Thunder and lightning accompany thunderstorms, volcanoes, and heat waves, but have you ever wondered what causes thunder and lightning. The short answer is that an unequal distribution of electrical charges cause a static discharge, which we call lightning, while thunder is the sound that results from the rapid expansion and contraction of air around a lightning strike.
- Lightning causes thunder.
- In a thunderstorm, lightning occurs when an electrical discharge happens within or between clouds or between a cloud and the ground. Charged dust particles work like charged ice particles in volcanic eruptions and heat lightning.
- While the two events occur simultaneously, you see lightning before you hear thunder because the speed of light is much faster than the speed of sound.
How Lightning Works
Lightning in thunderstorms comes from cumulonimbus clouds. The average duration of a lightning strike is 0.52 seconds, but it consists of a series of shorter strokes each lasting between 60 and 70 microseconds. On average, a lightning strike releases a gigajoule of energy and heats air to temperatures five times hotter than the surface of the Sun.
Positive and negative electrical charges (ice crystals that have lost electrons and hail/graupel that has gained electrons) form pools within cumulonimbus clouds. Lighter ice crystals rise, while heavier hail falls. When the two forms of ice collide, they transfer electrical charge. The upper part of the cloud (the anvil) has a high concentration of positive charge, while the lower portion has a high concentration of negative charge. The bottom of the cloud has a small buildup of positive charge from rain precipitation at a warmer temperature. Positive charges from the air and ground feel attraction to the lower part of the cloud, while negative charges feel repulsion toward the lower part of the cloud and attraction to the upper portion.
Eventually, there is a large enough accumulation of charge that the attraction between positive and negative charges overcomes the insulating effect of air. Initially, a channel of ionized air called a “leader” forms between opposite regions of charge. Leaders often split into branching shapes (Lichtenberg figures) or form steps. The leader is visible in photographs, but the brightest part of a lightning bolt is the return stroke. This occurs when the leader completes a conductive path for charge, the resistance drops, and electrons travel the path at up to a third the speed of light.
There are three path options for lightning in a thunderstorm:
- Cloud to ground lighting forms between the cloud and the surface.
- Cloud to cloud lightning occurs between two clouds.
- Intracloud lightning occurs within two points of a single cloud.
Usually in cloud to ground lightning, negative lightning occurs. This means the electrons travel from the cloud toward the ground. Once a strike occurs, there are several strokes. So, lightning usually strikes the same place twice because there is less resistance. About 5% of the time, positive lightning occurs. In positive lightning, electrons travel from the ground toward the cloud. (It’s not a scenario where protons or positive ions move.) Positive lightning typically connects the ground to the anvil portion of a thunderhead.
How Thunder Works
Thunder is the sound of the shockwave produced by the rapid heating and expansion of air, followed by cooling and flowing into the vacuum formed by the expansion. While it isn’t a perfect analogy, consider the loud sound you hear from popping a balloon, as pressurized air rushes out. The shockwave also resembles that of an explosion.
Thunder is loud. Close to its source, it is about 165 to 180 decibels (dB), although it can exceed 200 dB.
If you listen carefully, there are different types of thunder:
- Claps or Thunderclaps: Claps are very loud, last between 0.2 and 2 seconds, and contain higher pitches of sound.
- Peals: A peal of thunder changes irregularly in loudness and pitch.
- Roll: A roll of thunder has a regular variation of loudness and pitch.
- Rumbles: As the name suggests, rumbles are low pitch and not very loud, but they last a long time (up to 30 seconds).
A few different factors play into the sound of thunder, including the presence or absence of a temperature inversion and whether the thunder comes from the first lightning strike (louder) or return strokes (quieter).
Seeing Lightning Before Hearing Thunder
You see lightning before you hear thunder. The speed of light in air is much greater than the speed of sound. If you are very close to a lightning strike, you see lightning, hear the “snick” sound of the electrical discharge, and then both hear and feel the booming shockwave of the thunder.
While you can’t reliably tell the direction of lightning based on the sound of thunder, the time between seeing lightning and hearing thunder provides a good estimate of distance from the lightning strike. All you do is count the number of seconds between seeing lightning and hearing thunder. Divide this number by 5 and you have an approximate distance in miles to the lightning strike.
References
- Graneau, P. (1989). “The cause of thunder”. J. Phys. D: Appl. Phys. 22 (8): 1083–1094. doi:10.1088/0022-3727/22/8/012
- Jennings, S. G.; Latham, J. (1972). “The charging of water drops falling and colliding in an electric field”. Archiv für Meteorologie, Geophysik und Bioklimatologie, Serie A. Springer Science and Business Media LLC. 21 (2–3): 299–306. doi:10.1007/bf02247978
- Rakov, Vladimir A.; Uman, Martin A. (2007). Lightning: Physics and Effects. Cambridge, England: Cambridge University Press. ISBN 978-0-521-03541-5.