Clouds can be white and wispy—or gray and lumpy. They might look like a puppy, a castle, or even a UFO! Did you know the shape of a cloud holds clues about what’s happening in the sky?

Clouds form when water evaporates, or turns into a gas called water vapor. This gas floats up into the atmosphere. There, cool temperatures turn water vapor into liquid droplets in a process called condensation. Tiny particles like dust help these droplets form. When many drops come together, they form a cloud! (See diagram below.)

Scientists group clouds into four main types. Cumulus clouds are tall and fluffy. Cirrus clouds look wispy and form high in the sky. Stratus clouds are wide and flat. Nimbus clouds look dark and create rain or snow.

“But some clouds are so rare, they get special names,” says Flor Vanessa Maciel. She’s a scientist who studies clouds at the University of California, Los Angeles. Read on to learn more about some of these strange-looking clouds and what they tell us about the weather.                     

These clouds form when warm air collides with cold air in the atmosphere. This temperature difference creates strong winds that disturb flat low-lying clouds. This cloud is one of the newest cloud types. It wasn’t documented by scientists until 2008!

These oval-shaped clouds get their name because they look like the curved glass in a camera lens. The clouds form when moist wind blows over a mountain or tall building. Some moisture condenses above the structure, forming a flying saucer-like cloud.

These bubble-like clouds are a sign that stormy weather may be near. Scientists think that they form when cold air pockets sink toward the ground, making downward-facing pouches. This is unlike most clouds we see in the sky, which puff upward with warm air.

These wavelike clouds form when the top cloud layer is warmer, lighter, and faster moving than the bottom one. (This is similar to how ocean waves form!) The clouds are named after two scientists who studied them, Lord Kelvin and Hermann von Helmholtz.