Thunder is the sound generated due to lightning. The flash of lightning and the accompanying thunder occur around the same time. Nonetheless, we all are aware that lightning is seen first, followed by the sound of thunder after few seconds. This phenomenon occurs as light wave travels much faster than the sound waves. In order to understand thunder causes, one needs to know about lightning first. Well! Lightning is the electricity discharged during a thunderstorm. Lightning is caused due to build-up and discharge of electrical energy in the thunderstorm clouds, which average about 15,000 to 25,000 feet above sea level. Lightning usually occurs within clouds, between the cloud and air or between the ground and cloud. Based on the nature of lightning, it is of various types, such as in-cloud lightning, cloud-to-ground lightning, cloud-to-cloud lightning, sheet lightning, bead lightning, ribbon lightning, ball lightning and bolt from the blue. Cause of Thunder So, what causes thunder? A lot of theories has been discussed concerning causes and creation of thunder. In the third century BC, it was believed that thunder is caused due to collision of clouds. However, the most accepted theory was conceived in the 20th century. According to this theory, lightning causes thunder sound. The bolts of lightning that consist of electron streams are very hot, much hotter than the surface of the sun. It is estimated that the bolt has a temperature of 30,000 to 50,000 °F (i.e., 28,000 °C). When the high temperature bolt hits the surrounding air, sudden rise in temperature takes place. It brings along an instant expansion of heated air in the outward direction. Following this, elevated pressure is obvious due to compression of heated air. The rapid expansion of compressed air then sends out shock wave or vibration, which we hear as a sound of explosion. In short, thunder is caused due to rapid heating and cooling of the air, near the stroke of lightning. The sound intensity of thunder varies, depending upon the nature of lightning and the distance of the hearer from the origin of the sound. Thunder can be perceived clearly, if lightning occurs within a distance of 10 miles or more than that. You can estimate the distance from the strike; first count the interval between the flash of lightning and the sound of thunder in seconds, and then divide the interval by five. What you get is the rough distance in miles. In case, you are near to the flash of lightning, you will hear thunder as a sharp crack; whereas, if you are far from the lightning stroke, then the sound will seem like a low rumble. The rumbling sound is because of the echo, occurred due to reflection of sound waves from the buildings, trees and hillsides. Lightning and Thunder: Dangers As we have seen, thundering booms are created by lightning. Some cases of lightning may not be accompanied with thunder. There have been incidences, in which lightning occurs without the accompanying sound. The reason may be farther distance from the lightning bolt (more than 15 miles). But, one word of caution is, thunderstorm can be dangerous, depending on how close we are from the stroke of lightning. If the thunder sound is very loud, it can hurt our ears and cause internal injury. However, it is lightning that is more dangerous. On an average, about one hundred people die every year in the United States and several people suffer from lifelong disability due to lightning. Most of the lightning casualties occur, when people are caught outdoors during rainy weather. In case, you are outside during a thunderstorm, it is advisable to avoid open fields, beaches and lakes, and also stay away from tall trees. The safest way is to get inside your home or stay in the car. In the latter situation, you should not touch metal, as it is a good conductor of electricity.