are caused this way. Thunderstorms usually begin as great updrafts caused by warm, moist air rising from heated soil. As air rises, it cools, and rain or hail begins to fall downward, producing downdrafts to conflict with the updrafts. If the winds become very strong, they twist themselves into the tight spiral called a tornado.