What Are Clouds?

Clouds — Elementary

What is a cloud?

A cloud is made of water drops or ice crystals.

They float in the sky.

There are many kinds of clouds.

Clouds are a big part of our weather.

How does a cloud form?

The sky can be full of water.

But you can not see most of it.

The drops are too small to see.

The water has turned into a gas.

The gas is called water vapor.

The vapor goes up high in the sky.

Up high, the air gets cooler.

Then the tiny drops start to stick to small bits.

The bits can be dust, ice, or sea salt.

Lots of tiny drops together make a cloud.

The drops are so light that they float in the air.

What happens when drops get big?

Sometimes drops join with more drops.

Then they turn into bigger drops.

Big drops fall to Earth.

We call the falling drops rain.

When the air is colder, the water may form snowflakes.

Freezing rain, sleet, or hail can also fall from clouds.

Some kinds of clouds

Cirrus clouds are high up. They look like feathers.

Cumulus clouds look like giant cotton balls in the sky.

Stratus clouds are low. They cover the sky like bed sheets.

Clouds get names from where they are in the sky.

Clouds also get names from their shape.

A low cloud can touch the ground.

Then it is called fog.

Why do clouds matter?

Clouds bring rain and snow.

In the day, clouds make shade. Shade can keep us cooler.

At night, clouds keep heat near the ground. The ground stays warmer.

NASA studies clouds to learn about Earth’s weather.

NASA uses satellites in space to study clouds.

Clouds — Middle

What is a cloud?

A cloud is a mass of water drops or ice crystals suspended in the atmosphere.

Clouds form when water condenses in the sky.

That condensation is what lets us see water vapor.

There are many different types of clouds.

Clouds are an important part of Earth’s weather and climate.

How does a cloud form, step by step?

Water evaporates from the ground, or it moves in from other areas.

Water vapor is always in the sky in some amount, but it is invisible.

NASA’s Climate Kids page explains it this way: liquid water gets extra energy, for example from the Sun, and changes into the invisible gas called water vapor.

Next, an area of air becomes cooler.

When the air cools enough, the water vapor condenses into liquid form.

At that point, NASA says the air is “saturated” with water vapor.

The vapor condenses around tiny bits called condensation nuclei.

NASA’s Grades 5–8 page names dust, ice, and sea salt as condensation nuclei.

NASA’s Climate Kids page adds more examples of tiny particles: dust, salt crystals from sea spray, bacteria, and even ash from volcanoes.

Those particles give the vapor a surface to change on, into liquid droplets or ice crystals.

A large group of those droplets or ice crystals is a cloud.

The Climate Kids page also names two change processes. Condensation is gas changing into a liquid. Deposition is gas changing directly into a solid, which is ice.

Temperature, wind, and other conditions where a cloud forms decide what type of cloud it will be.

How clouds are grouped

NASA groups clouds mainly by two things: location and shape.

High clouds form several kilometers up in the sky.

Low clouds generally form within a kilometer or two of Earth’s surface.

Middle-level clouds form between low and high clouds.

A low cloud can even form touching the ground. Then it is called fog.

The Climate Kids page makes the same point in its own words: if you have walked through fog, you have walked through a cloud.

Three common kinds to spot

Cirrus clouds are thin and wispy, and they often curve with the wind. NOAA SciJinks calls them delicate and feathery, made mostly of ice crystals, and says wind currents twist and spread the crystals into strands.

Cumulus clouds tend to be big and fluffy. NASA and NOAA SciJinks both compare them to giant cotton balls in the sky.

Stratus clouds form sheets of clouds that cover the sky. NOAA SciJinks says stratus clouds often look like thin, white sheets covering the whole sky, and that they seldom produce much rain or snow.

From cloud to rain, snow, and hail

Most of the water in clouds is in very small droplets.

Sometimes those droplets collect more water and turn into larger drops.

Then gravity causes them to fall through the air faster. The falling water drops are rain.

When it is colder, the water may form snowflakes instead.

Freezing rain and sleet happen when snow melts on the way to the ground but then gets colder again.

Hail falls during more severe weather. Air currents move rain and snow around in the sky, they get colder and turn into ice, and the ice chunks get bigger before they fall.

Why NASA studies clouds

Precipitation, like rain or snow, is one reason clouds are important.

At night, clouds reflect heat back to the surface and keep it warmer.

During the day, clouds can shade us from the Sun and keep Earth cooler.

Studying clouds helps NASA better understand Earth’s weather and climate.

NASA uses satellites in space, as well as computers, to study clouds.

NASA also studies clouds on other planets. Mars has clouds that are similar to some clouds on Earth. Other planets have clouds made of materials other than water. For example, Jupiter has clouds made of ammonia.