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)

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.

Results of severe thunderstorms Lightning Thunder Wind Hail Floods Tornados

Lightening 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.

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 features air diving toward the surface, whereas a tornado is composed of rising air.

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.

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.