From Academic Kids
Thunderstorms form when significant condensation, resulting in the production of a wide range of water droplets and ice crystals, occurs in an atmosphere that is unstable and supports deep, rapid upward motion. This often occurs in the presence of three conditions: sufficient moisture accumulated in the lower atmosphere, reflected by high dewpoint temperatures; a significant fall in air temperature with increasing height, known as a steep lapse rate; and a force such as mechanical convergence along a cold front that will focus the lift.
Thunderstorms have had a lasting and powerful influence on early civilizations. Romans thought them to be battles waged by Jupiter, who hurled lightning bolts forged by Vulcan. Thunderstorms were associated with the Thunderbirds, held by Indians to be a servant of the Great Spirit. In more contemporary times, thunderstorms now have taken on the role of a curiosity. Every spring, storm chasers head to the Great Plains to explore the visual and scientific aspects of storms and tornadoes.
There are three main types of thunderstorm, single-cell, multi-cell, and supercell. Which type forms depends on the instability and relative wind conditions at different layers of the atmosphere ("wind shear"):
- Single-cell storms form when the atmosphere is unstable, but there is little or no wind shear, meaning precipitation falls back down through the updraft that led to it, cooling it and eventually killing it. These storms are short lived, and last for less than an hour after becoming strong enough to produce lightning. Days with suitable weather conditions often see the repeated forming and dissipation of such storms, leading them to be known as "pulse" storms.
- Multi-cell storms are groups of cells in different stages of development which have merged into a larger system. The cloud becomes divided into updraft and downdraft regions separated by a gust front. The gust front may extend for several miles ahead of the storm, bringing with it increases in wind speed and atmospheric pressure, decreases in temperature, and shifts in wind direction. The storm itself will have different portions sequentially going through the various thunderstorm stages. In many cases the immature cells develop along a line known as a flanking line, resulting in what is known as a line multicell.
- Supercell storms are large, severe steady-state storms which form when the wind changes speed or direction with height ("wind shear") producing a separate downdraft from the updraft (i.e., precipitation is not falling down through the updraft) and contain a strong, rotating updraft (a "mesocyclone"). These are the most damaging type of thunderstorm, and 30% produce tornadoes.
Multicell or squall line systems may form a meteorologically important feature known as mesoscale convective system (MCS) stretching for hundreds of miles. They are large enough to have a pronounced effect on the upper-level and surface weather pattern, and may influence forecasts over half of a continent. MCS systems are common in the Midwest region of the United States during the summer months and produce much of the region's important agricultural rainfall.
Geographic features (such as mountain ranges) or linear boundaries (such as warm or cold fronts) may create lines of thunderstorms which move across the landscape. A special case of this is the squall line, which usually occurs in the warm sector of a cyclone. The squall line is propelled by its own outflow, which reinforces continuous development of updrafts along the leading edge.
A severe thunderstorm is a thunderstorm with winds 58 mph (90 km/h) or greater, 3/4 inch (2 cm) or larger hail, or tornadoes. These storms also can contain deadly cloud-to-ground lightning, heavy downpours which can lead to localized flooding, gusty winds, and possible tornadoes.
Severe thunderstorms may occur as a Supercell thunderstorm or in a line with other non-severe storms.
A given cell of a thunderstorm goes through three stages: the cumulus stage, the mature stage, and the dissipation stage. This life cycle was identified in 1949 as the result of the U.S. Weather Bureau's landmark Thunderstorm Project.
In the cumulus stage of a thunderstorm cell, masses of moisture are pushed upwards. The trigger for this can be solar insolation heating the ground producing thermals, areas where two winds converge forcing air upwards, or where winds blow over areas of high ground. The moisture rapidly cools into liquid drops of water, which appears as cumulus clouds. As the water vapour condenses into liquid, latent heat is released which warms the air, causing it to become less dense than dry air, and so the air will tend to rise in an updraft due to the process of convection. This creates a low-pressure zone beneath the forming thunderstorm. In a typical thunderstorm, some 5×108 kg of water vapour are lifted and the amount of energy released when this condenses is about equal to the energy used by a city (US-2002) of 100,000 over a month.
In the mature stage, the warmed air continues to rise until it reaches existing air which is itself warmer, and the air can rise no further. Often this 'cap' is the tropopause. The air is instead forced to spread out, giving the storm a characteristic anvil shape. The resulting cloud is called cumulonimbus. The water droplets will coalesce into heavy droplets and freeze to become ice particles. As these fall they melt, to become rain. If the updraft is strong enough, the droplets are held aloft long enough to be so large that they do not melt completely as they fall and fall as hail. While updrafts are still present, the falling rain creates downdrafts as well. The presence of both updrafts and downdrafts during this stage can cause considerable internal turbulence in the storm system, which sometimes manifests as strong winds, severe lightning, and even tornadoes. If there is little wind shear, the storm will rapidly 'rain itself out', but if there is sufficient change in wind speed and/or direction the downdraft will be separated from the updraft, and the storm may become a supercell.
Finally, in the dissipation stage, updraft conditions no longer exist, and the storm is characterized largely by weak downdrafts. Because most of the moisture has precipitated out there is no longer sufficient moisture in the lower air to sustain the cycle.