Clouds look white because sunlight is white. Their droplets are much bigger than the particles that make the sky blue, so the droplets scatter all colors of sunlight almost equally and the light stays white. The bases of clouds often look gray because most light in a cloud is scattered back up or out to the sides, while the tops always get direct sunlight, so they stay white. At sunrise and sunset clouds can turn red or orange, because the low Sun's light travels through more air and more of the blue light is scattered away.

Clouds can also be shaped by moving air. 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. 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.

Arcus clouds form when a cold downdraft from a cumulonimbus cloud reaches the ground and spreads out, pushing warm moist air upward. A funnel cloud does not reach the ground. When it touches land it becomes a tornado, and when it touches water it becomes a waterspout. Clouds can also be thin and high; cirrostratus clouds can make a ring of light, called a halo, around the Sun or Moon, and thin cirrostratus clouds can warn that rain or snow may come within a day.

Meteorologists measure cloud cover in oktas, or eighths of the sky. A clear sky is zero oktas and a totally overcast sky is eight. A sky full of small cirrocumulus clouds can look like fish scales, so it is called a mackerel sky. Clouds are sorted into high, middle and low groups by how high they are in the sky.

Cumulonimbus is the only cloud type that can produce hail, thunder and lightning. If there is thunder, lightning or hail, the cloud is a cumulonimbus, not a nimbostratus. 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; as of November 2026, an estimated 16 million thunderstorms happen worldwide each year, and at any moment roughly 2,000 are in progress.

You may have heard that lightning never strikes the same place twice. In fact, 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 can usually be heard about 10 miles from a lightning strike. 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

• scatter: spread out
• virga: trails of rain
• meteorologists: weather scientists
• oktas: eighths of sky