It can destroy buildings and pick up cars and trains, throwing them hundreds of metres into the air. It can make a cool day feel bitterly cold and it can make a hot day feel nice and cool. But what is wind and why does it occur?

Wind is simply air - the stuff we breathe- moving across the surface of the Earth. It does this because the Sun warms the air over the middle part of the Earth - the Equator - and this sets up a series of wind patterns across the globe.

Winds are recorded in kilometres (or miles) per hour. The weather instrument used to measure wind is called an anemometer. It has a series of cups which rotate as the wind hits them and the faster they rotate, the higher the wind speed.

The wind direction is the direction from which the wind blows. (So a westerly wind blows from the west towards the east.) Windy weather is common in the West Country. The prevailing wind (the direction from which the wind usually blows) is southwesterly. Winds of up to 90 miles per hour have been recorded in Bristol. These very strong winds are called gales. Much stronger winds are found in other parts of the world.

Hurricanes are the biggest storms on Earth and winds of 150 MPH may be found within their swirling cloud systems. The very strongest winds have been measured in tornadoes. These are violent swirls of air, similar to the way water drains down the plughole in the bath. The biggest of these are usually found in the United States of America. A gust of 286 MPH was once recorded in one of these ‘twisters’. But winds are of great importance.

Most plants need wind to transport their seeds. Many species of bird need the wind to soar above the oceans or to help them on their migrations. The wind can help us too.

The power of the wind can be harnessed to provide us with energy. Wind farms change the energy of the wind into electrical energy which can then be used to power our homes and schools. This is a non-polluting form of energy which does not produce any of the gases which cause global warming.