thunderstorm

an atmospheric phenomenon, during which powerful electrical discharges—lightning accompanied by thunder—arise in large cumulonimbus clouds and between the clouds and the earth. As a rule, intensive downpours and often hail occur during thunderstorms and the wind increases, often to squall intensity.

Thunderstorms are classified as air-mass and frontal storms, depending on the conditions under which they develop. Air-mass storms over a continent arise as a result of local heating of the air from the earth’s surface, which leads to the development of ascending currents of local convection and the formation of powerful cumulonimbus clouds. For this reason, air-mass thunderstorms over land develop mostly after midday. Over the ocean, thunderstorms arise as a result of the development of convection in cold air masses, which move toward low latitudes over the warm water surface, so that in this case a certain maximum over a 24-hour period occurs during the night hours.

Frontal thunderstorms arise on atmospheric fronts—that is, on the boundaries between warm and cold air masses— and do not have a regular 24-hour rhythm. Over land areas of the temperate zone thunderstorms are most frequent and intensive in summer, but in arid regions they are most frequent in spring and autumn. Winter thunderstorms occur in the temperate zone in exceptional circumstances—when especially severe cold fronts pass through. Over the oceans, which are warmer in winter than land, thunderstorms (mostly air-mass storms) predominate in winter.

Thunderstorms are very unevenly distributed over the earth. In the arctic they occur once in several years, in the temperate zone, in any given spot there are several dozen days a year with thunderstorms, and in certain regions that are world thunderstorm centers (Indonesia. Central America, etc.) the number of days a year with thunderstorms is in excess of 200.

Thunderstorms arise in powerful cumulus clouds with peaks in the region where temperatures are below -15° to -20° C—that is, at altitudes on the order of 7 to 15 km. These clouds consist of a mixture of drops (supercooled in high strata) and crystals. Strong ascending and descending streams at speeds reaching dozens of meters per second penetrate the storm cloud, which is several hundred or even thousands of cubic meters in volume. The weight of water and ice particles in this volume totals 10⁶ to 10⁷ tons. The potential energy stored up in a storm cloud is in excess of 10¹³to 10¹⁴ joules—that is, an amount equal to the energy of a thermonuclear megaton bomb. The electrical charges of a storm cloud that produce lightning are equal to 10–100 coulombs and are spaced over distances of from 1 to 2 up to 10 km; the electrical currents that create these charges reach 10–100 amperes. The electrical field intensity within a storm cloud is equal to (1–3)·10⁵ volts per m, and the effective electrical conductivity in the cloud is almost 100 times greater than in the surrounding atmosphere. The frequency of lightning in thunderstorms varies from several occurrences a second to one in several minutes. Under these conditions. 0.1–0.01 of the current flowing in storm clouds is expended on the current of lightning flashes.

The electrical characteristics of thunderstorms are due to the interaction of two groups of processes, which help to bring about the accumulation of electrical charges and to prevent them. Among the first are processes leading to the enlargement of particles in the storm cloud and the growth of electrical charges on them: increased supply of water vapor, increased speed of vertical air currents and increased capacity of clouds, and the presence of liquid and solid particles in the clouds. Among the second group is the electrical conductivity of the cloud. The relationship of the intensities of both groups of processes explains both the characteristic features of thunderstorms (seasonal rhythm, geographical distribution, structure, etc.) and their anomalies (the occurrence of thunderstorms in warm clouds, the development of unusually intensive thunderstorms, etc.).

During thunderstorms the state of the atmosphere is unstable; for this reason it is possible to expect the development of effective methods for controlling thunderstorms based on controlling one or another group of the processes that cause them.