Processes responsible for the separation of positive and negative electric charges in the atmosphere during storms, including the spectacular manifestation of this charge separation: lightning discharges. Cloud electrification is almost invariably associated with convective activity and with the formation of precipitation in the form of liquid water (rain) and ice particles (graupel and hail). The most vigorous convection and active lightning occurs in the summertime, when the energy source for convection, water vapor, is most prevalent. Winter snowstorms can also be strongly electrified, but they produce far less lightning than summer storms. Electrified storm clouds occasionally occur in complete isolation; more commonly they are found in convective clusters or in lines that may extend horizontally for hundreds of kilometers. See also Precipitation (meteorology).
Measurements of electric field at the ground and from instrumented balloons within thunderclouds have disclosed an electrostatic structure that appears to be fairly systematic throughout the world. The measurements show that the principal variations in charge occur in the vertical and are affected by the temperature of the cloud. The charge structure within a thundercloud is tripolar, with a region of dominant negative charge sandwiched between an upper region of positive charge and a subsidiary lower region of positive charge. In addition to the charge accumulations described within the cloud, electrical measurements disclose the existence of charge-screening layers at the upper cloud boundary and a layer of positive charge near the Earth's surface beneath the cloud. These secondary charge accumulations arising from charge migration outside the cloud are caused by electrostatic forces of attraction set up by the charges within the cloud.
Large differences of electric potential are associated with the distribution of charge maintained by active thunderclouds. These large differences in potential are maintained by charging currents that result from the motions of air and particles. The charging currents range from milliamperes in small clouds that are not producing lightning to several amperes for large storms with high rates of lightning. See also Cloud physics.
In response to charge separation within a thundercloud, the electric field increases to a value of approximately 106 V/m (300,000 V/ft) at which point dielectric breakdown occurs and lightning is initiated. Most lightning extends through the cloud at speeds of 10,000–100,000 m/s (22,000–220,000 mi/h). The peak temperature of lightning, which is a highly ionized plasma, may exceed 30,000 K (54,000°F). The acoustic disturbance caused by the sudden heating of the atmosphere by lightning is thunder. See also Lightning.
Meteorologists have shown a growing interest in the large-scale display of real-time lightning activity, since lightning is one of the most sensitive indicators of convective activity. Research has expanded into relationships between lightning characteristics and the meteorological evolution of different types of storms. The discovery of the sensitive dependence of local lightning activity on the temperature of surface air has led to research focused on the use of the global electrical circuit as a diagnostic for global temperature change. See also Atmospheric electricity; Storm detection.
