Four different types of lightning between cloud and Earth have been identified, the ways by which these are initiatedbeing shown in Fig. 1. [25]. CG flashes initiated by downwardmoving negatively-charged leaders probably account for about 90% of the CG discharges worldwide (Fig. 1, category I ) , while less than 10% of lightning discharges are initiated by a downward-moving positive leaders (category 3). Ground-tocloud discharges are also initiated by leaders of either polarity that move upward from the Earth (categories 2 and 4). These upward-initiated flashes are relatively rare and usually occur from mountain peaks and tall man-made structures. The number of cloud-to-ground flashes per square kilometer per year has a maximum of 30 to 50, and a typical over-land value of 2-5. Between 10 and 20 million CG flashes strike the continental United States annually. Worldwide there are about 100 total (cloud and ground) flashes per second for a worldwide average flash density of about 6 km-2yr-' [26].