Moisture in the Air   

Along with sunshine, the moisture in the air is what creates all of our weather. These two features work together to form clouds, to make rain and thunderstorms, and to cause winds to blow. Without the sun or moisture, the Earth would have no weather.

Even on the sunniest day and over the hottest deserts, the air is full of water. Without realising it, we are surrounded by water all the time. The air acts like a sponge and continually soaks up water. This water is in the form of invisible vapour, and has been evaporated from oceans, lakes and rivers. The actual amount of water vapour in the air (its humidity) depends on how hot and dry the area is.

Although this water is usually invisible vapour, there are times when we can see it. When air rises and cools, some of the water vapour will turn into drops of liquid, forming the clouds, mist and haze that continually surrounds the Earth. This process is called condensation. Water vapour will only form water droplets if the air contains plenty of dust, smoke, salt and other particles. These are called condensation nuclei and are needed as a base for the water to condense on to. If the air is very pure, there will not be enough nuclei and clouds and mist will not form.

Rising air.

Air can rise for a number of reasons. It may rise over mountainous terrain, when it is heated by warm ground and so made lighter, or if cold, heavier air pushes under it and forces it upwards.

As the air rises and cools and the water vapour turns to drops of liquid, these drops gather together to form clouds. Whilst this process is standard, the clouds that result vary in a number of ways – including their shape, colour and height. These characteristics can give us some idea of what the weather will be like over the next few hours or days.

Dew drops.

Moisture condenses as air cools. This is because the cooler the air is, the less water vapour it can hold. The limit to the amount of water air can hold is called its saturation or dew point. Once it reaches this limit, water vapour condenses into droplets and this is why dew is formed. After a cool night, leaves, grass and spiders’ webs are often covered in dew drops, the water that the air could not hold. Next time you see dew on the ground, try to remember why this has formed.

Breathing Clouds.  

When you breathe out, you are filling the air with water vapour. If the air is very cold, the vapour will turn into lots of tiny water droplets and your breath looks "steamy".

Water Vision.

Without the moisture in the air, we would be able to see into the distance much more clearly. We know that visibility is dramatically reduced by fog and mist, but are you aware that moisture can effect how far you can see even on a clear day? Next time the day seems to be clear, try and see the slight haze in the air that will make distant objects look pale and indistinct.

Wet Hair

Human hair stretches in moist air and contracts in dry air. A hair hygrometer can therefore be used to measure the moisture content of the air, its humidity.

It is now time to look at the most well-known form of moisture in the air - clouds.  Let's go to the next page to find out more about them.