Moisture and evaporation

On a rainy day do puddles form on your playground? You may enjoy splashing in them, when a teacher isn't looking, but after a time they can spoil your games. Some of the larger puddles may empty into a drain but the smaller ones just stay there in hollows in the tarmac. When the rain stops the puddles no longer increase in size. In fact a strange thing happens - they shrink. If you mark out the edge of the water in a puddle and come back a couple of hours later, you will see there is a gap between the water's edge and the mark. The water has not flowed anywhere but some of it has vanished. If the weather remains dry the puddle will completely disappear.

The change in the size of the puddle is due to a change, which takes place in water. It changes from the liquid state to the gas or gaseous state. The name for the gas that water forms is water vapour. When the rain stops, some of the water at the surface of the puddle changes into water vapour and mixes with the air above it.

Water is made of tiny particles that slide around each other. This movement allows water to flow and take up the shape of any container into which it is placed. In the centre of a liquid the particles are surrounded by others and all are held together. At the surface of the liquid the particles are only held by particles below them or to their sides. All particles have energy, which allows them to move. Some of the particles at the surface use their energy to try and leave the liquid. As they are not held down as tightly as particles deep in the liquid some of them make it. They fly off from the surface into the air. The particles of water vapour gather just above the surface of the water but they are soon knocked about by air particles. This action spreads out the particles of water vapour and creates space for more to make the leap into the air.

The process of changing from a liquid to a gas at normal temperatures is called evaporation. It is this process that dries up the puddles in your playground.
When the particles of water vapour enter the air, the moisture content of the air rises. We call this moisture content humidity. If the air is warm and moving the moisture content will remain low as the particles of water vapour are whisked away. If the air is warm and still, the number of particles in the air rises and the air can feel clammy. We also may say that the weather is humid. One of the best places to find humid air is in a bathroom. Just go in after someone has had a bath and you will feel the moisture in the air due to large amounts of water vapour.

Does evaporation cause washing to dry?
Yes, it does. When the washing is pegged out, the material in the clothes, sheets and towels is soaking. The water on the surface of the materials evaporates first and water moves out of the materials to replace it. This water evaporates and the washing dries.

Why are some days better for drying washing than other days?
The speed at which evaporation takes place depends on three things. They are warmth of the air, dryness of the air and speed of the wind. If conditions are warm the water particles at the water surface have more energy and can change quickly into water vapour. If the air is dry there is plenty of space for the particles of water vapour. If there is a wind it can whisk the particles of water vapour away and make more space for more particles to evaporate. The best days for drying clothes outside are warm, dry, windy days.

What are the worst days for drying washing outside?
Cold, humid, still days. If the weather is cold the particles have less energy and fewer can change into water vapour. Humid air is already full of particles of water vapour. This means that there is no room for more particles of water from the washing so they have to stay where they are. Still conditions cause the air full of water particles to stay where it is. None is moved away to create more space. As a consequence of these three weather conditions the washing stays wet on the line.

How does a tumble drier work?
It works by providing the correct conditions for quick evaporation. Dry air is drawn into the machine and warmed, then passed over the tumbling washing. Water particles receive extra energy from the warm air so many can change into water vapour. The dry air provides plenty of space into which the water vapour particles can move and the current of air moves them out of the drier so that even more water particles can escape from the clothing.

Does a hair drier work in the same way as a tumble drier?
Yes, it does. It provides hot, dry air moving at speed. The air causes rapid evaporation so your hair is dry in a few minutes.

Can hair be affected by the humidity of the air?
Yes, it can. If the air is humid the hair gets longer. If the hair is dry the hair gets shorter. There used to be a simple humidity measurer in the form of a toy house. A wooden man and woman were attached to a turntable on which hair was stretched. When the air was humid the stretched hair made the turntable move and the man appeared outside the house. When the air was dry the hair shrunk and pulled the turntable in the opposite direction. This made the woman appear outside the house. Devices like this were used to forecast the weather.

Can pine cones and seaweed be used to detect changes in the air?
Yes, they can. When the air is dry a pine cone opens up. It if contains seeds they may be released. When the air is humid the cone closes up. Seaweed becomes dry in dry air but absorbs moisture from humid air and becomes damp.

Why do you shiver if you do not dry yourself after swimming?
When you dry yourself the material in the towel soaks up the water and takes it from your skin. If you do not dry yourself the water begins to evaporate from your skin. Water particles need energy to change from liquid to a gas so the liquid takes heat energy from whatever it is touching. If the water is on your skin it takes water from your skin and makes you cold. It could make you cold enough to shiver.

Why do elephants squirt water over themselves in the heat of the day?
They do this to cool off. Large bodies lose heat more slowly than small bodies (that is why care has to be taken to keep babies warm). As the elephant has a very large body it loses heat slowly. It must not let its temperature get too high or it will die so it covers its skin with water. As the water evaporates it cools the skin, the blood underneath and the whole of the elephant's body.