A spark several times hotter than the Sun’s surface

A flash, a pause, then a rumble that rolls across the sky. Lightning is one of the most familiar sights in nature — and one of the most extreme. In a few thousandths of a second, it heats a narrow channel of air to temperatures hotter than the surface of the Sun.

A giant spark

The U.S. National Severe Storms Laboratory (NSSL) defines lightning simply: it is a giant spark of electricity in the atmosphere, between clouds, the air or the ground.

The spark happens because a thunderstorm separates electric charge. Over time, regions of positive and negative charge build up in different parts of the cloud. When the difference becomes large enough, the air can no longer keep the charges apart and a discharge rushes through it.

How exactly the charges separate is still debated. NSSL describes two main ideas. In one, collisions between particles of different sizes inside the cloud transfer charge, with heavier particles carrying negative charge down to the lower part of the cloud. In the other, upward air currents carry positive charge up while downdrafts carry negative charge down. Scientists continue to study the details.

Not all lightning reaches the ground

Most of us think of lightning as a bolt striking the ground, but much of it never leaves the clouds. Lightning can occur between opposite charges inside a single thundercloud — intra-cloud lightning. NSSL notes that there are roughly five to ten times as many cloud flashes as flashes to the ground.

Even so, ground strikes are very common. In the United States alone, NSSL reports an average of about 20 million cloud-to-ground flashes detected every year.

Hotter than the Sun’s surface

The energy in a lightning channel heats the surrounding air enormously. According to NSSL, lightning heats the air to anywhere from about 18,000 °F up to 60,000 °F — roughly 10,000 to 33,000 °C. For comparison, the visible surface of the Sun is around 5,500 °C. The flash itself happens in just a few thousandths of a second.

Where thunder comes from

That sudden heating is what makes thunder. As NSSL explains, the heat causes the air around the lightning channel to expand very quickly. That rapid expansion creates a sound wave — the crack and rumble we hear as thunder.

Because light travels much faster than sound, we see the flash before we hear the thunder. Thunder can be heard up to about 25 miles (40 kilometres) from the lightning. When you see lightning but hear no thunder, the storm is usually just too far away. NSSL notes that this is sometimes called "heat lightning", because it is most often seen on summer evenings — but it is ordinary lightning from a distant storm.

The same place twice

A popular saying claims that lightning never strikes the same place twice. NSSL is clear that this is wrong: lightning does hit the same spot, or almost the same spot, more than once. Tall, isolated objects are especially likely to be struck again and again.

Staying safe

The distance at which you can hear thunder is also a useful safety rule. If you can hear thunder, you are close enough to the storm to be struck. The safest places are inside a substantial building or a hard-topped vehicle — not under a tree, and not in open fields or on the water.

Why it matters

Lightning connects the smallest particles in a cloud to the largest storms on the planet. It is beautiful to watch from a safe place, and worth understanding, because the same physics that makes it spectacular also makes it dangerous.