Fog is essentially a cloud at ground level, and it cuts visibility to less than 1,000 meters (about 3,300 feet); radiation fog forms on calm, clear nights when the land cools, chilling the air near the ground until it cannot hold its moisture. It usually clears soon after sunrise as the ground warms. Advection fog forms when moist, warm air passes over a colder surface and is cooled; it is common at sea when moist tropical air moves over cooler water. Freezing fog is made of supercooled droplets that stay liquid below freezing. When they freeze onto surfaces they build feathery ice crystals called rime; dense fog is fog with visibility below 100 meters, and it can cause severe travel disruption. The thickest fogs tend to occur in industrial areas where more pollution particles let droplets grow.

Clouds are sorted into high, middle and low groups by how high they are in the sky, and 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. Far above them, polar stratospheric clouds form in the stratosphere at 15 to 25 km (about 50,000 to 80,000 feet); they need temperatures near -78°C (-108°F), so they only occur during cold winter months. Noctilucent clouds form very high up in the mesosphere, about 85,300 meters above Earth, and are likely made of ice crystals.

Some clouds are shaped by the air around them. Kelvin-Helmholtz clouds form when there is a difference in wind speed or direction between two air currents. Lenticular clouds seem to stay in one place because air keeps flowing through them. Air rises over a mountain and makes the cloud just past the top, and the cloud evaporates on the downwind side; altocumulus castellanus clouds are named for their resemblance to castle turrets and are often a warning of thunderstorms. Virga are trails of rain or ice that fall from the underside of a cloud but evaporate before they reach the ground, which happens when the falling drops pass through dry or warm air.

Cloud seeding puts tiny particles into clouds to act as artificial condensation nuclei, in the hope of influencing when and where rain or snow falls; clouds form when water vapor in the air turns into tiny drops of liquid water. Cloud drops form around tiny specks such as dust, pollen or salt. Clouds form when air rises and cools; cloud droplets are too small to fall as rain, so they float in the air. Rain falls when cloud droplets grow big and heavy enough to leave the cloud.

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. 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; 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. 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

• rime: feathery ice
• stratospheric: upper-air
• mesosphere: high air layer
• seeding: adding particles