Thunderstorm

A deep rolling, booming peal of thunder, a rush of wind and dark clouds whose menacing aspect is increased by the pallor of leaves turned over to expose their pale underbellies by the gust, flashes of lightning, a sudden drop in temperature, and a spattering of heavy raindrops becoming a deluge, are all the visible signs that a thunderstorm has arrived. Dangerous and often beautiful, thunderstorms are a regular part of the world's weather cycles, occurring in isolation when hot, unstable air rises on a summer afternoon, or coming in long lines as part of an advancing cold front.

A thunderstorm is any storm that produces lightning and the accompanying thunder. There are various types of thunderstorm, but all of them contain lightning, and all are created by powerful updrafts that can lift the summit as high as the stratosphere, where the thunderstorm will cap out in a classic anvil shape. Thunderstorms are usually tropical or summer phenomena, since they need a lot of heat and a strong supply of moisture to take shape. Winter thunderstorms, called thundersnow, are possible under certain conditions, however.

Types of thunderstorm

Thunderstorms share certain common features -- a tall, often anvil-topped shape, lightning, the potential to create deadly weather such as hail, powerful winds, tornados, and downbursts -- but they come in several different varieties depending on where and how they form, and whether other storms are present. Each type of storm has different traits and follows a different life cycle. The four kinds are known as single cell, supercell, multicell, and squall line.

• Single cell thunderstorms are the most basic type, and are short-lived, isolated thunderstorms that occur when the air is unstable -- that is, when there is a lot of convection (upward movement) and plenty of moisture is present to form clouds. A single cell thunderstorm can occur on a hot summer afternoon, or it can form in cooler but still unstable air in the wake of a cold front. These storms last less than hour on average and typically don't produce any weather more severe than a few lightning strikes and some rain. They end when a downdraft -- which can sometimes turn into a microburst -- robs them of the upward air flow that maintains them, and dissipate shortly thereafter.
• Supercell thunderstorms are extremely powerful, isolated thunderstorms that are higher than single cell thunderstorms and can therefore survive much longer, since their updrafts and downdrafts reach an almost 'steady state' equilibrium. A supercell thunderstorm usually produces severe weather, ranging from large hail to violent tornados, and is the most common source of tornadic activity. Supercell thunderstorms can last for many hours and include a mesocyclone -- a whirling updraft in the southwest quadrant of the storm that produces a cloud dome above the anvil and is the usual spot for tornados to form. Strangely enough, mesocyclones can survive a supercell thunderstorm's dissipation as an upper-air vortex, and, if conditions are right, can serve as the nucleus around which another supercell develops a day or two later.

• Multicell clusters are groups of thunderstorms made by a cold front boundary, or, sometimes, from large-scale updrafts near mountains. The individual thunderstorms are brief, as with single cell thunderstorms, but the cluster can persist for much longer,with new storms forming as older ones melt away. Multicell clusters are not as powerful as supercells, and any tornados or hail they produce tend to be on the weak side.
• Squall lines are long-lasting, fast-moving lines of thunderstorms that are produced by cold fronts. They include heavy rain, strong straight-line winds, and sometimes weak tornadoes. They often develop into a bow echo storm, where wind pressure on the ground causes the line of storms to form a bow shape. When this happens, the conditions exist for a rare but powerful type of windstorm called a derecho to develop, in which straight line winds can exceed 100 miles per hour and cause incredible damage.

Special thunderstorms

Although most thunderstorms fall into one of the four types described above, there are occasional oddballs. Two of these special cases are dry thunderstorms and back-building thunderstorms.

• A dry thunderstorm is a thunderstorm without rain falling from its base -- or whose rain doesn't reach the ground because of evaporation. These thunderstorms happen mostly in dry climates such as deserts or semi-arid regions. They can cause fires with their lightning in dry grass or brush, which is then spread quickly by the thunderstorm winds.
• A back-building thunderstorm is a series of short-lived thunderstorms which form and dissipate over a single place for many hours without moving forward. They usually do not produce tornados or extremely severe weather, but they can and do result in huge rainfall totals over a specific area, leading to erosion or flooding, depending on the topography.