How does dew appear on grass?

1. Early in the morning the lawn sparkles, and your shoes get wet. But the sky was clear all night. There was no rain. So where did the water come from?
2. During the day, water evaporates from ponds, soil and plants. It mixes into the air as water vapor, an invisible gas. The dots here are only pretend.
3. At night, grass gives off its heat to the sky. With no clouds to send the heat back, the thin blades get cold fast. They become even colder than the air around them.
4. Vapor molecules zip around in the air. When they hit the cold blade, they slow down and cling together. Gas turns into liquid. This is called condensation.
5. All night more vapor arrives, and the droplets grow. When two touch, they pull together into one bigger drop. On a waxy blade, the drops sit up as round beads.
6. When the sun warms the grass, the dew evaporates and becomes invisible vapor again. Tonight, if the sky is clear, it can all happen once more.

Tap each part to see how dew forms on a blade of grass at night.

• Clear night sky. A clear sky lets the heat from the grass escape into space. On a cloudy night the clouds act like a blanket and send the heat back down.
• Cold blade. A blade of grass is thin, so it loses its warmth quickly. By the middle of the night it is the coldest thing around.
• Chilled air. The air right next to the cold blade gets chilled. Cold air cannot hold as much vapor, so some of it has to turn into liquid.
• Water vapor. Water vapor is water as an invisible gas. On a damp night there is lots of it, so more dew forms.
• First tiny drops. Dew starts as droplets too small to see. They form where the blade has tiny bumps and hairs, then slowly grow.
• Big dewdrop. Small drops that touch pull together into bigger ones. A big drop can roll down the blade and water the soil underneath.
• Waxy coat. Grass has a thin waxy skin that water cannot soak into. So dew sits on top in round beads instead of spreading out.

Drag the cloud blanket away or back, and change how damp the air is. On which nights does dew grow on the leaf?

Experiment: On which night will the grass get more dew?

1. Leave the cloud over the sky and watch the leaf for a moment.
2. Drag the cloud away and watch the thermometer and the leaf.
3. Put the cloud back. What happens to the drops?

What we found: Clear nights give more dew. Without clouds, the leaf's heat escapes to the sky, so the leaf gets cold and vapor turns to drops on it. Clouds work like a blanket and keep the leaf too warm.

Turn on JavaScript to try it yourself.

Fill the dew meter before the sun comes up. Clouds keep drifting back, so keep the sky clear!

Dew is water that comes out of the air. Air always holds some water vapor, which is water as an invisible gas. Warm air can hold a lot of vapor. Cold air can hold only a little.

At night, everything outdoors gives off heat toward the sky. If the sky is clear, that heat escapes, and thin things such as blades of grass get cold quickly. They can get several degrees colder than the air.

The air that touches a cold blade is chilled too. Now it cannot hold all its vapor, so the extra turns into tiny drops of liquid water on the blade. This change from gas to liquid is called condensation. All night the drops grow, and when two touch, they join. By morning the grass is covered in beads of dew.

You get the most dew on nights that are clear, calm and damp. Clouds act like a blanket and keep the grass warm. A strong wind keeps stirring warmer air down onto the grass. And if the air is very dry, the grass cannot get cold enough. When the sun rises, the dew evaporates and goes back into the air.

Key words

Water vapor

Water as an invisible gas in the air.

Condensation

When a gas turns into a liquid, as when vapor becomes drops of water.

Dew point

How cold something must be for dew to form on it.

Humidity

How much water vapor is in the air.

Radiate

To give off heat as invisible rays.

Check your thinking

1. Where does the water in dew come from?

   Show the answer

   From water vapor in the air. Invisible vapor in the air touches the cold grass and turns into liquid drops.

2. Which night gives the most dew?

   Show the answer

   A clear, calm, damp night. A clear sky lets the grass get cold, calm air lets it stay cold, and damp air has plenty of vapor to turn into drops.

3. Why is there less dew after a cloudy night?

   Show the answer

   Clouds send the grass's heat back down, so it stays warmer. Clouds work like a blanket. The grass does not get cold enough for vapor to condense on it.

4. What happens to dew after sunrise?

   Show the answer

   It evaporates and becomes vapor again. The sun warms the grass, and the liquid water turns back into invisible vapor.

5. Why do dewdrops sit on grass as round beads?

   Show the answer

   The leaf's waxy coat repels water, and surface tension pulls each drop into a ball. Water cannot spread across the waxy surface, and surface tension makes the smallest surface it can: a rounded bead.