About 30 percent of the Sun's incoming energy is reflected back to space. High, thin clouds let sunlight through but trap some of the infrared radiation rising from Earth and send it back down, warming the surface. 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. Averaged over the whole globe, clouds cool Earth's surface: it is cooler than it would be if the atmosphere had no clouds; cumulonimbus clouds have a neutral effect on temperature. Their tall, cold tops trap heat and their thickness reflects sunlight, and the two effects almost balance.

Lightning can carry 100 million to 1 billion volts and billions of watts. 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; 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. 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.

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. These bolts have been documented several miles from the storm; 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. At night the lightning looks brighter as it reflects off snowflakes, and the snow dampens the thunder; 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.

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; cumulonimbus clouds are thunderclouds, and a cumulonimbus cloud can be about 10 kilometers (33,000 feet or 6 miles) tall. Arcus clouds form when a cold downdraft from a cumulonimbus cloud reaches the ground and spreads out, pushing warm moist air upward. Cumulonimbus is the only cloud type that can produce hail, thunder and lightning; as of November 2026, an estimated 16 million thunderstorms happen worldwide each year, and at any moment roughly 2,000 are in progress. Far above these storm clouds, polar stratospheric clouds form in the stratosphere at 15 to 25 km (about 50,000 to 80,000 feet); noctilucent clouds form even higher, in the mesosphere, and are likely made of ice crystals.

On Jupiter, the stripes and swirls are clouds, made of ammonia and other chemicals. Beneath Jupiter's thick layer of ammonia clouds lies what some astrophysicists believe is a thin layer of water clouds; scientists think so because bursts of lightning have been spotted in Jupiter's atmosphere. Venus is a world of intense heat and crushing atmospheric pressure, with clouds of corrosive acid.

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. 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

• infrared: heat rays
• stratosphere: upper air layer
• astrophysicists: space scientists
• corrosive: eating away