The horizontal movement of air along the earth’s surface is called a Wind. The vertical movement of the air is known as an air current. Winds and air current together comprise a system of circulation in the atmosphere. The movement and the speed of wind are affected by three main factors:
- Pressure gradient
- Rotation of the Earth
- Friction of the Earth.
We know that a wind always blows from a region of high-pressure to a region of low-pressure. Steeper the pressure gradient, higher is the speed or force of the blowing wind. Slower the pressure gradient, slower is the force of the blowing wind.
Rotation of the Earth
If the Earth did not rotate upon its axis, winds would follow the direction of the pressure gradient. But the rotation produces another force other than the pressure force. It is called the ‘Coriolis force’. This tends to turn the flow of air by changing its direction from its original straight path. The wind starts deflecting more and more to its right from its original path in the northern hemisphere. In the southern hemisphere it starts deflecting more and more to its left from its original path. Thus a wind blowing from north becomes north-easterly in the northern hemisphere. A wind blowing from south becomes south-easterly in the southern hemisphere. The effect of the Coriolis force on wind is stated in Ferrel’s law as follows: “Any object or fluid, moving horizontally in the Northern Hemisphere, tends to be deflected to the right of its path of motion regardless of the compass direction of the path. In the Southern’ Hemisphere, a similar deflection is towards the left of the path of motion.” The deflection is the least at the equator and the greatest at the poles.
Friction of the Earth
The friction along the Earth’s surface decreases both the angular deflection and velocity of the wind. It is very little over vast free surface of oceans and is considerable over the mountains and the heavily forested areas.
Types of Winds
On the earth’s surface, certain winds blow constantly in a particular direction throughout the year. These are known as the ‘Prevailing Winds’. They are also called the Permanent or the Planetary Winds. Certain winds blow in one direction in one season and in the opposite direction in another. They are known as Periodic Winds. Then, there are Local Winds in different parts of the world.
Planetary or Permanent Winds
The planetary wind system of the world accompanies the presence of the High and Low-Pressure Belts. We know that winds tend to blow from the high-pressure centres to the low-pressure centres. The effect of the earth’s rotation (Coriolis Force) tends to deflect the direction of these winds. The deflection in the direction of these winds take place according to Ferrel’s Law. Two sets of surface winds, the Trades and the Westerlies are the main planetary winds of the world.
North and South of the Equatorial Belt of Calms, are the Trade Winds covering roughly the zone lying between 5° and 30° North and South. In other words they cover almost the entire area between 30°N and 30°S latitudes on both sides of the equator. The Trade Winds are a result of a pressure gradient from the Sub-Tropical Belt of High Pressure to the Equatorial Belt of Low Pressure. In the Northern Hemisphere, the wind moving equatorward, is deflected by the earth’s rotation to flow south-westward. Thus, the prevailing wind there is from the North-East, and it has been named as the ‘North-East Trades’. In the Southern Hemisphere, deflection of the wind is towards the left, this causes the ‘South-East Trades’. Trade Winds are noted for their steadiness and persistent direction. But the system of Doldrums and trades shifts seasonally north and south, through several degrees of latitudes, as do the pressure belts that causes them. The trades are best developed over the Pacific and the Atlantic Oceans, but are upset in the Indian Ocean because of nearness of the great Asian landmass. They are named after the Latin word ‘trado’ which means blowing steadily in a constant direction; hence, the name Trade Winds. As these Trade Winds blow from the warmer, sub-tropical latitudes to the hot tropics, they have a great capacity for holding water-vapour or moisture. When they cross the open oceans, they pick up a lot of moisture. They bring heavy rainfall to the eastern coasts of continents lying within the tropics because they blow on-shore. On the western coasts of continents, these Trade Winds do not bring any rainfall. It is because here there are ‘off-shore’ winds or winds blowing just parallel to the shores, as they blow off-shore. As such, the western areas within the tropics suffer from aridity. The great deserts of the Sahara, Kalahari, Atacama and the Great Australian Deserts all lie on the western mar¬gins of the continents, lying within the tropical latitudes.
The Westerlies or the Prevailing Westerly Winds blow between 35° and 60° North and South latitudes from the Sub-Tropical High-Pressure Belts towards the Sub-Polar Low-Pressure Belts. We know that the high-pressure belt is a zone of divergence for these outgoing winds. In the Northern Hemisphere, the Westerlies generally blow from the south-west to the north-east, and in the Southern Hemisphere from the north-west to the south-east. These are on-shore winds on the west coasts and off-shore winds on their east coasts. The on-shore winds bring rainfall while the off-shore winds are lacking in it. These winds are not as constant in strength and direction as the Trade Winds. They are rather stormy and variable though the main direction remains from west to east. But as their general direction is from the west, they are called the “Westerlies”. They are also known as “Anti-Trade Winds”, because their movement is in the opposite direction from that of the Trade Winds. In the Northern Hemisphere, land-masses cause considerable disruption in the westerly winds. But in the Southern Hemisphere, between 40°S and 60°S, the westerlies gain great strength and persistence because of the vast expanse of oceans in their belt. This made the mariners of old call them the “Roaring Forties”, the “Furious Fifties” and the “Screaming Sixties”. In olden days, sailing vessels had to face great danger while sailing in the opposite direction in the face of the prevailing westerly winds. It is to be rioted that the westerlies bring warmth and rainfall throughout the year to all the western coasts of the Temperate Zone. But the areas, which lies in the Mediterranean type of region, get rainfall only in winter. At that time, in December, the Mediterranean parts of Europe and California (U.S.A.) come under the influence of the westerlies and receive rainfall. In the Southern Hemisphere, in this month, the Mediterranean regions (Central Chile, Southern Africa, S.W. Australian coast) do not receive any rainfall, as they shift away from the influence of the westerlies. In June, the Mediterranean parts of the southern continents come under the influence of the westerlies and receive rainfall. At that time, the Mediterranean areas of the Northern Hemisphere do not receive any rainfall from the westerlies, because they shift away from their influence.
The winds blowing in the Arctic and the Antarctic latitudes are known as the Polar Winds. They have been termed the ‘Polar Easterlies’, as they blow from the Polar High Pressure Centres towards the Sub-Polar Low-Pressure Belts. In the Northern Hemisphere, they blow in general from the north-east, and are called the North-East Polar Winds; and in the Southern Hemisphere, they blow from the south-east and are called the South-East Polar Winds. As these winds blow from the ice-capped landmass, they are extremely cold. They are more regular in the Southern Hemisphere than in the Northern Hemisphere.
Land and sea breezes and monsoon winds are winds of a periodic type. Both are caused by the unequal heating of land and sea. Land and sea breezes occur daily, whereas the occurrence of monsoon winds is seasonal.
During the day, the greater heating of the land causes the air to ascend, causing a low pressure over land and the cool heavy air from the sea moves in to take its place. The strength of the sea breeze depends on the topography of the coast and the regions.
During the night the land cools quickly so that it is colder than the sea. A low pressure area is caused over the sea and the cooler heavier air from the land begins to flow towards the sea. The general effect of the contrast in heating of land and sea is to produce cooler winters and warmer summers in the centres of continents than along coasts.
Monsoon winds are seasonal winds. For six months they blow from land to sea, and for the other six months from sea to land. Thus, they may be regarded as land and sea breezes on a large-scale in which the period is a year instead of a day. The word ‘monsoon’ has been derived from the Arabic word ‘Mousim’, which means ‘season’. These seasonal winds greatly disturb the belted system of the earth’s permanent winds.
- Summer Monsoons
In summer the continents get more heated than the sea, hence there develops a centre of low pressure. This may be called a “heat-low” or a “thermal low”. Over the adjoining sea, the air is comparatively cool and heavy, and a high pressure area develops there. This causes the wind to blow from the sea to the land. It is the ‘Summer Monsoon’. In May, June and July, the huge plains of India and China are intensely heated by the vertical rays of the Sun. The intense heat develops a continental low pressure area over hot land-mass of Central Asia and north-western Indian Sub-continent. During these months, over the Indian Ocean, where the air is comparatively cool and heavy, a high pressure area develops. So, the winds blow from the Indian Ocean northward and north-westward into Asia, passing over Indian Sub-continent, Indo-China, southern China and south-eastern Asia. As they blow from the sea to the land, they bring heavy rainfall to South-East Asia. The summer monsoon winds blow south-west; so they are known as the ‘South-West Summer Monsoon’.
- Winter Monsoons
In winter, conditions are reversed. The continents become rapidly cool, and the air above it becomes cool and heavier. Because of this, an area of high-pressure develops over the land. Over the sea, because of the comparatively higher temperature, a centre of low pressure develops. This causes the wind to blow from the land to the sea. It is known as the ‘Winter Monsoon’. In South-East Asia, the winter monsoon travels southward and southeastward towards the Indian Ocean. As the winds blow from the land to the sea, they bring cold dry weather. They are incapable of giving rain. But sometimes these winds blow over seas and then pass over the adjoining land. In such a situation, they bring some rainfall to that area. The Southern Coromandel Coast (Tamil Nadu) in India and the Vietnamese Coast and the west coast of Japan get sufficient rainfall from winter monsoons. The winter monsoon winds blow north-east; so the monsoon is known as the ‘North-East Winter Monsoons’. Besides the South-East Asian countries, mon¬soons are also experienced in Northern Australia, Malagasy Republic, Ghana and Nigeria, but there they have modified type of monsoon winds.
Local winds are the result of a variety of causes. The mountain and valley winds follow a daily alteration of direction in a manner like the land and sea breezes. During the day, when slopes are intensely heated by the Sun, the air moves from the valleys upward over rising mountain slopes towards the summits. This is known as a Valley Wind. It decreases the cold of the areas on higher summits. At night, when the same slopes have been cooled by radiation from ground to the air, the air moves valleywards down the ground slopes and reduces the temperature of the valleys. These winds, thus, respond to local pressure gradients, caused by the heating or cooling of the ground. The Katabalic Type of local winds flow, under the influence of gravity, from higher to lower regions. In winter, a cold and dense air may accumulate over highland and flow out upon adjacent lowlands as a strong cold wind. The Bora of the northern France are well known examples of this type. Foehn and Chinook are still other types of local winds, passing over a mountain range, are forced to descend on the leeward side with the result that the air is heated and dried.