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November 4, 2013
Winds of the World: Factors affecting them, Types: Permanent Winds, Periodic Winds & Variable Winds
What are Winds?
Air flows from areas of high pressure to areas of low pressure. Horizontal movement of the air is called wind. It is nature's attempt to balance inequalities in air pressure. The vertical or nearly vertical movement of air is referred to as air current. Winds and air currents manage a system of circulation in the atmosphere.
What are the FACTORS AFFECTING DIRECTION AND VELOCITY OF WIND?
Direction and speed of wind are controlled by a combination of factors. These are the pressure gradient force, the coriolis force, altitude, latitude, rotation of earth, the centripetal acceleration and friction.
1. Pressure Gradient: We have already studied about the pressure gradient. The greater the difference in pressure between two points, the steeper is the pressure gradient and the higher is the wind speed. Since the direction of the force is from higher to lower pressure area and perpendicular to the isobars, the wind blows parallel to the gradient and at right angles to the isobars.
2. Coriolis Force: The earth rotates on its inclined axis. If it did not, 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', which deflects the air.
The deflection is the least at the Equator and greatest at the Poles. This tends to turn the flow of air by changing its direction from its original straight path. The wind starts deflecting to its right in the Northern Hemisphere. In the Southern Hemisphere it starts deflecting 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, e.g.. North East Trade winds. This is due to maximum speed of rotation at the Equator, hence the deflection is less.
3. Rotation of the Earth: The rotation of the earth causes the air at the Poles to be thrown away towards the Equator. In theory, this should result in air piling up along the Equator to produce a belt of high pressure, whilst at the Poles low pressure should develop. But what actually happens is explained under the heading Pressure Belts in this Chapter. It is to be noted that temperature
and rotation together affect the pressure pattern.
4. Altitude: Vertically, pressure decreases with increase in altitude. The rate of decrease with height is however, not constant. Many factors like the slope of the land, presence of winds and the temperature affect the pressure of air vertically. As stated above, at ground level the pressure varies between 940 mb to 1040 mb. At a height of 3000 metres, the pressure may be just 700 mb. This means the rate of decrease is 100 millibars per 1000 metres of height gained. At high altitudes, there is a cover of thin air which has both low pressure and a low capacity to hold heat. This is one of the reasons that mountaineers take oxygen supplies with them while going up high altitudes.
5. Latitude: When the temperature of a place rises, air expands; this makes the air less dense, and so its pressure decreases. When the temperature falls, air becomes dense and its pressure increases. Temperature and pressure are inversely related. It is because of high temperature that we find a belt of low-pressure around the earth at the Equator. At the Poles, because of extremely low temperature, high pressure exists.
There are in-blowing winds and uplifting air currents in the hot regions of low pressure. There are out-blowing winds and sinking air currents in the cold regions of high pressure. The uplifting air currents are called convection currents.
The more the pressure, the more is the speed of the wind. Because of the 'migration' of the sun, the heat zones are not constant. So the winds direction and speed are affected.
6. Friction: Near the earth's surface wind does not move freely in a horizontal plane due to irregularities in the earth's surface. Friction determines the angle at which the air will flow across the isobars. It may also alter wind direction. Over relatively smooth ocean surface, friction will be low and the air will move at comparatively lower angle to the isobars and hence at a greater speed. Over rugged terrain, friction will be high and therefore, the angle of the air flow will also be high and the speed much reduced.
What are the TYPES OF WINDS?
There are some winds which blow throughout the year from one latitude to the other in response to the latitudinal differences in air pressure. These are known as Permanent or Prevailing or Planetary Winds. Certain winds reverse their direction periodically with season and are called Periodic Winds. There are certain winds in different parts of the world which flow in comparatively small area and have special characteristics. These are called Local Winds.
Discuss about PERMANENT WINDS?
The Trade Winds
The winds blowing from the Sub-Tropical High Pressure area (30°N and S) towards the Equatorial Low Pressure belt are the extremely steady winds known as the trade winds.
The name trade comes from the German word trade meaning 'track'. To blow trade means 'to blow steadily in the same direction and in a constant course'.
North and South of the Equatorial Belt are the Trade Winds in 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 towards the Equator, is deflected by the earth's rotation to flow south-westward. Thus, the prevailing wand there is from the North-East, and it has been named as the 'North-East Trade'. In the Southern Hemisphere, deflection of the wind is towards the left, this causes the 'South-East Trades'.
Characteristics of Trade Winds: They blow from Sub-tropical High Pressure to Equatorial Low Pressure.
(i) Since they are warm winds, they pick up moisture and are responsible for heavy rainfall on eastern sides of tropical lands.
(ii) They are called North-East Trades in Northern Hemisphere and South East Trades in Southern Hemisphere. The winds and pressure belts move a few degrees north and south along with the movement of the overhead sun.
(iii) They have fixed velocity and are regular.
(iv) They are permanent or prevailing winds.
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 Ix)w-Pressure Belts.
In the Northern Hemisphere, the Westerlies generally blow from the south-west to the northeast, 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 wands do not bring rainfall.
Westerlies bring heavy rainfall to the eastern coasts of continents lying within the Tropics. On the western coasts of continents, these winds do not bring any rainfall. It is because here they are 'off-shore' winds or winds blowing just parallel to the shores. Therefore, 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 margins of the continents, within the tropical latitudes. Characteristics of Westerlies
(i) They blow from Sub-tropical High Pressure to Sub-polar low.
(ii) They are very strong wands, and most often blow from the western side of land mass.
(iii) They are interspersed by cyclones and cause light drizzle.
(iv) They arc stronger in the southern hemisphere as there is absence of land mass.
THE POLAR EASTERLIES
The wands that originate in the North and South Polar regions and blow towards Circum-polar Low Pressure Zone are known as Polar Winds. They start from Polar High Pressure Zone, and originate from ice capped land-mass in Arctic and Antarctic latitudes. In the Northern Hemisphere, they blow from the north-east, and are called the North- East Polar Winds; and in the Southern Hemisphere, they blow from the south-east and arc callcd the South-East Polar Winds.
They are also deflected to the west in both hemispheres and hence are known as Polar Easterlies.
Characteristics of Polar Winds
(i) They are very cold winds.
(ii) They are also referred to as Polar easterlies from the direction in which they blow.
(iii) When they blow over oceans they become warm.
WHAT ARE PERIODIC WINDS?
Periodic winds blow at regular intervals or in regular cycles. They are winds that result from localised differences in pressure and temperature. For example, land and sea breezes and the seasonal wands.
LAND AND SEA BREEZES
They are caused by differential rate of heating of the land and the sea.
In day time land gets heated faster than the adjoining sea. This creates a low pressure zone
on the land and high pressure zone over the sea. Thus the winds blow from sea to land and are called Sea Breezes. At night reverse of this happens and winds blow from land to sea and are called Land Breezes.
Monsoons are periodic seasonal winds blowing in the regions of South East Asia and Northern Australia. The word 'monsoon' is derived from the Arabic word Mausim meaning 'season'. They develop because of differences in heating conditions of the continent and the oceans. They are divided into two wind systems—the Summer Monsoon and the Winter Monsoon.
In summer the land gets more heated than the sea. Hence there develops a centre of low pressure on the land. Over the adjoining sea. the air is comparatively cool, and a high pressure develops there. This causes the winds to blow from the sea to the land. It is the 'Summer Monsoon'.
In May, June and July, the plains of India and China are heated by the vertical rays of the sun. The intense heat develops a continental low pressure. During these months, over the Indian Ocean, a high pressure area develops. So, the winds blow from the Indian Ocean northward and north-westward into 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.'
During winter season, the conditions are just reverse of those of summers. A high pressure develops over a big land mass stretching from Central Asia upto north-west Indian plain. At the same time a low pressure zone develops in the Indian Ocean. As the winds blow from the land to the sea, they bring cold dry weather. They are incapable of producing rain.
When these winds blow over seas and pass over the adjoining land, they bring some rainfall. The Southern Coromandel Coast (Tamil Nadu) in India and the Vietnamese Coast and the west coast of Japan get rain from winter monsoons. The winter monsoon winds blow north-cast; so the monsoon is known as the 'North-East Winter Monsoons'.
Local winds are restricted to a certain place only. They may be warm or cold depending upon the area from which they blow. For example, Harmattan is a hot local wind of Sahara desert.
Loo: In the plains of northern India and Pakistan, sometimes a very hot and dry wind blows from the west in summer in the afternoons. It is known as loo. Its temperature invariably ranges between 45°C and 50°C. It may cause sunstroke to people.
Foehn and Chinook: A strong warm wind develops on the leeward side of the Alps. Due to regional pressure gradient, stable air is forced to cross the barrier. As the air ascends the southern slopes of the Alps, it expands and cools. Condensation takes place when the air is saturated, causing rain and snowfall on the higher slopes. However, on descending the northern slopes, the wind experiences an increase in pressure and temperature. Due to this air is compressed and warmed. Most of its moisture is lost and it reaches the valley bottom as a dry. hot wind, called the Foehn. The temperature of the wind is from 15°C to 20°C. The wind is of use for melting snow and it hastens the ripening of grapes. Similar kind of wind in the USA and Canada move down the west slopes of the Rockies and are known as chinooks. The word chinook literally means 'snow eater'. It is beneficial to ranches
east of the Rockies as it keeps the grasslands clear from snow.
Mistral: During winter, areas adjacent to highlands may experience a local cold wind which originates over the snowcapped mountains or highlands and blows down the valley. These winds have been given local names. The most famous is the mistral that blows from the Alps over France towards the Mediterranean Sea. Even though the skies are clear, the mistral brings down the temperature below freezing point.
These local winds often have a considerable effect on climatic conditions, notably on the temperature of a place.
WHAT ARE VARIABLE WINDS?
These winds are related to pressure systems and blow in small areas. They are called variable because they do not blow in any definite direction and their direction varies with the movement of the pressure system. Their speed also depends on the intensity of the depression. They last only for a few days. Two chief types of variable winds are—Cyclones and Anticyclones.
In low latitudes, an intense depression with a low pressure centre is known as a Tropical cyclone in the Indian Ocean area, as hurricanes in the Caribbean, typhoons in China and willy-willies in Australia.
Cyclones on account of the Coriolis Force blow in an anti-clockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Cyclones are associated with turbulent weather conditions, cloudy weather, strong winds and rainfall. Tropical cyclones, thus, cause heavy damage to property and loss of human lives. The cyclones generally originate in the tropical region between 8° and 20° N and S. They are more frequent in summer because of the movement of the Doldrum belt away from the Equator. Most often they originate in the South China Sea and cause a lot of damage to life and property In the countries bordering that region. In the Bay of Bengal and the Arabian Sea, they cause great damage as they are strong winds even though their range is small.
The central part of a cyclone is a calm region and is known as Lhe eye of the cyclone surrounded by a turbulent vortex . The force of the winds depends on the intensity of the low pressure centre and the surrounding high pressure. The greater the difference, the stronger is the wind.
The weather conditions associated with a tropical cyclone:
1. When there is a tropical cyclone the air is still, but the temperature and the humidity are high. There is sudden drop in air pressure.
2. When the front of the vortex arrives, there are strong winds and thick clouds, then the winds become violent with great speed. Dense clouds and heavy rain reduce visibility.
3. The eye of the cyclone heralds a calm condition.
4. When the rear of the vortex arrives, there are violent winds, thick clouds and heavy rain. The wind blows in the opposite direction.
A TORNADO: A tornado is a tropical cyclone, which occurs over land. Its diameter is hardly 300 or 400 metres. It looks like a dark, funnel- shaped cloud extending downward from the base of the thunderstorm. It has low pressure in the centre, with very strong winds blowing inwards. The speed of the winds may be as high as 500 to 800 km per hour. Tornadoes arc generally experienced over tropical waters in North America.
Temperate Cyclones: Temperate cyclones are active over mid-latitudinal region between 35° latitude and 65° latitude in both hemispheres. They are also known as extra-tropical or wave cyclones. Unlike tropical cyclones which are produced and developed mainly over the sea, temperate cyclones are produced both on land and on sea. Whereas the tropical cyclones are limited to a small area and usually travel from east to west, the temperate cyclones occupy areas measuring a couple of thousands of kilometres and move from west to east. In a temperate cyclone, wind speed is low and all the sectors of the cyclone have different temperatures. The rainfall is slow and continues for many days.
The centre of a tropical cyclones is known as the eye and the wind is calm at the centre with no rainfall. In a temperate cyclone, there is not a single place where winds and rains are inactive. Direction of winds rapidly change at the front. The temperate cyclones are associated with anticyclones which precede and succeed a cyclone. The approach of a temperate cyclone is marked by fall in temperature, fall in the mercury level, wind shifts and a halo around the sun and the moon. A light drizzle follows which turns into a heavy downpour.
ANTICYCLONES: It is a fine atmospheric condition. There is High Pressure in the core or centre and Low pressure around it. Winds blow gently outwards. These winds are clockwise in Northern hemisphere and anticlockwise in the southern hemisphere. Since they are centres of high pressure they are found in region of descending air. currents.
Jet streams refer to the concentrated bands of rapid air movement found at the tropopause and the stratosphere, located at 10-15 kilometres above the surface of the Earth. They are formed near boundaries of adjacent air masses with significant differences in temperature, such as the polar region and the warmer air to the south. They are associated with latitudes where the poleward temperature gradient is particularly strong. Two such zones occur in each hemisphere. The sub-tropical jet stream occurs at about 30° latitude and the other, the polar front jet stream is associated with the polar front zone in each hemisphere. The major jet streams are westerly winds (flowing west to east) in the Northern Hemisphere.
During the summer, easterly jets are formed in tropical regions, typically in a region where dry air encounters more humid air at high altitudes. Low level jets can form wherever low level winds are squeezed together, typically between an oncoming front and a high pressure cell.
Jet streams transport moisture in the stratosphere, causes horizontal convergence and divergence which helps in the formation of cyclones and anti-cyclones, control monsoon rains in tropics and their vertical air motion.
Meteorologists use the location of the jet stream as an aid in weather forecasting. The main commercial use of the jet stream is in air travel, as flight time can be dramatically affected by either flying with or against the stream. One type of clear-air turbulence is found in a jet stream's vicinity, which can be a hazard to aircraft.