Wind

Wind is the result of air moving from one point to another. Air moves due to differences in temperature and pressure.

Ancient mariners were very aware of these winds. Winds carried traders to the Americas from Europe; they are now known as 'Trade winds'. On the equator there is little wind, mariners called this region the doldrums (after an old English word meaning dull) because they feared being stranded there.

Air can move in different directions and at different speeds, so wind is always described as a vector with direction given in degrees and speed usually inmph. Gust strength of wind is also reported. A gust of wind usually only lasts from a few seconds to a few minutes.

The strongest gust of wind ever reported on Earth was 231mph at Mount Washington in New Hampshire, USA, on 12th April 1934. It was caused by air being funnelled between the top of the mountain and the troposphere, (the region of the atmosphere where all our weather takes place).

Wind strengths
Jet streams occur about 10-12km (30,000-35,000ft) above the earth's surface, where cold air from the poles meets hot air from the equator. The winds in the centre of the jet stream can be as much as 250mph. They sometimes help strengthen and steer areas of low pressure and are therefore very important to weather forecasters as well as airline pilots.

Wind scales
The monsoon winds are not as strong but affect a vast area. They are most commonly associated with South Asia. The south-westerly monsoon winds bring copious amounts of rain from June onwards. During the winter months the winds turn North-easterly and South Asia is mainly dry.

There are also very localised, small scale winds, for example, sea breezes. These occur in summer when hot air rises over the land and is replaced by cooler air from the sea. On a hot day sea breezes can bring refreshingly cooler conditions to coastal towns and cities.

Localised winds
Sometimes localised winds have been given names by the people who live where these winds blow, especially it would seem in desert or mountainous regions.

In mountainous areas winds are either forced over mountains or through them. If the wind is forced over them on the leeward side of the mountain the air sinks, creating an area of high pressure and clear skies. As the air on this side of the mountain is compressed the temperature rises, resulting in a warm wind. This type of effect can be seen over the Scottish Mountains and the Pennines but on a far grander scale to the east of the Rockies in the USA.

When wind is forced through the valleys in mountainous regions the mountain chain acts as a funnel and therefore the wind becomes stronger, such as the Mistral in France.

Other winds result from intense heating of inland areas which creates an area of low pressure. The Sirocco brings hot, dry, dust laden winds to the Mediterranean from the Sahara.

There are also cold winds. Cold air is dense and so moves down mountain sides creating what is call katabatic (Greek word to mean go down) winds. A good example is the Bora wind which is a cold north-easterly wind which blows in winter down the east coast of the Adriatic from the Balkan mountains. At its most intense it can reach 115mph.

The most extreme type of katabatic wind is found in the Antarctic where cold gusts of wind surge across the polar ice caps. They can reach 120mph! Chilly for the penguins!