Severe Weather

Thunderstorms

Most violent of all weather systems. Fortunately, small and short lived.

Mature thunderstorm with typical anvil shaped cloud. (Source: NOAA Photo Collection Website)

Thunderstorms form when moist, unstable air is lifted vertically into the atmosphere.

Lifting of this air results in condensation and the release of latent heat. Immediately after lifting begins, the 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.

With the vertical extension of the air parcel, 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 some these clouds includes hail, strong winds, thunder, lightning, intense rain, and tornadoes.

Some thunderstorms can develop into more severe storms if the conditions exist to enhance and prolong the mature stage of development.

A Thunderstorm's life cycle

Developing thunderstorm cloud at the cumulus stage.

Extensive studies indicate that thunderstorms go through a cycle of development from birth to maturity to decay.

The first stage is known as the cumulus stage. As humid air rises, it cools and condenses into a single cumulus cloud or a cluster of clouds.

As the cloud builds, the transformation of water vapor into liquid or solid cloud particles releases large quantities of latent heat.

As the cloud builds well above the freezing level, the cloud particles grow larger. They also become heavier. Eventually, the rising air is no longer able to keep them suspended, and they begin to fall.

As the particles fall drier air from around the cloud is being drawn into it. This dier air causes some raindrops to evaporate, which chills the air. This air, cooler and thus heavier, begins to descend as a downdraft.

The appearance of the downdraft makes the begining of the mature thunderstorm. The downdraft and updraft within the mature thunderstorm constitute a cell.

During this mature stage the thunderstorm is most intense. The top of the cloud, having reached a stable region of the atmosphere, begins to take the familiar anvil shape, as the strong upper level winds spread the cloud's ice crystals horizontally.

After the storm enters the mature stage, it begins to dissipate in about 15 to 30 minutes. The dissipating stage occurs when the updrafts weaken and downdrafts tend to dominate throughout much of the cloud. Deprived of the rich supply of warm moist air, cloud droplets no longer form.

A single ordinary thunderstorm may go through its three stages in an hour or less. The reason it does not last very long is that the storm's downdraft may cut off the storm's fuel supply by destroying the humid downdrafts.

Distribution of thunderstorms

It is estimates that more than 40,000 thunderstorms occur each day throughout the world.

Thunderstorms form from the equator to as far north as Alaska. They occur most commonly in the tropics were 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 these surfaces do not warm rapidly. 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.

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 southeast and in parts of Colorado, Arizona, and New Mexico. This particular spatial distribution suggests that extreme solar heating is not the only requirement for thunderstorm formation. Another important prerequisite is the availability of warm moist air. 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 another climatic factor. All of these areas are on the leeward side of the Rocky Mountains. Mountain slopes in these areas that face the sun absorb more direct solar radiation and become relatively warmer creating strong updrafts that form into cumulus clouds. If the differential heating is also supplemented by winds from the east, the cumulus clouds are further enhanced to become thunderstorms. 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.