Clouds and Rain

Core Idea

Clouds form when water vapor rises, cools, and condenses into tiny droplets around microscopic particles in the air. Different types of clouds bring different weather. Fluffy white cumulus clouds usually mean fair weather. Flat gray stratus clouds often bring steady drizzle. Tall dark cumulonimbus clouds bring thunderstorms with heavy rain. Rain falls when cloud droplets grow large enough to overcome the air's resistance and drop to the ground.

How It's Best Learned

Go outside and observe clouds, naming basic types (cumulus, stratus, cirrus). Keep a cloud journal alongside a weather journal and look for connections between cloud types and subsequent weather. Use cotton balls at different heights and densities on blue paper to model cloud types. Make a cloud in a jar using hot water, ice, and hairspray (as condensation nuclei).

Common Misconceptions

• All clouds produce rain. Most clouds are made of droplets far too small to fall -- only certain cloud types and conditions produce precipitation.
• Clouds are empty space filled with water. Clouds have physical structure -- they are collections of billions of tiny water droplets or ice crystals.
• Dark clouds are dirty. Dark clouds appear dark because they are thick and block more sunlight from passing through, not because they contain dirt.

Explainer

You already know that clouds are made of tiny water droplets and that rain falls from clouds. Now let's look more closely at how clouds form and why only some of them produce rain.

Clouds form through a three-step process. First, the sun heats water on Earth's surface, causing some of it to evaporate into invisible water vapor. Second, the warm, moist air rises because warm air is lighter than cool air. Third, as the air rises higher, it cools down, and the water vapor condenses -- turns back into tiny liquid droplets -- around microscopic particles (dust, pollen, salt) floating in the atmosphere. Those billions of tiny droplets together make a visible cloud.

Not all clouds look the same, and the way a cloud looks tells you a lot about the weather it might bring. Cumulus clouds are the puffy, white clouds that look like cotton balls scattered across a blue sky. They usually form on sunny days and mean fair weather. Stratus clouds are flat, gray layers that cover the whole sky like a blanket. They often bring light, steady drizzle or an overcast day. Cirrus clouds are thin, wispy streaks high in the sky, made of ice crystals -- they usually mean fair weather now but may signal a change coming. Cumulonimbus clouds are the giants -- towering columns of cloud that reach high into the atmosphere. They are dark at the bottom because they are so thick that sunlight cannot get through. These are the thunderstorm clouds that bring heavy rain, lightning, and sometimes hail.

Most clouds never produce rain. The droplets inside them are incredibly small -- about 100 times thinner than a human hair. Droplets this tiny are so light that the air holds them up. Rain only happens when droplets collide with each other and merge into larger, heavier drops. When a drop gets big enough -- roughly a million times the volume of a cloud droplet -- it is too heavy for the air to support, and it falls. That is rain.