Severe Weather

Thunderstorms

Mature thunderstorm with typical anvil shaped cloud.

Thunderstorms are violent weather systems. Fortunately, most thunderstorms are spatially small and temporally short-lived. Only a small fraction of all thunderstorms are classified as severe thunderstorms.

Thunderstorms form when moist, unstable air is lifted vertically into the atmosphere. The processes that initiate the vertical lifting were discussed in an earlier lecture. These are unequal heating of the Earth's surface, orographic (mountain) lifting, dynamic lifting, which occurs near areas of lower level convergence and/or upper level divergence, and lifting because of the presence of a frontal zone -- each of these can initiate thunderstorm development. You may want to review the Conditions which force air to rise because each of these can initiate thunderstorm development.

Immediately after lifting begins, a rising parcel of warm moist air begins to cool. At a certain elevation the dew point is reached resulting in condensation and the formation of a cumulus cloud. During condensation, large quantities of latent heat are released. Much of the energy used in thunderstorm development comes from the release of latent heat. It is the heat released by condensation within a cloud that permits the rising air to stay warmer than its surroundings, and thus to be buoyant through great depths.

With the vertical extension of air parcels, the cumulus cloud grows into a cumulonimbus cloud. Cumulonimbus clouds can reach heights of 20 kilometers above the Earth's surface. Severe weather associated with thunderstorms includes hail, strong winds, lightning, intense rain, flash flooding, and tornadoes.

Thunderstorm development requires an unstable atmosphere. The more unstable the atmosphere, the more violent the potential thunderstorm. There are three basic ways to make the atmosphere more unstable:

1. Warm the air at the surface. Rising parcels which start out warmer have a better chance of becoming warmer than the surrounding environment as they are lifted.
2. Cool the air aloft (above the surface). Colder environmental conditions above the surface make it more likely that lifted surface parcels will be warmer than the surrounding environment.
3. Add more water vapor at the surface. More water vapor in a rising parcel means that more latent heat will be released during cloud formation and increases the likelihood that the parcel will become warmer than the surrounding environment.

Distribution of thunderstorms

It is estimated that more than 40,000 thunderstorms occur each day throughout the world. Annually, this adds up to 14,500,000 thunderstorms per year.

Distribution of world-wide lightning strikes from satellite-based optical sensors. Units are #flashes per square kilometer per year. Figure provided by NSSTC lightning team.

Thunderstorms form from the equator to as far north as Alaska. They occur most commonly in the tropics where convectional heating of moist surface air occurs year round. Many tropical land based locations experience over 100 thunderstorm days per year. Thunderstorm formation over tropical oceans is less frequent because ocean surfaces do not warm as rapidly as land surfaces. Thus, in general, thunderstorms (especially severe thunderstorms) are much more common over land areas than over the oceans. Outside the tropics, thunderstorm formation is more seasonal occurring in those months where heating is most intense.

The figure bellow shows the average number of days each year having thunderstorms in various parts of the United States. Note that this figure includes all thunderstorms. Most thunderstorms are not severe thunderstorms.

The average number of days each year on which thunderstorms are observed thoughout the United States. (Due to the scarcity of data, the number of thunderstorms is underestimated in the mountainous west.)

The figure above describes the annual average number of thunderstorm days across the United States. According to this map, the greatest incidence of thunderstorms occurs in the southeastern United States. This particular spatial distribution suggests that extreme solar heating is not the only requirement for thunderstorm formation. Another important prerequisite is the availability of large quantities of water vapor at the surface. In the United States, the Gulf of Mexico supplies adjacent continental areas with moist maritime tropical air masses. These air masses are relatively unstable quickly forming cumulonimbus clouds when surface heating is intense. The secondary maximums found in Colorado, Arizona, and New Mexico are due to the mountainous terrain. Mountain slopes that face the sun absorb more direct solar radiation and become relatively warmer creating strong updrafts that form into cumulus clouds. These areas are much less humid than the southeastern United States, thus the thunderstorms in these areas are characterized by higher cloud bases and less rainfall. During the summer monsoon season, east winds bring some moisture from the Gulf of Mexico resulting in an increase in thunderstorm frequency. Few thunderstorms occur along the west coast of the United States. This region is dominated by cool maritime polar air masses which suppress convectional uplift over land. That is the atmosphere in these regions is typically stable because surface air usually comes from the relatively cold eastern Pacific Ocean.