Lightning is caused by the buildup of electrostatic charge in clouds. Scientists have found that the upper portion of the cloud builds up a positive charge and the lower portion a negative charge. The negative charges are concentrated in one or several clusters within the cloud as depicted in Figure 11.1. If the charges are dense enough, the negative charges may leap to the positive side of another cloud, or it may leap to the ground in the form of a lightning strike. The lightning strike is formed when the negative leader from the cloud travels toward earth while the highest grounded conductive object generates a positive leader that travels toward the negative leader leaping from the cloud. The two leaders travel at a speed of about 60 km/s and they meet at approximately one-third of the height of the cloud, creating a lightning channel which is a high-current ¢ow. Each lightning strike is packed with 10-100 GJ of energy. Most of this energy is converted into thunder noise, ¢ashing light, and heat. The channel’s temperature can be as high as 30,000°C. Although the energy reaching the earth is a small fraction of the total lightning energy, it is suf“cient to cause “res, make considerable damage to structures and equipment, disturb communications, and cause physical harm to living beings.