Severe Thunderstorms

Each day throughout the world it is estimated that more than 40,000 thunderstorms occur, therefore over 14 million occur annually. The combination of moisture and warmth makes the land masses around the equator, especially conducive to thunderstorm formation. Thunderstorms are prevalent over water along the intertropical convergence zone, where the low-level convergence of air helps to initiate uplift. Near the equator thunderstorms can occur on average about every third day. Heat energy liberated during these storms allows the earth to maintain its heat balance by distributing heat poleward. Thunderstorms rarely occur in dry climates, such as the polar regions of the earth and the desert areas of the subtropical highs.

The Earth’s climate is constantly evolving and undergoing change. Evidence suggests that throughout much of the earth’s history, the earth’s climate was much warmer than it is today.

Over the last hundred years or so, the earth’s surface temperature has increased by about 0.7°C (1.2°F). These conditions can help explain why the frequency of severe thunderstorms has increased all over the world.

We examined thunderstorm form and causes of storms. What about the impacts of the storms?

Due to the intense nature of these thunderstorms, they are often associated with flash floods – floods that rise rapidly with little or no advanced warning. Such flooding often results when thunderstorms stall or move very slowly, causing heavy rainfall over a relatively small area. Lightning is simply a discharge of electricity, a giant spark, which occurs in mature thunderstorms. Lightning may take place within a cloud, from one cloud to another, from a cloud to the surrounding air, or from a cloud to the ground. Given the convective environment (very large CAPE, high cloud base, dry air below 700hPa and high freezing level) storms have potential for generating strong downdraught gusts, due to evaporative cooling leading to negative buoyancy of associated air.

The majority of severe thunderstorms are likely to occur between April and September. We examined thunderstorms and the atmospheric conditions that produce them. In order for an isolated ordinary (air-mass) thunderstorm to form there needs to be a conditionally unstable atmosphere, humid surface air and plenty of sunlight to heat the ground. With these conditions what starts as small cumulus clouds may grow into towering clouds and thunderstorms rapidly in under 20 minutes.

The stage is set for the generation of severe thunderstorms, when the conditions are optimal for thunderstorm development and a strong vertical wind shear exists. Supercell thunderstorms may exist for many hours, as their updrafts and downdrafts are nearly equal and perfectly balanced. Thunderstorms that form in a line, ahead or alongside an advancing cold front, are called a squall line.

Lightning is a discharge of electricity that occurs during mature thunderstorms. The lightning strike momentarily heats the surrounding air to a very high temperature. The rapidly expanding air produces a sound called thunder. Due to the speed of sound being much slower than the speed of light we always see the lighting first. The speed of sound can be used to calculate how far away the lighting strike was. Along with lightning and thunder, severe thunderstorms produce violent weather, such as destructive hail, strong downdrafts, and the most feared of all atmospheric storms, the tornado.

Tornadoes are rapidly rotating columns of air that extend downward from the base of a thunderstorm. The majority of tornadoes are less than a few hundred meters wide with wind speeds less than 100 knots, although violent tornadoes have reported wind speeds that exceeded 250 knots. A violent tornado could potentially have smaller whirls (suction vortices) rotating within it. Scientists are probing tornado spawning thunderstorms, with theassistance of equipment such as the Doppler radar in the hopes of being able to better predict tornadoes and to better understand all aspects of their formation.