Clouds are sorted into low, middle and high levels. Each level is the range of heights where certain genera occur most often, and the levels overlap and change with latitude; cloud heights depend on latitude: in polar regions clouds are lower, and in the tropics clouds are higher. In the middle latitudes, high clouds are found roughly 5 to 13 kilometers (about 16,000 to 43,000 feet) up; some clouds do not stay in one level. Altostratus often extends higher, nimbostratus usually reaches into the other two levels, and cumulus and cumulonimbus can have tops that reach the middle and high levels; the system itself goes back to 1802, when Luke Howard first proposed sorting clouds into types, and the World Meteorological Organization later extended his three main types to 10 main groups called genera.

About 30 percent of the Sun's incoming energy is reflected back to space. Low clouds are much thicker than high cirrus clouds, so they let less sunlight reach Earth's surface and reflect much of it back to space; low, thick clouds mainly reflect sunlight and cool the surface. High, thin clouds let sunlight through but trap some of the infrared radiation rising from Earth and send it back down, warming the surface; high cirrus clouds are cold, so they radiate less energy to space than clear air would, an effect known as large cloud greenhouse forcing. Cumulonimbus clouds have a neutral effect on temperature, because their tall, cold tops trap heat and their thickness reflects sunlight, and the two effects almost balance; averaged over the whole globe, clouds cool Earth's surface: it is cooler than it would be if the atmosphere had no clouds.

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. In the thunderstorm cloud, rising air carries lighter, positively charged ice crystals upward, while heavier negatively charged hail falls toward the lower part of the storm; laboratory studies suggest graupel gains positive charge at temperatures a little colder than 32°F but gains negative charge at colder temperatures a little higher in the storm. Scientists know what conditions produce lightning, but they still debate exactly how a cloud builds up electrical charge and how lightning forms.

Negatively charged lightning channels do not grow continuously but in short steps. 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. In cloud-to-ground lightning the stroke comes from the sky down, but the bright part you see comes from the ground up; lightning can carry 100 million to 1 billion volts and billions of watts. Positive lightning bolts may be more dangerous than the more common negative ones because the peak current is often stronger, the flash lasts longer and the charge can be much greater; a bolt from the blue is a cloud-to-ground flash that leaves the side of a storm, travels a long way through clear air, then angles down, and these bolts have been documented several miles from the storm.

The lightning channel heats the air above 50,000°F in a few millionths of a second. The air has no time to expand, so it reaches very high pressure, then expands outward into the surrounding air. Thunder starts as a shock wave from the explosively expanding lightning channel, and after the first 10 yards it becomes an ordinary sound wave. Thunder seems to go on and on because each point along the channel makes its own shock and sound wave, so thunder is a pile-up of many sound waves from different parts of the flash.

Cumulonimbus is the only cloud type that can produce hail, thunder and lightning. Hailstones grow as strong updrafts hold them up inside a thundercloud, adding layer after layer of ice, and 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. Lightning does not need a thunderstorm: it can also occur in volcanic eruptions and heavy snowstorms; thundersnow is a thunderstorm in wintry weather with heavy snow, and at night the lightning looks brighter as it reflects off snowflakes while the snow dampens the thunder. As of November 2026, an estimated 16 million thunderstorms happen worldwide each year, and at any moment roughly 2,000 are in progress.

Clouds form when air rises and cools, and when water vapor in the air turns into tiny drops of liquid water around tiny specks such as dust, pollen or salt. Air can be lifted in several ways; 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. 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.

All of this matters most in one moment: when you hear thunder. 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. It is not possible to have thunder without lightning, and people who see lightning but hear no thunder are too far away to hear it; 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

• genera: main groups
• graupel: soft hail
• infrared: heat rays
• updrafts: rising air