How hot is lightning, and what does that heat do?

01

What temperature does lightning reach?[1]

Around 30,000 kelvin at peak, roughly five times the temperature of the sun's visible surface. The channel is only a few centimeters wide, so the total heat energy is modest even though the temperature is extreme.

Temperature and heat are different quantities, and lightning is a good illustration of why that matters. The channel is extraordinarily hot but extremely thin and short-lived, so it does not deliver anything like the energy that number suggests. A campfire is far cooler and will heat a room better.

The temperature comes from resistive heating. Air is a poor conductor, so forcing tens of thousands of amps through a narrow channel dumps enormous power into a small volume in a few tens of microseconds.

02

Why does that heat make thunder?[1][2]

The air in the channel is heated so fast that it cannot expand smoothly. It bursts outward as a shock wave, which decays into the sound wave you hear as thunder. The bang is the air itself, not the electricity.

Because the heating is effectively instantaneous relative to how fast air can move, the pressure spike is enormous. Close to the channel that pulse is a genuine shock front travelling faster than sound, which is why a nearby strike is felt as much as heard.

Within a few meters the shock slows to the ordinary speed of sound, around 343 m/s at 20 °C. From that point on it behaves like any other sound wave, which is what makes the flash-to-thunder count work as a distance measure.

03

Does lightning turn sand into glass?[1]

Sometimes. Fulgurites are hollow glass tubes formed when a strike fuses silica sand along its path underground. They are real but uncommon, and far smaller and more fragile than the dramatic photographs suggest.

Formation needs the right conditions: dry, silica-rich sand and a strike with enough continuing current to sustain the heat. Most strikes into sand leave nothing at all. The ones that do produce a thin, branching, rough-surfaced tube, usually a few centimeters across and easily broken during excavation.

04

What does it do to trees and buildings?[1][3]

It flash-boils moisture. Sap inside a tree and water inside masonry turn to steam almost instantly, and the pressure splits the material apart. Most lightning damage to trees and stonework is steam pressure rather than burning.

That explains the characteristic long vertical scar down a struck tree, with bark thrown meters away. The current travels in the moist layer just under the bark, boils it, and the expanding steam blows the bark off from the inside.

The same mechanism damages chimneys and stone walls. Water in mortar joints or pores vaporises and pushes the material apart, which is why a struck chimney often loses bricks in a line rather than showing scorch marks.