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Clouds are made up of water droplets or tiny ice crystals.
As part of the water cycle, water evaporates from oceans, rivers and lakes. This moisture rises up into the atmosphere. The farther up it travels, the colder it gets. When the air gets cold enough, the water starts to condense around small particles in the air. This condensation creates visible clouds.
Did you know?
Although clouds float in the air, they can still be very heavy. A single cumulus cloud can weigh hundreds of tons!
Clouds are classified based on their shape and their altitude. In fact, clouds often have compound names that describe both these features.
Most clouds form in the troposphere. That’s the lowest layer of the Earth’s atmosphere. The height of the troposphere varies between 6 km and 18 km.
Location of Clouds
Clouds can be classified based on where they are found.
- Stratus clouds are low-level clouds that form at an altitude of less than 2 000 m. This group includes stratus, cumulus and stratocumulus clouds. It can also include the bases of cumulonimbus clouds. Although they form below 2 000 m, some stratus clouds can grow very tall. The tops of cumulonimbus can reach an altitude of up to 12 000 m!
- Alto clouds are mid-level clouds that form at an altitude between 2 000 m and 7 500 m. This group includes altocumulus, altostratus and nimbostratus clouds.
- Cirrus clouds are high-level clouds that form at an altitude of between 3 000 m and 18 000 m. This group includes cirrus, cirrocumulus and cirrostratus clouds.
Did you know?
Along with the height of the troposphere, cloud heights vary in different parts of the world. In warmer regions near the equator, the top of the troposphere is much higher. As a result, clouds form much higher in the sky!
Shapes and Colours of Clouds
Based on their appearance, most clouds fall into one of ten categories.
Thin and Wispy
- High-level cirrus clouds are made of ice. They appear in thin, delicate wisps.
- Cirrocumulus clouds are also made of ice. They appear in thin, patchy rows. Cirrocumulus clouds are sometimes called cloudlets.
- Cirrostratus clouds appear in a thin sheet that seems to cover the sky. You can usually see the Sun through these clouds.
Fluffy and White
- Cumulus clouds are white and puffy. They sometimes look like towers in the sky. Many people call them fair weather clouds. Cumulus clouds have a flat base and a top that resembles floating pieces of cotton or cauliflower.
- Stratocumulus clouds appear in a patchy greyish-white layer across the sky.
- Altocumulus clouds appear either in white or grey patches or in layered sheets. You often see them with other types of clouds.
Grayish and Layered
- Stratus clouds are flat and greyish. They extend over large sections of the sky and sometimes bring drizzle or light snow. Fog develops when stratus clouds touch the ground.
- Altostratus clouds appear in a grey or bluish sheet that covers the entire sky. Although you can see the Sun through these clouds, they sometimes bring light precipitation.
Dark and Stormy
- Nimbostratus clouds appear in a thick, dark layer. They often bring rain or snow. They are usually classified as low-level clouds because they tend to be thicker at the base. However, their tops can reach further up into the troposphere.
- Cumulonimbus clouds form heavy, dense towers. They are often anvil-shaped, with very dark bottoms. These clouds are responsible for thunderstorms, which can develop into other forms of extreme weather like tornadoes and hurricanes.
Did you know?
“Contrails” is short for “condensation trails.” At high altitudes, cold air temperature causes water vapour to condense around the particles in hot, humid airplane exhaust. This creates the line-shaped clouds you see in the sky.
So the next time you look at clouds, see if you can predict what type they are!