What Causes Thunder?
QUICK ANSWER
Thunder is caused by lightning rapidly heating the surrounding air to about 50,000°F (28,000°C) in microseconds. The intense heat causes the air to expand explosively, creating a shockwave that propagates outward through the atmosphere. We hear that shockwave as thunder, with sound traveling much slower than the light from the lightning.
Thunder is one of the most familiar sounds in nature, yet most people don't know exactly how it's made. The answer involves remarkable extremes of temperature and pressure happening on incredibly short timescales. Lightning is one of the hottest natural phenomena on Earth, briefly hotter than the sun's surface, and that intense heating creates the explosive expansion that produces thunder.
How does lightning make thunder?
Lightning makes thunder by heating air so suddenly that the air explodes outward. As a lightning bolt passes through the atmosphere, electrical current of tens of thousands of amperes flows through a narrow channel for a few millionths of a second. The current heats the channel of air to about 50,000°F (28,000°C) almost instantly. The air can't expand fast enough to release that energy normally, so it expands explosively, creating a shockwave that propagates outward. The shockwave is the sound we hear as thunder.
How hot does lightning get?
Lightning briefly heats the air in its channel to about 50,000°F (28,000°C), roughly five times hotter than the surface of the sun. This extreme temperature exists for only millionths of a second within a channel maybe an inch across, so the total energy is enormous but very localized. The light from lightning that we see is the superheated air glowing brightly. The brief heating is what causes the explosive air expansion that creates thunder, and it's why lightning bolts cause fires when they strike flammable materials.
Why does thunder sometimes rumble and sometimes crack?
Close lightning produces a sharp crack or bang because the entire shockwave reaches you within a fraction of a second. Distant lightning produces a long rumble because the sound from different parts of the long lightning bolt arrives at different times. A lightning bolt is typically several miles long, and parts closer to you reach your ears sooner than parts farther away. The result is a sound spread out over several seconds, which sounds like rolling thunder. Echoes off mountains or buildings can add additional rumbling.
How far away is lightning based on thunder?
You can estimate the distance to lightning by counting the seconds between the flash and the thunder. Sound travels about 1 mile every 5 seconds. So if you count 5 seconds, the lightning was about 1 mile away; 10 seconds means 2 miles; 15 seconds means 3 miles, and so on. The 30/30 rule for lightning safety: if you count less than 30 seconds between lightning and thunder, the storm is close enough to be dangerous, and you should seek shelter and stay there until 30 minutes after the last thunder.
How does lightning make thunder?
Lightning makes thunder by heating air so suddenly that the air explodes outward. As a lightning bolt passes through the atmosphere, electrical current of tens of thousands of amperes flows through a narrow channel for a few millionths of a second. The current heats the channel of air to about 50,000°F (28,000°C) almost instantly. The air can't expand fast enough to release that energy normally, so it expands explosively, creating a shockwave that propagates outward. The shockwave is the sound we hear as thunder.
How hot does lightning get?
Lightning briefly heats the air in its channel to about 50,000°F (28,000°C), roughly five times hotter than the surface of the sun. This extreme temperature exists for only millionths of a second within a channel maybe an inch across, so the total energy is enormous but very localized. The light from lightning that we see is the superheated air glowing brightly. The brief heating is what causes the explosive air expansion that creates thunder, and it's why lightning bolts cause fires when they strike flammable materials.
Why does thunder sometimes rumble and sometimes crack?
Close lightning produces a sharp crack or bang because the entire shockwave reaches you within a fraction of a second. Distant lightning produces a long rumble because the sound from different parts of the long lightning bolt arrives at different times. A lightning bolt is typically several miles long, and parts closer to you reach your ears sooner than parts farther away. The result is a sound spread out over several seconds, which sounds like rolling thunder. Echoes off mountains or buildings can add additional rumbling.
How far away is lightning based on thunder?
You can estimate the distance to lightning by counting the seconds between the flash and the thunder. Sound travels about 1 mile every 5 seconds. So if you count 5 seconds, the lightning was about 1 mile away; 10 seconds means 2 miles; 15 seconds means 3 miles, and so on. The 30/30 rule for lightning safety: if you count less than 30 seconds between lightning and thunder, the storm is close enough to be dangerous, and you should seek shelter and stay there until 30 minutes after the last thunder.
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