Images Videos Interactive Structure of a thunderstormWhen the atmosphere becomes unstable enough to form large, powerful updrafts and downdrafts (as indicated by the red and blue arrows), a towering thundercloud is built up. At times the updrafts are strong enough to extend the top of the cloud into the tropopause, the boundary between the troposphere (or lowest layer of the atmosphere) and the stratosphere.Click on the icons along the left-hand side of the figure to view illustrations of other phenomena associated with thunderstorms. World patterns of thunderstorm frequencyThunderstorms occur most often in the tropical latitudes over land, where the air is most likely to heat quickly and form strong updrafts. Rain and lightning during a thunderstorm in Arizona. Cloud-to-ground lightning discharge in a field from a cumulonimbus cloud. Hail-producing thunderstorm(Left) A hailstone can travel through much of the height of the storm during its development and may make multiple vertical loops. (Right) Most hailstones are formed by accretion around a nucleus (spherical embryo). Peculiarly shaped hailstones are generally the product of multiple stones fusing together. (Left) Tornadic thunderstormThe rotating updraft that produces the tornado extends high into the main body of the cloud.(Right) Anatomy of a tornadoAir feeds into the base of a tornado and meets the tornado’s central downflow. These flows mix and spiral upward around the central axis. The tornado’s diameter can be much greater than that of the visible condensation funnel. At times the tornado may be hidden by a shroud of debris lifted from the ground. Thunderstorm microburst(Left) The air that forms the microburst is initially “dammed” aloft by the strength of the storm’s updraft then cascades downward in a high-velocity, narrow column (less than 4 km, or 2.5 miles, in diameter). (Right, inset) Microbursts are very dangerous to aircraft and can create great damage on the ground. In the absence of observers, microburst damage can often be distinguished from that of a tornado by the presence of a “starburst” pattern of destruction radiating from a central point. Evolution of a gust front(Left) During a thunderstorm a large column of cold air, originating high in the thundercloud, can descend rapidly to form a gust front. (Right, inset) Fed by the main downdraft, the gust front flows in a turbulent layer along the ground and can extend far from the main body of the storm. A gust front is often felt by observers as a sudden cool wind arriving well in advance of a storm. When the electrical charges become sufficiently separated in a thundercloud, with some regions acquiring a negative charge and others a positive, a discharge of lightning becomes likely. About one-third of lightning flashes travel from the cloud to the ground; most of these originate in negatively charged regions of the cloud. Lightning flash striking a tree at a distance of 60 metres (200 feet) from the camera. Lightning discharge triggered by the presence of a tall tower atop Mount San Salvatore, near Lugano, Switzerland. Cloud-to-ground lightning discharge showing a bright main channel and secondary branches. (Top) As shown in the chart, the elapsed time between seeing a flash of lightning and hearing the thunder is roughly three seconds for each kilometre, or five seconds for each mile. (Bottom) An observer’s relative distance from the main lightning channel and its secondary branches determines whether thunder is heard to start with a sudden clap or a softer rumbling. Lightning rod protection system for a residential buildingThe flow of electricity from a lightning strike is channeled harmlessly around the outside of the building and into the ground. Lightning rod types(Left top) Vertical rods or masts up to 15 metres in height create lightning protection zones that extend in a 45° cone from the rod’s tip. (Left bottom) Connecting two rods with a wire extends the zone of protection. (Right) Towers taller than 30 metres provide protection for an area 30 metres high and 60 metres wide. The protected zone is in the shape of an inverted funnel with inward-curving sides. Towers between 15 and 30 metres high create protected zones of similar shape but with height and width equal to tower height. Formation of a thunderstorm. The density of lightning flashes worldwide in a typical yearAs demonstrated by the animation, lightning activity year-round is greatest over continental areas in the tropics, particularly in South America, Africa, and Australasia. Lightning strikes in the higher latitudes rise during the spring and summer months (May–September in the Northern Hemisphere and November–March in the Southern Hemisphere). Time lapse of clouds followed by a thunderstorm. An interactive diagram depicting thunderstorm structure, gust front evolution, microburst development, hail production, and tornado formation.