A thunderstorm needs three things: moisture, rising unstable air, and something to give the air a push. Thunderstorms have three stages: developing, mature and dissipating. In the developing stage a cumulus cloud is pushed upward by a rising column of air called an updraft; scientists think thunderstorm charge starts when small hail called graupel collides with smaller ice particles. The graupel gains one sign of charge and the smaller ice gains the other. The smaller ice particles rise faster in updrafts than the graupel, so the two kinds of charge separate: the charge on the ice collects above the charge on the graupel. Scientists know what conditions produce lightning, but they still debate exactly how a cloud builds up electrical charge and how lightning forms.
When a downward lightning channel comes within about a hundred yards of the ground, trees, bushes and buildings send up sparks to meet it. When one connects, a huge current surges down the channel. Lightning heats the air around it to about five times hotter than the surface of the Sun; lightning often strikes the same place again and again, especially a tall, pointy, isolated object. As of November 2026, the Empire State Building is hit about 23 times a year on average. Thunder starts as a shock wave from the explosively expanding lightning channel. After the first 10 yards it becomes an ordinary sound wave; thunder rumbles because you hear the nearest part of the lightning channel first and then the sound from parts farther and farther away.
Hail is solid balls of ice that fall from clouds. Hailstones grow as strong updrafts hold them up inside a thundercloud, adding layer after layer of ice. They fall when they get too heavy for the updraft; hailstones range from about 5 mm to over 50 mm across, though most are smaller than 25 mm. Cumulonimbus clouds are thunderclouds, and a cumulonimbus cloud can be about 10 kilometers (33,000 feet or 6 miles) tall. A tall thundercloud can flatten at the top into the shape of an anvil. Cumulonimbus is the only cloud type that can produce hail, thunder and lightning.
Rain does not always begin as liquid water. Some rain starts as ice. In the Bergeron-Findeisen process, ice crystals grow quickly at the expense of the water vapor in a cloud, then fall as snow or melt into rain; rain and snow fall out of clouds. The water droplets in a cloud are only about a hundredth of a millimeter across, and cloud droplets are too small to fall as rain, so they float in the air. It takes millions of cloud droplets to make a single raindrop, and rain falls when cloud droplets grow big and heavy enough to leave the cloud.
As of November 2026, one study of nearly a decade of satellite data estimated that about 67 percent of Earth's surface is typically covered by clouds. Clouds form when air rises and cools. The Sun heats the ground, the warm air above it rises, and cumulus clouds can form; air pushed up a mountain cools and can make clouds. At a weather front, warm air rises over cold air, and clouds form. Clouds also form where winds meet at the center of a low-pressure system and are forced to go up. Meteorologists measure cloud cover in oktas, or eighths of the sky; a clear sky is zero oktas and a totally overcast sky is eight.
It is not possible to have thunder without lightning. People who see lightning but hear no thunder are too far away to hear it. You see lightning before you hear its thunder, because light travels faster than sound; to find how far away lightning is, count the seconds between the flash and the thunder and divide by 5 to get miles. Thunder can usually be heard about 10 miles from a lightning strike, and since lightning can strike outward 10 miles from a storm, hearing thunder means you are likely within striking distance. After the last sound of thunder, stay in a safe place for at least 30 minutes. When you hear thunder, go indoors, into a building or a car with a roof.
Words to know
- graupel: soft hail
- updraft: rising air
- dissipating: fading away
- oktas: eighths of sky
