How do clouds form, and why does it rain?

1. Meet Dewy, a drop of water in the sea. Every day the sun shines on the ocean and warms the water near the top.
2. Warm water molecules move fast. Some escape from the surface and float into the air as water vapor, an invisible gas. This is called evaporation.
3. As the air rises it cools down. Cold air can't hold as much vapor, so the vapor turns back into tiny droplets around specks of dust or sea salt. This is condensation.
4. Billions of tiny droplets together make a cloud. Inside, droplets bump into each other and join up, growing bigger and bigger. The cloud turns gray.
5. When droplets get too heavy for the rising air to hold up, they fall as rain. It takes about a million cloud droplets to make one raindrop! If it is cold enough, they fall as snow.
6. Rain runs downhill into streams and rivers, and some soaks into the ground. The rivers carry the water back to the sea, and the trip starts all over again. That's the water cycle!

Tap each part of the water cycle. Watch what the water does there!

• The sun. The sun powers the whole water cycle. Its heat gives water the energy to evaporate and makes air rise.
• Ocean and evaporation. The ocean holds almost all of Earth's water. Warm water at the top evaporates and becomes invisible water vapor in the air.
• Water vapor. Water vapor is water as a gas. You can't see it, but it is in the air around you right now.
• Cloud and condensation. Rising air cools. Vapor turns back into tiny liquid droplets around specks of dust or salt. Billions of them make a cloud.
• Rain. Cloud droplets bump together and grow. When they are too heavy to float, they fall as rain.
• Snow on the mountain. High on the mountain it is cold, so water falls as snow. The snow is stored there until warm weather melts it into streams.
• River (runoff). Water always flows downhill. Rain that runs over the ground gathers in streams and rivers that carry it back to the sea.
• Groundwater. Some rain soaks into the soil and trickles down between rocks. This groundwater moves slowly and can fill wells and springs.
• Plants (transpiration). Plants pull water up from the ground. Some escapes from tiny holes in their leaves as vapor. This is called transpiration.

Turn the sun up and down. Watch the invisible vapor rise, the cloud grow and the rain fall. Follow Dewy all the way around!

Experiment: What happens to the clouds and the rain when the sun shines hotter on the sea?

1. Turn the sun down low and watch for a while.
2. Now turn the sun up high.
3. Watch how fast the vapor rises, how big the cloud grows and when it rains.

What we found: A hotter sun warms the water more, so more water evaporates. More vapor rises, cools and condenses, so the cloud grows bigger and heavier until it rains. With a cool sun, only a little water evaporates and the cloud stays small and dry.

Turn on JavaScript to try it yourself.

Fill the cloud over the sea, then blow it over the farm so the crops get rain. Keep the picnic dry!

Water can be a solid (ice), a liquid (water) or a gas (water vapor). When the sun warms the sea, lakes or even a puddle, some of the water turns into vapor and mixes into the air. You can't see vapor, but it is all around you.

Warm air rises. As it goes up, it spreads out and cools. Cool air can't hold as much vapor, so some of the vapor turns back into liquid. It forms tiny droplets around specks of dust, smoke or sea salt. Billions of these droplets together make a cloud.

Cloud droplets are so small that rising air keeps them floating. Inside the cloud they bump into each other and join together. When a drop gets big and heavy enough, it falls. That is rain! In cold weather the water falls as snow, sleet or hail instead.

The rain soaks into the ground, waters plants or runs into rivers that carry it back to the sea. Then the sun warms it again. The same water has been going around this cycle for billions of years, so the water you drink may once have been in a dinosaur's puddle!

Key words

Evaporation

When liquid water turns into an invisible gas called water vapor.

Condensation

When water vapor cools and turns back into tiny liquid droplets.

Precipitation

Water falling from clouds as rain, snow, sleet or hail.

Dew point

The temperature at which air is so full of vapor that droplets start to form.

Runoff

Rainwater that flows over the ground into streams and rivers.

Humidity

How much water vapor is in the air.

Check your thinking

1. What gives water the energy to evaporate from the sea?

   Show the answer

   Heat from the sun. The sun's heat gives water molecules enough energy to escape into the air.

2. What is a cloud made of?

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   Billions of tiny water droplets or ice crystals. A cloud is made of tiny liquid droplets or ice crystals. Vapor itself is invisible.

3. Why does rising air make clouds?

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   It cools down, and cool air can't hold as much vapor. Air cools as it rises. Cooler air can't hold as much vapor, so droplets form.

4. Air at the ground is 24 °C with a dew point of 16 °C. About how high is the cloud base?

   Show the answer

   About 1,000 m. The difference is 8 °C, and 8 × 125 m = 1,000 m.

5. Where does most rainwater end up after it runs into rivers?

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   Back in the ocean. Rivers carry the water back to the ocean, ready to evaporate again.

6. Why does it take a while for a cloud to rain?

   Show the answer

   Tiny droplets must bump and join until they are heavy enough to fall. Cloud droplets are too small to fall. They have to grow much bigger first.