G3_Water and the Three States of Matter 400 – 500L_05

Condensation

The next morning you wake up and look outside. It has not rained, so why is the grass wet? No, the sprinklers have not been on. Your dad says this is called dew.
Dew usually forms overnight. When the Sun is not out, the Earth’s surface cools.

The grass will become cool too. If the air around the grass is still warm and moist, dew will form. When warm, moist air touches the cooler grass, the water vapor in the air cools down and changes back into liquid water. When this happens, tiny water droplets will form on the blades of grass. Water vapor turns back into liquid water. This change, when a gas turns into a liquid, is called condensation.

Sometimes, air is very moist. You may have heard the term ‘humidity’. Humid air holds a lot of water vapor. Warm air holds more water vapor than cold air.  When air cools down, it holds less water vapor. This results in some of the water vapor being changed into liquid water. It doesn’t have to cool very much for water vapor to start condensing. That’s why glasses of cold drinks “sweat” on a hot, humid day.

You can see this process for yourself. Try breathing on a window or mirror. Notice how the glass fogs up. The air coming from your lungs is warm and moist. The glass is cooler. The warm, moist air touches the cooler surface and the water vapor turns into liquid water.