Some clouds are flat. Others are puffy. Some are pretty and turn orange or red at sunrise or sunset. Others signal stormy weather and can produce precipitation, such as rain, hail or snow.
Meteorologists, or weather forecasters, spend lots of time predicting clouds. That’s because clouds are behind all storm systems — from tornadoes and hurricanes to rain and snowstorms.
Technically speaking, a cloud is nothing more than a clump of aerosols — small solid or liquid particles suspended in the air. In some clouds, those aerosols can be bits of wildfire smoke or even desert dust.
But most clouds are made of water. A typical cloud droplet is only 20 micrometers (0.00008 inch) in diameter. That’s less than a thousandth the size of an average raindrop. Since cloud droplets are so tiny, they are very light. And that’s part of why clouds float.
These water droplets start to form through evaporation. Evaporation occurs when liquid water molecules become energized and change phase into a gas. Energy from the sun heats Earth’s surface — including water molecules in the ground as well as in oceans, lakes and rivers. Some water molecules become warmer, and more energized, than others. The most energized ones become a gas, known as water vapor.
Most air is made of N2 — or two nitrogen atoms clumped together. A molecule of water vapor weighs less than this nitrogen molecule. That’s also partly why clouds float.
Why can we see clouds?
We can’t see water vapor on its own. Clouds become visible when water vapor condenses, or transforms back into a liquid. That happens when water vapor rises high enough in the atmosphere that it cools.
Gravity pulls those droplets toward the ground. But they’re so tiny that warm rising air keeps them suspended aloft. And even if a single cloud weighs tens of millions of kilograms (pounds), the air outside the cloud is a bit heavier still. So the cloud floats.
Sometimes, water vapor rises extra high. At 12,200 meters (40,000 feet) above the ground, the average temperature is –57° Celsius (–71° Fahrenheit). Here, it’s too cold for water droplets to form. Any water turns into tiny ice crystals.
That’s why clouds at high altitudes aren’t puffy. They aren’t made of liquid droplets. Made of ice crystals, they’re wispy-looking.
Different types of clouds
Some clouds form near the ground. Fog is a cloud that forms on the ground.
“When we see fog, that is simply a cloud at the surface,” wrote Brandon Richards, a meteorologist for Spectrum News Texas in Austin, in a text message. “Fog happens when the air is saturated.” That means the air is holding as much water as it can. The extra water condenses, forming a cloud.
Fog is one type of stratus cloud. The word stratus comes from the Latin word for “layer.” Other stratus clouds may form a few thousand meters (feet) above the ground. They’re usually pretty flat.
In summer, the sky might host puffy clouds. It can be fun to look for shapes in these cumulus clouds. Such clouds usually form a couple kilometers (miles) above the ground. They come together when pockets of warm air rise.
The highest types of common clouds are cirrus. They form 9,100 meters (30,000 feet) or more above the ground. At that height, they’re made of ice crystals, which gives them a wispy, hairlike appearance.
Different types of clouds can be present at the same time. And some clouds are actually a combination of different cloud types. Cirrostratus clouds are wispy like cirrus clouds, but flat and layered like stratus clouds. Stratocumulus clouds are layered but a bit puffy.
Rare clouds
Some clouds form in weird ways. Consider noctilucent clouds. The term noctilucent comes from Latin words that mean “light at night.” Earth’s highest clouds, these form at heights of 76 to 85 kilometers (47 to 53 miles).
There’s little water vapor at such altitudes. So how can clouds form there? Meteor smoke!
Meteors burn up in this part of the atmosphere, leaving behind smoke particles. Water droplets can glom onto these particles and turn into ice crystals. And being so high, these clouds can catch sunlight even after the sun has set (at ground level). These clouds shimmer, like blue, glow-in-the-dark curtains.
Another type of strange cloud is the pyrocumulonimbus (PY-roh-KEW-mew-lo-NIM-bus), or smoke cloud. Wildfires can release lots of smoke. As smoke and hot gases rise, they can form a thundercloud. From these clouds, wildfires sometimes produce their own thunderstorms, windstorms — or even fire tornadoes.
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Precipitation and thunderstorms
Precipitation happens when a cloud can’t hold any more water. Tiny, light cloud droplets coalesce, or merge, into bigger droplets. Those that become heavy enough will fall out as rain.
Something similar happens when it’s cold. Tiny, chilly water droplets merge to form ice crystals. That’s what makes snowflakes in the wintertime. Snow falls more slowly than raindrops. While raindrops plummet as fast as 10 meters per second (22.4 miles per hour), snowflakes tend to glide down at a pokey 0.4 to 1.8 meters per second (0.9 to 4 miles per hour).
Thunderstorms can produce dangerous precipitation. These storms come from cumulonimbus clouds. That’s the technical name for thunderclouds.
Thunderclouds start as cumulus clouds — puffy clouds fairly close to the ground. Warm, wet air helps them grow taller. Some thunderclouds can rise into towers 16 kilometers (10 miles) tall. While the tops of these clouds are made of ice, their bottoms consist of water droplets.
Cumulonimbus clouds can produce thunder and lighting. Because they’re tall, they can lift water droplets very high before they finally fall. That’s how hail forms. High-up water droplets can cool and freeze into an ice pellet. As more droplets freeze onto the pellet, the hailstone grows. Some thunderclouds can produce chunks of ice the size of baseballs or bigger.
The most extreme thunderclouds become supercells. That’s a thundercloud that rotates. Changing winds blow different layers of the cloud in different directions. That causes the entire cloud to spin. Supercells occasionally spawn tornadoes.
But there’s a catch. Sometimes, clouds can limit the intensity of storms. Why? “Clouds, even light wispy ones, can affect how much [sunshine] reaches the ground,” explains Joey Krastel. He’s a meteorologist for the Maryland Department of Emergency Management in Hanover.
If you look out from a plane window, you can sometimes see shadows on the ground cast by individual clouds. That’s one sign of how much sunlight clouds can keep from reaching the ground. With less sunlight, the Earth’s surface cools. Now there’s less energy to evaporate water. With less warm and wet air, storms can’t form as easily.
“During storm outbreaks, thick cloud cover can limit surface heating and reduce storm energy and potential,” Krastel wrote via text message.
That’s why the stormiest days often begin with sunshine. It seems strange to think that sunshine actually helps storms, but it’s true. The more the air is heated by sunlight, the more it can rise — and the bigger storms will grow!