onexistent. In these regions the air near the surface is cold and the atmosphere is generally stable. There are also few thunderstorms in regions that are dominated by semipermanent high-pressure centres, such as southern California. In these regions air from higher altitudes is descending and warming, which lowers the relative humidity and causes stable stratification of the lower atmosphere. As a result, thunderstorm development is inhibited.
Lightning distribution in the United States
Every year, most of the United States experiences at least two cloud-to-ground strikes per square kilometre (about five per square mile). Most of the interior of the country east of the Rocky Mountains has four or more strikes per square kilometre (about 10 discharges per square mile). Summer thunderstorms are frequent in northern Mexico and the states of Arizona, New Mexico, and Colorado when warm, humid air is forced to rise by mountainous terrain.
Maximum flash densities are found along the Gulf Coast and Florida peninsula, where over a year’s time, values exceeding 10 strikes per square kilometre (25 strikes per square mile) have been measured. More than 20 million cloud-to-ground flashes strike the United States annually, and lightning is clearly among the country’s most severe weather hazards.
A typical flash of cloud-to-ground lightning is initiated by electrical breakdown between the small positive charge region near the base of the cloud and the negative charge region in the middle of the cloud. The preliminary breakdown creates channels of air that have undergone partial ionization—the conversion of neutral atoms and molecules to electrically charged ones.
On timescales measured in fractions of a second, high-speed cameras can record luminous events in the flash. Initially, a faint luminous process descends in a downward-branching pattern in regular distinct steps, typically 30 metres (100 feet) in length, though they can range from 10 to 100 metres (33 to 330 feet). The time interval between steps ranges from 10 to 50 microseconds (millionths of a second). Carrying currents on the order of hundreds to thousands of amperes, the stepped leader propagates toward the ground at an average velocity of 1.5 × 105 metres per second, or about one two-thousandth the speed of light. It is called a stepped leader because of its downward-moving “stepped” pulses of luminosity. Diameter estimates for the stepped leader range from a few centimetres to a few metres. The current-carrying core has a diameter on the order of 1 or 2 cm (0.4 or 0.8 inch), and photographic measurements indicate that a corona sheath of electric charge with a diameter of 1 to 10 metres (3 to 33 feet) surrounds the core.
As the stepped leader nears the ground, approximately five coulombs of charge have been deposited along the channel, inducing an opposite charge on the ground and increasing the electric field between the leader and the point to be struck. An upward discharge starts at the ground, church steeple, house, or other object, and rises to meet the stepped leader about 15 to 50 metres (50 to 160 feet) above the surface. At this moment of junction the cloud is short-circuited to the ground and a highly luminous return stroke of high current occurs. It is this return stroke, rather than the stepped leader, that is perceived as lightning because it is so much brighter and follows so quickly after the stepped leader. Portions of the stepped leader that have not reached the ground become the branches of the return stroke, and charge on the branches flows into the main channel. The five coulombs of charge typically deposited along the stepped leader flow to ground in a few hundred microseconds and produce peak currents that are usually on the order of 30,000 amperes but may range from a few thousand to over 200,000 amperes. Peak temperatures in the channel are on the order of 30,000 °C (50,000 °F), about five times hotter than the surface of the Sun. Because the junction process occurs near the ground, the time to peak current measured at the ground is typically onl