THUNDERSTORMS

Severe Thunderstorms Most thunderstorms include heavy rains, strong winds, lightning, and sometimes hail. Storm clouds and darkening skies indicate that a thunderstorm is approaching.

How thunderstorms form For a thunderstorm to form, three conditions must exist.

#1 There must be an abundant source of moisture in the lower levels of the atmosphere As the moisture condenses, it releases latent heat The release of latent heat keeps the cloud warmer than the air around it, which is crucial in maintaining the upward motion of the cloud

#2 Some mechanism (orographic lifting, temperature differences, fronts) must lift the air so that the moisture can condense and release heat.

#3 The portion of the atmosphere through which the cloud grows must be unstable. (It must continue to cool with increasing altitude for the growing cloud to stay warmer than the surrounding air.)

Most Important! Two of the most important ingredients for thunderstorm formation are instability (unstable air) and moisture.

Types of Thunderstorms There are three main types of thunderstorms; orographic, air mass, and frontal.

Air mass thunderstorms are the result of localized convection in an unstable air mass.

Frontal thunderstorms occur along the boundaries of weather fronts (e Frontal thunderstorms occur along the boundaries of weather fronts (e.g. cold front).

Orographic thunderstorms are caused by air that is forced up by a mountain or hillside.

Stages of Development A thunderstorm usually has three stages: The cumulus stage The mature stage The dissipation stage (the stages are classified according to the direction in which the air is moving) Click here for Thunderstorm formation video

The Cumulus Stage The cumulus stage - when the updraft reaches the condensation point begins to form the cumulus cloud.

The Mature Stage The mature stage begins when the first drop of precipitation from the cumulus clouds reaches the ground. In the convection cycle, this is when the water droplets become too heavy for the updraft to hold aloft.  During the mature stage, cloud tops begin to exceed 60,000 feet. Strong winds at these altitudes cause the tops of the clouds to level off, and take an anvil shape. The "anvil" is so high and temperatures are so low that the top of the cloud is composed entirely of ice crystals. 

The mature stage continued A thunderstorm is strongest toward the end of the mature stage. Rain will be the heaviest and lightning is abundant. This is when hail, strong winds and even tornadoes may form. 

The Dissipation Stage   The end of a thunderstorm. This is when precipitation falls through the cloud, breaking it up. During the dissipating stage, the humidity in the air drops and the precipitation ends. 

The cumulus stage of a thunderstorm is characterized mainly by updrafts. The mature stage is characterized by strong updrafts and downdrafts. The storm loses energy in the dissipation stage.

What causes a storm to be severe? Greater temperature differences form between upper and lower parts of the storm This causes air to become more unstable. As instability of the air increases, strength of the storm’s updrafts/downdrafts intensifies and the storm is said to be “severe”

Supercells Severe thunderstorms may develop into self-sustaining, extremely powerful storms called “supercells,” which are characterized by intense, rotating updrafts. click here for "How do Thunderstorms Form?" video

Click here for supercell video

Results of severe thunderstorms Lightning Thunder Wind Hail Floods Tornados

Lightning The sky is filled with electric charge. The positively and negatively charged particles are separated by updrafts and downdrafts, separating the charges into two levels.

Click here for “Bluemoon Lightning Storm” video

Click here for “How Does Lightning Form?” video

Thunder Lightning is a giant spark. A single stroke of lightning can heat the air around it to 54,000 degrees Fahrenheit! This extreme heating causes the air to expand at an explosive rate. The expansion creates a shock wave that turns into a booming sound wave, better known as thunder.

Wind During strong or severe thunderstorms, a burst of intense winds often flatten buildings and knock down trees. These winds, known as "downbursts," are often mistaken for tornadoes due to the severity of the damage. A downburst (or microburst) features air diving toward the surface, whereas a tornado is composed of rising air. Click here for “Tucson Wet Microburst” video

Hail Hail forms when: strong currents of rising air, known as updrafts, carry water droplets high enough in a thunderstorm for the water droplets to freeze. Click here for “How Does Hail Form?” video

Floods When a storm dumps its rain over a limited location, rather than spreading it over a large area. If rain falls faster than the ground can absorb it, or faster than streams and rivers can transport it out of the area, flooding can occur. Click here for “Floods 101” video

Hurricanes The most severe of all thunderstorms are hurricanes. Sept. 20, 2005: Hurricane Katrina hits the southern coast of Louisiana. It became the most destructive hurricane in U. S. history. Click here for “How Hurricane Katrine Formed” video