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Introduction to Thunderstorms

[thunderstorm cloud]
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 previously discussed. These are unequal heating of the Earth's surface (surface heating and free convection), orographic (mountain) lifting, dynamical 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 as each of these can initiate thunderstorm development.

Immediately after lifting begins, a rising parcel of warm moist air begins to expand and cool. At a certain elevation the parcel will reach saturation with respect to water vapor 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. A picture of a large thunderstorm is shown on the left. Shallow clouds in the foreground have not reached the unstable layer. The tall cumulonimbus cloud traces the path taken by unstable parcels of air. The anvil at the top of the cloud indicates where rising parcels become stable and begin to spread out horizontally.

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. You should understand how each of these changes to the atmosphere tend to make it more unstable.

Three changes that can make the atmosphere more unstable:

  1. Warm the air at the surface (just above the ground). Rising parcels which start out warmer have a better chance of becoming warmer than the surrounding environment as they are lifted. This explains why thunderstorms are most common in the mid to late afternoon when surface temperatures are highest.
  2. Cool the air aloft (4000 - 7000 meters 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, i.e., increase the dew point temperature of the air near the ground. More water vapor in a rising parcel means that more latent heat will be released during cloud formation and this increases the likelihood that the parcel will become warmer than the surrounding environment.

Distribution of thunderstorms -- Global

It is estimated that more than 40,000 thunderstorms occur each day throughout the world. Annually, this adds up to nearly 15 million thunderstorms per year across the surface of the Earth. The figure below shows the spatial density of annual lightning flashes around the world as measured with satellite instrumentation.

[Worldwide lightning flashes from satellites]
Average number of lightning flashes per year (per square km). Color key is at the bottom of the image. Data are based on satellite-borne optical lightning detectors LIS and OTD. (Image courtesy of NASA.)

Thunderstorms form from the equator to as far north as northern Alaska and Siberia. Thunderstorms are most common over land areas in and near the tropics, with the global hot spot over central Africa. Tropical regions receive intense sunshine all year round, and the heating contrast between the land and ocean surface causes warm unstable air over land to rise up and form thunderstorms (see this hand drawn diagram.) Thunderstorm formation over tropical oceans far from land areas is less frequent because ocean surfaces do not warm as rapidly as land surfaces. In general, thunderstorms are more common over land areas and over oceans adjacent to land than over the oceans far from land. Outside the tropics, thunderstorm formation is more seasonal occurring most often in those months where heating from the sun is most intense (late spring through early fall). While tropical areas do produce the most thunderstorms per year, the most common place for severe thunderstorms is in the middle latitude regions of the Earth. In these regions, warm moist tropical air can mix with cold polar air, which sets the stage for severe thunderstorms.

Distribution of thunderstorms -- United States

[thunderstorm frequency - US]
The average number of days each year on which thunderstorms are observed thoughout the United States. Taken from Koehler, T. L., (2020). Cloud-to-Ground Lightning Flash Density and Thunderstorm Day Distributions over the Contiguous United States Derived from NLDN Measurements: 1993 – 2018. Mon. Wea. Rev., 148, 313 – 332, https://doi.org/10.1175/MWR-D-19-0211.1.

The figure above describes the annual average number of thunderstorm days across the United States. This figure includes all thunderstorms. Most thunderstorms are not severe thunderstorms. You should be familar with the thunderstorm characteristics of three distinct regions, labeled (1) - (3) below.

  1. The greatest incidence of thunderstorms occurs in the southeastern quarter of the United States. The largest numbers occur over the immediate gulf coast with a the maximum numbers over the state of Florida. 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. The southeastern states are surrounded by the warm ocean waters of the Gulf of Mexico and the warm Gulf Stream Current along the east coast, which supplies vast quantities of water vapor for the adjacent land areas. The atmosphere is often relatively unstable, particularly during the day when the ground is warmed by the sun, quickly forming cumulonimbus clouds when surface heating is intense. Florida is the thunderstorm capital of the United States as it is surrounded on three sides by warm ocean water often resulting in convergent sea breezes.
  2. A secondary maximum in thunderstorm days is found in Colorado, New Mexico, and eastern Arizona. Many of these thunderstorms happen during the summer monsoon season. In this region the mountainous terrain is an important factor. 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, some tropical moisture is imported into this region resulting in a strong seasonal increase in thunderstorm frequency. In fact the majority of Tucson thunderstorms take place during the summer monsoon season.
  3. Few thunderstorms occur along the west coast of the United States. The main reason is that this region is adjacent to the cold surface ocean water of the eastern Pacific Ocean. The cold air in the lower atmosphere makes the atmosphere stable and not conducive to thunderstorm formation. In other words, the atmosphere along the west coast of the United States is typically stable because surface air usually comes from the relatively cold eastern Pacific Ocean.
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