Throughout the 1930s, meteorologists often referred to thunderstorms as "heat thunderstorms" because they thought they were usually triggered by intense surface heating. That heat would cause thunderstorms actually made sense. Thunderstorms were typically most severe on summer afternoons after the ground had been baked by the Sun all day. As the hot air rose, taking moisture with it, large cumulus clouds would begin to form. Within a few hours, the cumulus would continue to grow vertically, forming bright, billowing white tops, called towering cumulus, stretching tens of thousands of feet (several kilometers) into the atmosphere. When their tops bumped into the tropopause and flattened out, they became cumulonimbus—commonly known as thunderclouds.
The relatively small size of thunderstorms was one reason meteorologists possessed only rudimentary knowledge of their structure and behavior. They often occurred near mountains as warm air rushed up the hillsides and were obscured by trees. Furthermore, unless a trained observer happened to be present while the thunderstorm clouds were building, it was impossible to get a good sense of what was happening.
The observational problem was reduced after World War II when the availability of radar and aircraft made it possible to examine thunderstorm mechanisms closely. The University of Chicago's Thunderstorm Project, largely funded by the airline industry, whose airborne assets were most likely to be affected by the driving rain, hail, icing, and turbulence associated with thunderstorms, undertook to unmask thunderstorms.
As a result of their research, University of Chicago researchers identified the three stages of a thunderstorm. In the first, or cumulus, stage, the air only moves up within the cloud. This leads to precipitation only at the top of the cloud, where the vertical velocity is at a minimum. At lower levels within the cloud, the air is moving up so rapidly that it holds droplets in it. In the second, or mature, stage, water droplets become too heavy to be carried within the cloud and rain starts to fall. As the rain falls, it carries air with it and forms a downdraft. Now air is moving both up and down within the cloud. At the highest levels (the anvil top), ice crystals form in the extremely cold temperatures, and as discussed earlier, grow at the expense of the water droplets. Moving up and down within the clouds, these ice crystals pick up more moisture and freeze again and again. Hail forms one layer at a time, much as onions are formed by layers of plant tissue. When the updrafts can no longer hold them, the hail falls out. Although most hail is small, if the cloud is very tall (up to 60,000 feet [18 km]) and the hailstones have made many trips, they can grow to become grapefruit-sized—and very
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