Winds blow in every corner of the earth’s surface from a region of high pressure to a region of lower pressure. They are named according to the direction from which they com. A wind coming from the west is said to be a westerly wind, those blowing from the east called easterlies. The Earth’s rotation deflects winds to the right of their flow in the Northern Hemisphere and to the left to their flow in the southern Hemisphere.
Various Types of Winds
There are three main planetary winds that constantly blow in the same direction all around the world. They are also called prevailing or permanent winds.
Blow from the subtropical high pressure belt towards the Equator. They are called the north-east trades in the northern hemisphere and south-east trades in the southern hemisphere.
Blow from the same subtropical high pressure belts, towards 60˚ S and 60˚ N latitude. They are called the sought Westerly wind sin the northern hemisphere and North Westerly winds in the southern hemispheres.
Blow from the polar high pressure to the sub polar low pressure area. In the northern hemisphere, their direction is from the north-east. In the southern hemisphere, they blow from the south-east.
These winds are known to blow for a certain time in a certain direction – it may be for a part of a day or a particular season of the year.
Land and Sea breeze:
During the day, near an ocean, sea or lake, the land heats up faster than the water. The air above the land also gets heated. As warm air rises, it draws cooler air from over the water to blow towards the land, creating a sea breeze.
At night, the opposite conditions prevail. The land loses heat rapidly while the sea is still warm. The air resting over the land is cold while the air resting over the sea is warm and rises creating a low pressure area. A land breeze thus blows in from the high pressure over the land towards the water. These land and sea breezes maintain air circulation in the coastal areas and have a moderating affect on the temperatures.
They are land and sea breezes on a large scale. The word ‘monsoon’ comes from the Arabic word ‘mausim’ meaning weather. They change or reverse their directions according to the seasons. Strong contrasts in temperature between summer and winter because great differences in pressure conditions over the interior parts of the big continents like Asia. Hence winds blow onshore from a sea to the land in summer and from land to the sea in winter. The onshore winds bring moisture and heavy rainfall while the offshore winds are relatively dry. Although the monsoons are associated with south-east USA, Australia, parts of South America and East Africa they are most effective over south-east Asia and India blows from June to September while the winter monsoons prevails from October to December.
On the earth’s surface, some local variations of temperature on the land may cause changes in air pressure. As a result local winds blow. They blow in a particular season and are known by the local names in that region. For instance, the hot dry, dusty winds that blow in the month of May and June over the northern plains in India are called Loo.
Some other examples of local winds that bring unusual changes in the temperature of the places are the warm Chinooks that devour the snow on the leeward side of the Rocky mountains of North America, the Foehn in the Swiss Alps and the hot, sand laden Siocco that blows over Southern Europe from the Sahara and causes ‘blood rain’ which is actually desert sand and dust that falls with the rain.
There are still other types of winds that are irregular and keep changing their direction and blow in an area for a very short time, such as tornadoes, typhoons and cyclones.