What is a cloud?
A cloud is a huge collection of tiny water droplets or ice crystals floating in the air. The droplets are so small (about 0.02 mm across) that air currents keep them up. An average puffy cloud weighs about 500 tonnes, about as much as 100 elephants, but it’s spread out over a huge area.
How clouds form
- Water evaporates from oceans, lakes, plants and soil, becoming invisible water vapour.
- Warm air rises. As it rises, the air expands and cools.
- The air reaches its dew point. The dew point is the temperature at which air can’t hold any more water vapour.
- Condensation happens. Water vapour condenses onto tiny particles of dust, salt or smoke called condensation nuclei.
- Billions of droplets together form a visible cloud.
What makes air rise?
| Cause | Example |
|---|---|
| Heating from the ground | Puffy clouds on summer afternoons |
| Air forced up mountains | Clouds and rain on the windward side of the Rockies |
| Air pushed up at weather fronts | Long bands of clouds |
| Air converging in low-pressure systems | Storms |
Types of clouds
In 1802, British scientist Luke Howard gave clouds Latin names that we still use.
| Word part | Meaning |
|---|---|
| Cirro- / cirrus | Curl of hair; high cloud |
| Cumulo- / cumulus | Heap or pile |
| Strato- / stratus | Layer |
| Nimbo- / nimbus | Rain |
| Alto- | Middle height |
High clouds (above about 6000 m)
| Cloud | Appearance | Weather |
|---|---|---|
| Cirrus | Thin, wispy "mares’ tails" of ice crystals | Fair now, but a change may come in a day or two |
| Cirrostratus | Thin white veil; causes a halo around the Sun or Moon | Rain or snow may be coming |
| Cirrocumulus | Small white ripples ("mackerel sky") | Fair, but changing |
Middle clouds (about 2000–6000 m)
| Cloud | Appearance | Weather |
|---|---|---|
| Altocumulus | Grey or white puffy patches | Often fair; can signal storms later |
| Altostratus | Grey sheet; the Sun looks dim and watery | Rain or snow may follow |
Low clouds (below about 2000 m)
| Cloud | Appearance | Weather |
|---|---|---|
| Stratus | Flat grey blanket | Overcast, drizzle |
| Stratocumulus | Lumpy grey patches | Usually dry |
| Nimbostratus | Thick, dark grey layer | Steady rain or snow |
Vertical clouds
| Cloud | Appearance | Weather |
|---|---|---|
| Cumulus | Puffy and white with flat bottoms | Fair weather |
| Cumulonimbus | Huge towering cloud, often with a flat "anvil" top, up to 12–15 km high | Thunderstorms, lightning, heavy rain, hail, sometimes tornadoes |
Fog
Fog is a cloud at ground level. It forms when air near the ground cools to its dew point. Newfoundland’s Grand Banks are among the foggiest places on Earth, where warm Gulf Stream air meets the cold Labrador Current.
Why are clouds white or grey?
Cloud droplets scatter all colours of sunlight equally, so clouds look white. Thick clouds block more sunlight from passing through, so their bottoms look grey or dark. A dark, heavy cloud is usually full of water.
Clouds and climate
Clouds affect Earth’s temperature in two ways:
- Low, thick clouds reflect sunlight back to space, cooling Earth.
- High, thin clouds let sunlight in but trap heat, warming Earth.
On a cloudy winter night, it usually stays warmer than on a clear night, because clouds act like a blanket.
Reading the sky
People have long used clouds to predict weather:
- "Mares’ tails and mackerel scales make tall ships take in their sails." (Cirrus and cirrocumulus can mean a storm is coming.)
- A halo around the Moon often means rain or snow within a day.
- Towering cumulus clouds growing in the afternoon can turn into thunderstorms.
Common mistakes
- Thinking clouds are made of water vapour. Vapour is invisible; clouds are liquid droplets or ice.
- Thinking clouds are weightless.
- Thinking all grey clouds bring rain.
Try it yourself
- Make a cloud in a jar: pour a little hot water into a jar, cover it with a plate of ice, and add a puff of hairspray or a struck match (with an adult). Watch the cloud form.
- Photograph clouds for a week and identify their types. Compare with the weather that followed.
- Breathe on a cold mirror. Explain what you see.
Quick recap
- Clouds are tiny water droplets or ice crystals in the air.
- They form when rising air cools to its dew point and water vapour condenses.
- Cirrus, cumulus and stratus are the main cloud families.
- Cumulonimbus clouds bring thunderstorms.
- Clouds help forecast weather and affect Earth’s temperature.