Have you ever wondered why one day is partly cloudy while the next day the sky explodes with severe thunderstorms? In the summer months, thunderstorms may develop without an associated surface front. All it takes is the right atmospheric conditions!

According to the National Weather Service, approximately 2,000 thunderstorms are occurring around the world at any given moment. Very few of these storms will be severe, however – generally less than one percent. A severe thunderstorm is defined as one that produces hail dime-size or larger or downburst wind gusts greater than 58 miles per hour. Among severe thunderstorms, only a small percentage produce tornadoes.

Several conditions are required to produce a thunderstorm. First, the lower and middle levels of the troposphere must contain significant moisture. Since warmer air can contain more water vapor, this often means warm or even hot temperatures at the surface. Warm air will rise because it is less dense than cold air. A warm air pocket expands as it rises; this expansion causes it to cool, forcing the water vapor in it to condense as droplets. We now have a cloud!

Second, the atmosphere must be unstable. This allows the air that has cooled (losing some of its water vapor) to keep rising, with even more condensation. The cloud becomes taller as this process goes on. Eventually (depending on the extent of the instability) equilibrium will be reached between the rising air pocket and the atmosphere around it. At this point the updraft slows down and the air pocket spreads out, creating the familiar flat anvil top of the thunderstorm.

The third condition is a source of lift. A thunderstorm needs a strong updraft to develop, and a source of lift will ensure that the air continues to move upwards. Once possible source is differential heating. Areas of the earth’s surface, such as cities, forests, fields, and lakes, all absorb the sun’s heat differently. The temperature of the air above each area will also vary, and this fuels the upward-moving air.

The first stage of thunderstorm development is the cumulus or towering cumulus stage.



The updraft is the most important component of this stage. When the air begins to reach equilibrium, precipitation forms in the upper levels of the cloud. Once its density is high enough, the precipitation falls from the cloud, creating a downdraft.

The mature thunderstorm (second stage) is characterized by the presence of a downdraft and an updraft. Hail may form if dust particles move up and down repeatedly, receiving another layer of ice with each upswing. The downdraft of rain-cooled air may be swift enough to create a gust called a downburst. Thus, the storm may become severe.

After a significant amount of precipitation has fallen, the downdraft dominates and the updraft disappears. The storm has reached the dissipating stage, but the downdrafts can trigger new thunderstorms, continuing the cycle.