Thunderstorms can happen anywhere in the world, with the greatest frequency in tropical Rainforest areas, where they may occur nearly daily. In temperate regions, they are most frequent in summer, though can occur in cold fronts at any time of year.
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 Native Americans 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 of the United States and Canada's prairie provinces to explore the visual and scientific aspects of storms and tornadoes.
Multicell or squall line systems may form a meteorologically important feature known as mesoscale convective system (MCS) stretching for hundreds of miles or a "mesoscale convective complex". 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.
Severe thunderstorms may occur as a Supercell thunderstorm or in a line with other non-severe storms.
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 incus. 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.