Thunder and Lightning Introduction
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Introduction When a black, gloomy cumulonimbus cloud unleashes its deluge of thunder, lightning, wind, and rain, the effect can be truly awe-inspiring. Big thunderclouds tower 10 miles (16 km) or more in the air and churn within them enough energy to light a small town for a year. A cloud of such phenomenal depth and power demands tremendously vigorous updrafts that typically occur along cold fronts or above areas of ground heated very strongly by hot sunshine. This is why, in the tropics, massive thunderstorms often break in the afternoon, after a morning’s sun has stirred up the air. Inland, in the temperature zone, a long spell of hot weather often ends in a tumult of thunder and lightning. Thunder Thunder is the sound made by lightning. Depending on the nature of the lightning and distance of the listener, thunder can range from a sharp, loud crack to a long, low rumble. The sudden increase in pressure and temperature from lightning produces rapid expansion of the air surrounding and within a bolt of lightning. In turn, this expansion of air creates a sonic shock wave which produces the sound of thunder. The distance of the lightning can be calculated by the listener based on the time interval from when the lightning is seen to when the sound is heard. Lightning Lightning striking Atlanta, Georgia Lightning is an electrical discharge accompanied by thunder, usually associated and produced by cumulonimbus clouds, but also occurring during volcanic eruptions or in dust storms. From this discharge of atmospheric electricity, a leader of a bolt of lightning can travel at speeds of 220,000 km/h (140,000 mph), and can reach temperatures approaching 30,000 °C (54,000 °F). There are some 16 million lightning storms in the world every year. Lightning causes ionization in the air through which it travels, leading to the formation of nitric oxide and ultimately, nitric acid, of benefit to plant life below. Lightning can also occur within the ash clouds from volcanic eruptions, or can be caused by violent forest fires which generate sufficient dust to create a static charge. Calculating distance A flash of lightning, followed after some seconds by a rumble of thunder is, for many people, the first illustration of the fact that sound travels significantly slower than light. Using this difference, one can estimate how far away the bolt of lightning is by timing the interval between seeing the flash and hearing thunder. The speed of sound in dry air is approximately 343 m/s or 1,127 feet per second or 768 mph (1,236 km/h) at 20°C (68 °F). However, this figure can only be used as an approximation of the speed of a thunder-clap, as you are unlikely to find dry air in a thunderstorm. The speed of light is high enough that it can be taken as infinite in this calculation because of the relatively small distance involved. Therefore, the lightning is approximately one kilometer distant for every 2.9 seconds that elapse between the visible flash and the first sound of thunder (or one mile for every 4.6 seconds). In the same five seconds, the light could have travelled the same distance as circling the globe 37 times. Thunder is seldom heard at distances over 20 kilometers (12 mi). A very bright flash of lightning and an almost simultaneous sharp "crack" of thunder, a thundercrack, therefore indicates that the lightning strike was very near. Questions: (Answer on a separate sheet!)
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