Blue Sky Science: Why do clouds have water?

Asha Bahls


Why do clouds have water?

Not only do clouds have water, they consist almost entirely of water. And that water comes from the earth’s surface, including the ocean, lakes and streams and even the ground.
How does that water get from the ground all the way up into the sky? Warm air rises while cold air sinks, which means that the warm air full of water vapor at the surface rises up into the atmosphere.

It’s important to know that as we get higher up into the atmosphere, it gets colder and colder. This is why even in warm areas, mountain peaks often still have snow on them.

The atmosphere can be thought of as a bucket that can hold water vapor, and the size of this bucket is in part dependent on its temperature. So as the warm surface air full of water vapor rises, it gets colder. And as it gets colder, that bucket holding that water vapor shrinks until eventually that water vapor can no longer be held in the bucket.

When that happens, we have a process called condensation. If you’ve ever walked outside in the morning and noticed that the grass is wet, this is a very good example of condensation. The night before it got cold, and the water vapor in the air condensed onto those grass blades.

In the atmosphere, instead of grass blades, there are tiny particles of dust and sand and salt, and the water vapor condenses onto these tiny particles and makes water droplets. And you have millions and millions of these tiny water droplets that all form in the sky. When viewed from land, they look like a big, white, fluffy mass. The reason these clouds stay up high is these tiny droplets are so small that they aren’t affected enough by gravity to actually fall down to the ground.