Transient Sources - Environmental

Lightning is an electrical discharge in the air between clouds, between separate charge centers within the same cloud, or between a cloud and the earth. More discharges occur between or within clouds than from cloud to earth, but enough strokes terminate on the earth to constitute serious hazards for sensitive electronic equipment. The precise details of the charge separation mechanism are not completely understood; although there are a number of theories, more than one of which may be active in different locations within the cloud.

Lightning frequency varies throughout the world. It also varies from month to month and from year to year. For a long time now, the U.S. Weather Service has collected data on the number of days that observers (located throughout the United States) heard thunder. This information is published in map form, showing the total number of storm days expected annually, averaged over a number of years. Such a map is called an isokeraunic chart. The chart does not give information on the severity of lightning, but it supplies valuable data on the relative probability of lightning in different regions.