Wind        

A big layer of air called the atmosphere surrounds the Earth. The air within this layer moves from place to place when it warms up or cools down. This moving air is known as wind.  Winds move moisture and heat around the world and also produce much of our weather.

As we know, the strength of the wind can vary enormously. Sometimes air moves slowly and the wind is barely noticeable. When the weather is clear we may experience a gentle breeze, when the wind is still very light but we can feel it on our faces and in our hair, and we may hear leaves rustling. At other times, the air can move very quickly and become a gale or hurricane, blowing down trees and damaging cars and buildings.

Global wind pattern

All of these winds are part of a global air circulation system that acts to balance temperature and pressure around the world. We already know that different parts of the world receive different amounts of heat from the sun (see sunshine page for more information). This differential heating in turn results in differences in temperature and air pressure around the world – which drives the world’s winds.

As equatorial areas are heated most, the air above them warms and rises as it becomes lighter than the surrounding air, causing an area of low pressure. In cooler areas, the air sinks because it is heavier and results in an area of high pressure. Winds will blow as air is squashed out by the sinking cold air and drawn in under the rising warm air. Any difference in temperature like this will always cause a difference in air pressure – and therefore winds will blow. A good expression to remember is that:

"winds blow from high to low" (ie: from high pressure to low pressure).

So if you know the temperature and pressure in different areas, you will always be able to predict the wind direction.

However, winds do not simply blow in straight lines from north to south. Instead, they are bent by the spinning of the Earth:

• to the right north of the equator, and
• to the left in the south.

This is called the Coriolis Effect and it bends every wind on Earth, resulting in a distinct pattern of winds around the world. In the mid-latitudes (30-60º north and south of the equator) most winds are westerlies, blowing from the west. Elsewhere they blow mainly from the east; for example the steady winds blowing towards the equator from the subtropics. These are known as trade winds because they were used by sailing ships carrying trading goods around the world. They blow consistently from the NE north of the equator (north easterlies) and from the SE south of the equator (south easterlies).

Remember to have a look on the Activities page, where there is a special experiment to demonstrate the Coriolis Effect.  To find it, scroll down to the Wind section and click on the Coriolis effect experiment link.

Sailing ships have also identified other areas of the world by their unique wind patterns – for example the Doldrums. The Doldrums are an area of low pressure occurring where the trade winds meet along the equator. Winds here are usually calm or very light and so ships would avoid the area because they would only be pushed along very slowly.

Another set of famous winds are known as the Roaring Forties. These are very strong westerly winds which blow almost continuously in the southern hemisphere. These fierce winds are found at a latitude of 40º - hence their name!

Named winds

Other named winds can also be found on a smaller scale. These local winds can also have quite an important effect on the weather in different parts of the world, for example the Chinook in North America. Some other examples are given below:

• The Mistral in France – a cold, north-westerly wind that blows down the Rhone valley.
• The Harmattan in West Africa, which blows south from the Sahara and brings dust storms and very dry air.
• The Levante wind in the Mediterranean – an easterly wind bringing mild, moist air to Gibraltar and the mainland of Spain and Africa.
• The Pampero in Argentina – a very cold south westerly wind formed, like many cold winds, in the middle of a continent - in this case South America. It blows across the Pampas grasslands in Argentina.

Wind chill               

Temperature is the main factor affecting how warm or cold you are. However, wind can also play an important role, making you feel colder than you would otherwise be. This is because the wind causes your body to lose heat faster. The higher the wind speed, the more heat is lost from your body and you therefore feel colder. The temperature your body feels as a result of the wind is called the wind chill temperature.

Use the table below to see how much colder the wind can make you feel. This measurement is particularly important for mountaineers and explorers in cold regions, where the wind chill can affect their health and survival.

In a similar way, you may have noticed that washing dries faster on a windy day. This is because the wind blows away the water vapour which is evaporated from the surface of the wet clothes.

Wind facts

- Wind legend - the Ancient Greeks used to think that wind was the Earth breathing in and out. We now know that it is just air on the move.

- Wind power - windmills usually face into the prevailing wind – ie: the direction the wind blows in most often.

- Wind strength - the world’s windiest place is Antarctica, where winds blow at more than 100 km/hr for five months of the year.

Measuring the Wind

We all know that the strength of the wind can vary enormously, but do you know how we measure these changes? To find out, let's move on to the measuring the wind section.