distinct boundary between the cool air and the warm, humid air in which the storm developed. The passage of such a gust front is easily recognized as the wind speed increases and the air temperature suddenly drops. Over a five-minute period, a cooling of more than 5 °C (9 °F) is not unusual, and cooling twice as great is not unknown.
In extreme circumstances, the gust front produced by a downburst may reach 50 metres (about 160 feet) per second or more and do extensive damage to property and vegetation. Severe winds occur most often when organized lines of thunderstorms develop in an environment where the middle-level winds are very strong. Under such conditions, people might think the winds were caused by a tornado. If a funnel cloud is not observed, the character of the wind damage can indicate the source. Tornadoes blow debris in a tight circular pattern, whereas the air from a thunderstorm outflow pushes it mostly in one direction.
By the time the cool air arrives, rain usually is reaching the surface. Sometimes all the raindrops evaporate while falling, and the result is a dry thunderstorm. At the other extreme, severe multiple-cell and supercell storms can produce torrential rain and hail and cause flash floods.
In small thunderstorms, peak five-minute rainfall rates can exceed 120 mm (4.7 inches) per hour, but most rainfalls are about one-tenth this amount. The average thunderstorm produces about 2,000 metric tons (220,000 short tons) of rain, but large storms can produce 10 times more rainfall. Large, organized storms that are associated with mesoscale convective systems can generate 1010 to 1012 kg of rainfall.
Within a single thunderstorm, there are updrafts and downdrafts and a variety of cloud particles and precipitation. Measurements show that thunderclouds in different geographic locations tend to produce an excess negative charge at altitudes where the ambient air temperature is between about −5 and −15 °C (23 to 5 °F). Positive charge accumulates at both higher and lower altitudes. The result is a division of charge across space that creates a high electric field and the possibility of significant electrical activity.
Many mechanisms have been proposed to explain the overall electrical structure of a thunderstorm, and cloud electrification is an active area of research. A leading hypothesis is that if the larger and heavier cloud particles charge preferentially with a negative polarity, and the smaller and lighter particles acquire a positive polarity, then the separation between positive and negative regions occurs simply because the larger particles fall faster than the lighter cloud constituents. Such a mechanism is generally consistent with laboratory studies that show electrical charging when soft hail, or graupel particles (porous amalgamations of frozen water droplets), collide with ice crystals in the presence of supercooled water droplets. The amount and polarity of the graupel charges depend on the ambient air temperature and on the liquid water content of the cloud, as well as on the ice crystal size, the velocity of the collision, and other factors. Other mechanisms of electrification are also possible.
When the accumulated electric charges in a thunderstorm become sufficiently large, lightning discharges take place between opposite charge regions, between charged regions and the ground, or from a charged region to the neutral atmosphere. In a typical thunderstorm, roughly two-thirds of all discharges occur within the cloud, from cloud to cloud, or from cloud to air. The rest are between the cloud and ground.
In recent years it has become clear that lightning can be artificially initiated, or triggered, in clouds that would not normally produce natural lightning discharges. Lightning can be triggered by a mountain or a tall structure when a thunderstorm is overhead and there is a high electric field in the vicinity or when an aircraft or large rocket flies into a high-field environment.
Global lightning distribution
Data from Earth-orbiting satellites show that, on average, about 80 percent of