Condensation
Many people enjoy camping. Perhaps you have spent some time sleeping out overnight in a tent. In the evening, before sleep, you may have sat round a campfire. It may have been a sunny day and when the sun set the sky was clear and perhaps even starry. If you had bare feet you may have found the ground dry and dusty as you walked to your tent. In the morning, however, things would be different.

Although it had not rained in the night, when you looked out of the tent the grass was covered in water. When you stood outside, your feet felt cold as they met the wet soil. The outside of the tent was covered in water droplets and glistened in the sunlight. Everywhere had a coating of water droplets. They had all silently appeared as you slept, but where had they come from?

Air is a mixture of gases. One of the gases which is present in small amounts is water vapour. Like all gases, water vapour is made from particles. They move around in all directions. In the air the particles of all the different gases mix freely. Air is crowded with billions of rapidly moving particles and just like people moving quickly through a crowd they bang into each other. When particles hit each other they bounce away in different directions and just keep moving. Particles need energy to move. If the particles of a gas have not got enough energy they don't bounce off each other when they meet. They stick together and form a liquid. The water on your tent had formed in this way but why had it happened at night?

When you are making observations on the weather, one of the instruments you use is a thermometer. You use it to tell the temperature of the air. The air gets its heat from the Sun in a surprising way. The heat passes from the Sun in rays. They strike the ground and warm it. The warm ground then passes heat to the air next to its surface. The air warms up, becomes lighter in weight and rises though the cooler air above it. Some cooler air sinks down to take its place, becomes warm and rises too. This process of heating and rising continues throughout the day and makes the air warm. At night something different happens.

In addition to warming the air, the ground also sends out rays of heat back through the atmosphere into space. During the day the ground receives more heat from the Sun than it is losing so it keeps warm. When the Sun sets, the ground loses its source of heat. It is still warm and releases its heat into the air and into space. Eventually the ground loses so much heat that it becomes cooler than the air. When this happens, the air passes heat to the ground. This heat passes out from the ground as heat rays. They pass through the air and into space. This change in the way heat moves makes the water droplets form on your tent.

The air close to the ground becomes cool as it loses its heat. The particles of water vapour lose so much energy that they cannot keep moving around freely. They group together and form droplets of water. This process of changing from a gas to a liquid is called condensation. As you slept in your tent the water vapour in the air around your tent condensed and formed liquid water that we call dew.

On some mornings there is a large amount of dew and on other mornings there is hardly any. Why is this?
The difference is due to the amount of cloud. If there is no cloud the rays of heat that the ground sends out can pass right through the atmosphere and into space. A large amount of heat can be lost from the ground and this makes it much cooler than the air. When this happens, the air becomes very cold and large amounts of water vapour condense and form a heavy dew. If there is a thick covering of cloud it stops the heat rays escaping. They stay in the atmosphere and the temperature of the air only falls a little. When this happens, only a small amount of dew forms.

Can you get dew in a desert?
Yes, you can. There is little or no cloud cover in the desert at night so the ground loses a great deal of heat. This causes a dew. Desert animals and plants use the water which forms this way in the desert. Without it they could not survive.

You can get water on the inside of windows after a cold night. Is that dew?
No. We call this condensation. It has formed from water vapour in the room. Heat passes from the air in the room, through the glass, to the air outside. This makes the air inside the room, close to the window, very cold. The water vapour in this cold air loses so much energy that its particles cannot keep apart. They join together to form liquid water which coats the inside of the window.

Where has the water vapour in the room come from?
It has come from your breath. The lining of the lungs and air passages in your body are covered in a layer of water. As you breathe out, some of this water evaporates and leaves your body as water vapour. After a night's sleep the air in the room can contain a good deal of water vapour and some of it condenses on a cold windowpane.

When windows get misty in a classroom is that condensation too?
Yes, it is. Everyone in the classroom gives out water vapour when they breathe out. If it is cold outside, heat passes from the classroom through the glass and the air next to the window becomes so cold that water vapour condenses.

When you are outside on a cold day your breath makes a cloud. Is that condensation?
Yes, it is. The water vapour inside your body is warm but when it meets cold air its particles lose so much energy they clump together to form water droplets. The droplets form a cloud that looks like the cloud over a boiling kettle.

Is the steam of a kettle condensation?
The cloud that people call steam is caused by condensation. It is made of water droplets. The real steam is a gas you cannot see. It is in the clear space between the kettle spout and the edge of the cloud.

When you bring a can of drink out of the fridge it gets wet. Is that condensation?
Yes, it is. Water vapour needs a cold surface on which to form water droplets and the cold can provides this.

How do droplets of water vapour form in the air without a cold surface?
There are cold surfaces in the air but they are too small for us to see. They are on dust particles in the air. The dust is the same temperature as the rest of the air. If the air is cold, the surfaces of the dust particles are cold and water vapour condenses on them.