- Atmospheric circulation and weather systems.
NCERT Questions:
Multiple choice questions:
(i) Surface air is 1000 mb the air pressure at 1 kilometre above the surface will be:
a) 700 mb
b) 1100 mb
c) 900 mb
d) 1300 mb
Ans. c) 900 mb
(ii) The Inter Tropical Convergence Zone normally occurs:
a) Near the equator
b) Near the Tropic of Cancer
c) Near the Tropic of Capricorn
d) Near the Arctic Circle
Ans. a) Near the equator
(iii) The direction of wind around a low pressure in northern hemisphere is:
a) Clockwise
b) Perpendicular to isobars
c) Anticlockwise
d) Parallel to isobars
Ans. c) Anticlockwise
(iv) Which one of the following is the source region for the formation of air masses?
a) The Equatorial forest
b) The Himalayas
c) The Siberian plain
d) The Deccan Plateau
Ans. c) The Siberian plain
- Answer the following questions in about 30 words:
(i) What is the unit used in measuring pressure? Why is the pressure measured at station level reduced to the sea level in preparation of weather maps?
Ans. Milliabar or Pascal is the unit of measuring pressure. Most widely used unit is kilopascal expressed as kPa. Horizontal distribution of pressure is studied by drawing isobars at constant levels. Isobars are lines connecting places having equal pressure. In order to eliminate the effect of altitude on pressure, it is measured at any station after being reduced to sea level for making it competitive. The pressure measured at station level is reduced to the sea level in preparation of weather maps.
(ii) While the pressure gradient force is from north to South, i.e. from the subtropical high pressure to the equator in the northern hemisphere, why are the winds north easterlies in the Tropics?
Ans. Rotation of the earth also affects the wind movement. The force exerted by the rotation of the earth is known as the Coriolis force. Due to this effect, winds move to the right from their original direction in northern hemisphere and to the left in the southern hemisphere. The deflection is more when the wind velocity is high. The Coriolis force is directly proportional to the angle of latitude. It is maximum at the poles and is absent at the equator. The Coriolis force acts perpendicular to the pressure gradient force.
The pressure gradient force is perpendicular to isobar. The higher the pressure gradient force, the more is the velocity of the wind and the larger is the deflection in the direction of wind. As a result of these two forces operating perpendicular to each other in the low pressure areas, the wind blows around it. Therefore, when pressure gradient force is from north to south, the winds move from north east to south west by the name easterlies.
(iii) What are the geostrophic winds?
Ans.
The velocity and direction of the wind are the net results of the wind generating forces. The wind in the upper atmosphere 2 to 3 kilometre above the surface are free from frictional effect of the surface and are controlled mainly by the pressure gradient and the Coriolis force. When isobars are straight and when there is no friction, the pressure gradient force is balanced by the Coriolis force and the resultant wind blows parallel to the isobars. This wind is known as the geostrophic wind.
(iv) Explain the land and sea breezes.
Ans. During the day, the land heats up faster and becomes warmer than the sea. Therefore, over the land the air ascends giving rise to a low pressure area. Whereas the sea is relatively cool and the pressure is high.
Thus, pressure gradient from sea to land is created and the wind blows from the sea to the land which is known as sea breeze. In night, the reversal of condition takes place. The land loses heat faster and it’s cooler than the sea. The pressure gradient is from the land to the sea. This breeze is known as land breeze.
Answer the following questions in about 150 words:
(i) Discuss the factors affecting the speed and direction of wind.
Air reset in motion due to the differences and atmospheric pressure, the air in motion is called wind, which blows from high pressure to low pressure. The wind at the surface experiences friction. In addition, rotation of the earth also affects the wind movement. The force exerted by the rotation of the earth is known as Coriolis force. Thus, the horizontal winds near the earth surface respond to the combined effect of three forces, the pressure gradient force, the frictional force and the Coriolis force. In addition, the gravitational force acts downward.
The pressure gradient force: The difference in atmospheric pressure produces a force. The rate of change pressure with respect to distance is the pressure gradient. The pressure gradient is a strong where the isobars are close to each other and is week where the isobars are apart.
Frictional force: It affects the speed of the wind. It is greatest at the surface, and its influence generally extends up to an elevation of 1 to 3 km over the sea surface, the friction is minimal.
Coriolis force: The rotation of the earth on its axis affects the direction of the wind. This force is called the Coriolis force. It deflects the wind to the right direction in the northern hemisphere.
They oscillate (ITCZ) with the apparent movement of the sun in the northern hemisphere and southern hemisphere. In winter it moves southward and in summer northward.
(ii) Draw a simplified diagram to show the general circulation of the atmosphere over the globe. What are the possible reasons for the formation of subtropical high pressure over 30˚ N and S latitudes?
Ans. The general circulation of the atmosphere also sets in motion the ocean water circulation this influence the Earth’s climate. The large scale winds of the atmosphere initiate large and slow moving currents on the ocean, which in turn provide input of energy and water vapour into the air. These interactions take place rather slowly over a large part of the ocean. The air at the Inter Tropical Convergence Zone (ITCZ) rises because of convection caused by high insolation and a low pressure is created. The winds from the tropics converge at this low pressure zone. The converged air rises along the convective cell. It reaches the top of the troposphere up to an altitude of 14 km and moves towards the poles. This causes accumulation of air at about 30˚ N and S. Hence, air sinks to the ground and forms a subtropical high. Another reason for sinking is the cooling of air when it reaches 30˚ N and S latitudes.
(iii) Why does tropical cyclone originate over the seas? In which part of the tropical cyclone do torrential rains and high velocity winds blow and why?
Ans. At the equator the Coriolis force is zero and the wind blows perpendicular to the isobars. The low pressure gets filled instead of getting intensified. That is the reason why tropical cyclones are not formed near the equator.
Torrential rain occurs in the eye of the cyclone. The strong spirally circulating wind around the centre is called the eye of the cyclone. The diameter of the circulating system can vary between 150 and 250 kilometre. The eye is a region of calm with subsiding air. Around the eye there is eye wall where there is a strong spiral ascent of air to greater height reaching the tropopause, the wind reaches maximum velocity in this region reaching as high as 250 km per hour.
From the eye wall, rain bands radiate, and trains of cumulus and cumulonimbus clouds may drift into the outer region. Due to torrential rain, wind blowing from these regions is humid. It brings precipitation in oceanic regions. Due to torrential rains heavy rain take place on eastern coast of India and north east coast of China.
Question Bank:
Multiple choice questions:
- What does a cell in tropics called?
a) Polar cell
b) Hadley cell
c) Descending winds
d) Frost Cell
Ans. b) Hadley Cell
- What is tropical cyclone in China and Japan called?
a) Hurricanes
b) Typhoons
c) Tornado
d) Willy Willies
Ans. c) Typhoons
- From severe thunderstorms sometimes spiral in went descends like a trunk of an elephant with great force, with very low pressure at the centre, causing massive distraction on its way. Such a phenomenon is called:
a) Hurricanes
b) Typhoons
c) Tornado
d) Willy Willies
Ans. b) Tornado
- Circular flow of air in low pressure area is called:
a) Cyclonic circulation
b) Pascal
c) Geomorphic winds
d) Western storm
Ans. a) Cyclonic circulation
- What is tropical cyclone in Australia called?
a) Hurricanes
b) Typhoons
c) Tornado
d) Willy Willies
Ans. d) Willy Willies
- The cool air of the high plateaus and ice fields draining into the valley is called:
a) Mountain wind
b) Valley breeze
c) Katabatic wind
d) Cold front wind
Ans. c) Katabatic wind
- How is unit kilopascal represented?
a) kPa
b) kpa
c) KPA
d) Kpa
Ans. a) kPa
- What is the unit of measuring pressure?
a) Barometer
b) Pascal and mb
c) Fahrenheit
d) Isobars
Ans. b) Pascal and mb
- Near the poles, along 60˚ N and 60˚ S, the low pressure belts are termed as the:
a) Polar high
b) Subtropical highs
c) Subpolar lows
d) Equator high
Ans. c) Subpolar lows
- Which instrument is used for measuring pressure?
a) Mercury barometer or the aneroid barometer
b) Pascal
c) Mbars
d) Seismograph
Ans. a) Mercury barometer or the aneroid barometer
Very short answer type questions:
- What do you mean by extra tropical cyclones?
Ans. The system developing in the mid and high latitude beyond the tropics are called the middle latitude or extra tropical cyclones.
- How many types of fronts are there?
Ans. There are 4 types of fronts:
a) Cold
b) Warm
c) Stationery
d) Occluded
- How do front affect weather and what is their important feature?
Ans. The important feature of front is that they occur in middle latitudes and are characterised by steep gradient in temperature and pressure. They bring abrupt change in temperature and cause the air to rise to form clouds and cause precipitation.
- By which different names are tropical cyclones known?
Ans. They are known as cyclones in the Indian Ocean, hurricanes in Atlantic Ocean, typhoons in west Pacific and South China Sea, and willy willies in the Western Australia.
- What is Coriolis force?
Ans. The rotation of the Earth on its axis effects the direction of the wind. This force is called Coriolis force after the French physicist and mathematician (Gaspard Gustave de Coriolis) who described it in 1844.
- How do the horizontal winds near the earth surface respond to the combined effect of forces?
Ans. The horizontal winds near the Earth surface respond to the combined effect of three forces:
a) The pressure gradient force
b) The frictional force
c) The Coriolis force
- At what rate does pressure decreases with rise in height?
Ans. In the lower atmosphere, the pressure decreases rapidly with height. The decrease amounts to about 1 mb for each 10 m increase in elevation; it does not always increase in the same rate.
- What is low pressure system?
Ans. Low pressure system is enclosed by one or more isobars with the lowest pressure in the centre.
- What is high pressure system?
Ans. High pressure system is enclosed by one or more isobars with the highest pressure in the centre.
- What is polar cell?
Ans. At polar latitudes, the cold dense air subsides near the poles and blows towards middle latitudes as the polar easterlies. This cell is called the polar cell.
- What are the causes behind differences in atmospheric pressure?
Ans. Air expands when heated and gets accomplished when cooled. This results in variation in the atmospheric pressure.
What determines the pattern of general circulation of the atmosphere?
Ans. The 3 cells set the pattern for the general circulation of the atmosphere
a) Hadley cell
b) Ferrell cell
c) Polar cell
What is Ferrell Cell?
Ans. In the middle latitudes, the circulation is that of sinking cold air that comes from the poles and the rising warm air that blows from the subtropical high at the surface. These winds are called westerlies, and the cell is known as the Ferrell cell.
- What factors determine the pattern of planetary winds?
Ans. The pattern of planetary winds largely depends on:
a) Latitudinal variation of atmospheric heating
b) Emergence of pressure belts
c) The migration of belts following apparent path of the sun
d) The distribution of continents and oceans
e) The rotation of the Earth
Short answer type questions:
- What is a front? How many types of fronts are there? Explain all of them.
Ans. When two different air masses meet the boundaries between them is called a front. The process of formation of the fronts is known as frontogenesis. There are four types of front cold, warm, stationery occluded.
a) Cold front: when the cold air moves towards the warm air mass, its contact zone is called the cold front.
b) Warm front: if the warm air mass moves towards the cold air and remarks the contact zone is a warm front.
c) Stationary front: when the front remains stationary, it is called a stationary front.
d) Occluded front: if an air mass is fully lifted above the land surface, it is called the occluded front.
The fronts occur in middle latitudes and are characterised by steep gradient in temperature and pressure.
They bring abrupt changes in temperature and cause the air to rise to form clouds and cause precipitation.
- What are winds? Explain different types of winds.
Ans.
Wind is the flow of a huge amount of air, usually from a high pressure area to a low pressure area.
The types of winds are:
a) Primary winds: (Global wind/ Planetary wind/ Permanent wind) as the name suggests, these winds move constantly on the planet, but at times weakens or may become stronger.
b) Secondary winds (Seasonal wind/ Periodic winds /Regional winds) the wind pattern keeps on changing in different seasons due to the shifting of regions of maximum heating, pressure and wind belts.
c) Tertiary winds / Local winds: Due to the difference in the heating and cooling of earth surface and the cycles those developed daily or annually, can create several common local or regional winds.
- What do you mean by geostrophic winds? On what factors does their pattern depend?
Ans. When isobars are straight and when there is no friction, the pressure gradient force is balanced by the Coriolis force and the resultant wind blows parallel to the isobar. This wind is known as the geostrophic wind.
The pattern of these winds largely depends on:
a) Latitudinal variation of atmospheric heating
b) Emergence of pressure belts
c) The migration of belts following apparent path of the sun
d) The distribution of continents and oceans
e) The rotation of Earth
- What is air mass? In how many categories are air masses categorized on the basis of their source regions?
Ans. The air with distinctive characteristics in terms of temperature and humidity is called an air mass. In other words, it is defined as a large body of air having little horizontal variation in temperature and moisture.
The homogeneous surfaces over which air masses form are called the source regions. The air masses are classified according to the source regions into 5 major source regions. These are.
a) Warm tropical and subtropical oceans. Maritime, tropical (mT)
b) The subtropical hot deserts, continental, tropical (cT)
c) The relatively cold, high latitude oceans: Maritime Polar (mP)
d) The very cold snow covered continents in high latitudes. Continental polar (cP)
e) Permanently eyes covered continents in the Arctic and Antarctic. Continental Arctic (cA)
- What is a tropical cyclone? Give its two examples. What are the favourable conditions for its formation?
Ans. Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas, bringing about large scale destruction caused by violent winds, very heavy rainfall and storm surges. This is one of the most devastating natural calamities. Cyclones in Indian Ocean and hurricanes in the Atlantic are its biggest examples. Tropical cyclones originate and intensify over warm tropical oceans. The conditions favourable for the formation and intensification of tropical storms are:
a) Large sea surface with temperature higher than 27˚ C.
b) Presence of the Coriolis force
c) Small variation in the vertical wind speed
d) A pre-existing weak, low pressure area or low level cyclonic circulation
e) Upper divergence above the sea level system
- Explain mountain and valley winds.
Ans. In mountainous regions, during the day the slopes get heated up and air moves up slope and to fill the resulting gap, the air from the valley blows up the valley. This wind is known as the valley breeze. During the night, the slopes get cold and the dense air descends into the valley as the mountain wind. The cool air of the high plateaus and ice fields draining into the valley is called katabatic wind. Another type of warm wind occurs on the leeward side of the mountain ranges. The moisture in these winds while crossing the mountain ranges condenses and precipitates. When it descends down the leeward side of the slope, the dry air gets warmed up by adiabatic process. This dry air may melt the snow in a very short time eg. Chinook in Rockies is an adiabatic wind.
- What is atmospheric pressure? Explain about different atmospheric belts?
Or
Explain horizontal distribution of atmospheric pressure on the Earth.
Ans. The weight of a column of air contained in a unit area from the main sea level to the top of the atmosphere is called atmospheric pressure. The atmospheric pressure is expressed in units of mb. At sea level, the average atmospheric pressure is 1013.2 mb. Due to gravity, the air at the surface is denser and hence has higher pressure.
Long answer type questions:
- Write a detailed note on Tornado.
Ans. Thunderstorm is a well grown cumulonimbus cloud producing thunder and lightning. When the clouds extend to heights where sub-zero temperature prevails. Hails are formed and they come down as hail storm. If there is insufficient moisture or thunderstorm can generate dust storm. A thunderstorm is characterised by intense up draft of rising warm air which causes the cloud to grow bigger and rise to greater height. This causes precipitation. Later, downdraft brings down to earth the cool air and the rain. From severe thunderstorms, sometimes spiralling wind descends like a trunk of an elephant with great force with very low pressure at the centre causing massive destruction on its way. Such a phenomenon is called tornado.
Features:
a) Tornadoes generally occur in middle latitudes
b) The tornado over the sea is called water spouts
c) These violent storms are the manifestation of the atmospheres adjustment to varying energy distribution.
d) The potential and heat energies are converted into kinetic energy in these storms, and the restless atmosphere again returns to its stable state.
- Differentiate between vertical variation of pressure and horizontal distribution of atmospheric pressure on Earth.
Ans. Vertical variation of pressure: In the lower atmosphere the pressure decreases rapidly with height. The decrease amounts to about 1mb for each 10 metre increase in elevation. It does not always decrease at the same rate.
Horizontal distribution of pressure: Small differences in pressure are highly significant in terms of the wind direction and purposes of comparison. The sea level pressure distribution is shown on whether maps. Low pressure system is enclosed by one or more isobars with the lowest pressure in the centre. High pressure system is also enclosed by one or more isobars with the highest pressure in the centre. The vertical pressure gradient force is much larger than that of the horizontal pressure gradient. But it is generally balanced by a nearly equal but opposite gravitational force. Hence, we do not experience a strong upward wind.
- What is Coriolis force write in detail?
Ans. The rotation of the Earth on its axis affects the direction of the wind. This force is called Coriolis force after the French physicist and mathematician (Gaspard Gustave de Coriolis) who described it in 1844. It deflects the wind to the right direction in the northern hemisphere and to the left in the southern hemisphere. The deflection is more when the wind velocity is high. The Coriolis force is directly proportional to the angle of latitude. It is maximum at the poles and is absent at the equator.
The Coriolis force acts perpendicular to the pressure gradient force. The pressure gradient force is perpendicular to an isobar. The higher the pressure gradient force, the more is the velocity of the wind and the larger is deflection in the direction of wind. As a result of these two forces operating perpendicular to each other. In the low pressure areas, the wind blows around it. At the equator, the Coriolis force is zero and the wind blows perpendicular to the isobars. The low pressure gets filled instead of getting intensified.
- What factors affect direction and velocity of winds?
Ans. Air set in motion Due to the differences in atmospheric pressure, the air in motion is called when the wind blows from high pressure to low pressure. The wind at the surface experiences friction. Following factors affect the direction and velocity of winds:
a) Pressure gradient force: The difference in atmospheric pressure produces a force. The rate of change of pressure with respect to distance is the pressure gradient.
b) Frictional force: It affects the speed of the wind. It is greatest at the surface, and its influence generally extends up to an elevation of 1 to 3 km. Over the sea surface, the friction is minimal.
c) Coriolis force: The rotation of the Earth about its axis affects the direction of the wind. This force is called the Coriolis force after the French physicist and mathematician (Gaspard Gustave de Coriolis) who described it in 1844? In addition, rotation of the earth also affects the wind movement. The force exerted by the rotation of the earth is known as the Coriolis force.
d) Pressure and wind: The velocity and direction of the wind are the net result of the wind generating forces. The wind in the upper atmosphere, 2 to 3 km above the surface are free from the frictional effect of the surface and are controlled mainly by the pressure gradient and the Coriolis force. When isobars are straight and when there is no friction, the pressure gradient force is balanced by the Coriolis force and the resultant wind blows parallel to the isobar. This wind is known as geostrophic wind.
HOTS (High Order Thinking Skills)
- What are the features of El Nino?
Ans. El Nino is a complex weather system that appears once every 3 to 7 years, bringing drought, floods and other weather extremes to different parts of the world.
Features:
a) The system involves oceanic and atmospheric phenomena with the appearance of warm currents off the coast of Peru in the eastern Pacific, and affects weather in many places, including India.
b) El Nino is merely an extension of the warm Equatorial current, which gets replaced temporarily by cold Peruvian Current or Humboldt Current.
c) This current increases the temperature of water on the Peruvian coast by 10˚ C.
This results in:
i) The destruction of Equatorial atmospheric circulation
ii) Irregularities in the evaporation of sea water
iii) Reduction in the amount of planktons, which further reduces the number of fish in the sea
- Use a diagram to differentiate the direction of land breeze and sea breeze.
Land and sea breeze develop because of differential heating and cooling of adjacent land and water surface.
Water has a greater heat capacity than land. That is, land absorbs and emits radiation more efficiently and faster. Whereas on the other hand water takes longer time to absorb the heat and release the temperature. At the shoreline, the cool sea air blows inland and is known as a sea breeze.
- Describe atmospheric pressure and winds.
Ans. Atmospheric pressure is defined as the force per unit area exerted against surface by weight of the air molecules above the Earth surface. In the figure, the pressure at point X increases as the weight of the air increases. The atmospheric pressure is not distributed uniformly over the Earth. The air pressure increases or decreases according to the amount of molecules that exert the pressure on the surface. When the temperature of the air increases, the air expands and reduces the number of molecules over the unit area. It leads to reduction in pressure. Similarly, when the temperature falls, the air contracts and the pressure increase. Therefore, the temperature and atmospheric pressure are inversely related. Atmospheric pressure is measured by an instrument called barometer.
a) Vertical and horizontal distribution of atmospheric pressure: The relationship analysis between altitude and atmospheric pressure is very peculiar. The upper atmosphere is thin and less dense. The pressure at sea level is the highest and keeps decreasing rapidly with increasing altitude because of the progressive reduction of the mass above the point where it is measured.
b) Horizontal distribution of the atmospheric pressure:
When the air gets heated, it expands because light and rises vertically. As air rises, the pressure it exists on the earth surface is reduced causing a low pressure area.
On the other hand, cool air is dense and heavy. As a consequence, it sinks vertically. It results in additional weight and pressure which caused a high pressure area to occur on the ground.
Coriolis force: The rotation of the earth affects the moving objects on the earth surface. Free moving objects affected by the rotation of the earth do not follow a straight line in the northern hemisphere. They drift towards right in northern hemisphere and towards left in the southern hemisphere. The tendency is called as Coriolis force as it was discovered by Coriolis. This is the reason why rocket launching stations are located on the eastern coastal areas. Eg Sriharikota, French Guyana.
- Define the pressure belts of the earth.
Or
Delineate tricellular atmospheric model.
Ans. The atmospheric pressure belts envelope on the surface of the Earth. They are Equatorial Low pressure belt, Subtropical high pressure belt, Sub polar low pressure belts and Polar high pressure belts.
Winds system:
When the horizontal movement of air molecules move from areas of high pressure to areas of low pressure to maintain the atmospheric equilibrium the wind always moves perpendicular to isobars. If the earth did not rotate, the winds would blow in a straight path. Then the rotation of the earth results in Coriolis force and it deflects the direction of the wind. Wind direction is identified by an instrument called wind vane and wind speed is measured by anemometer.
General atmospheric circulation, pressure belts and primary wind system:
From the equator to the poles, each hemisphere has four pressure belts and totally there are seven belts on the globe. The pressure belts lead to formation of primary wind system as follows:
The Equatorial low pressure belt: (5˚ North and 5˚ South of Equator)
This is the region of calm weak and changeable winds due to the high temperature over this region, the air gets heated, expands and become lighter and rises upward and creates low pressure over the region. This region is a belt of calm and referred to as the doldrums. The winds blow from the subtropical high pressure belt towards the Equatorial low pressure belt due to Coriolis force. These winds are deflected to the right in the northern hemisphere and to the left in the Southern hemisphere. As winds are named after the direction from which they originate, they are called as the North East and South East Trade Winds. As the winds favoured trading ships, they are called as trade winds.
Subtropical high pressure belt: (25˚ to 35˚, North and South)
Air begins to cool when it reaches higher altitude over Equatorial region and flows towards the poles. This wind collides with the wind coming from the polar region at higher altitude and subsides down over subtropical latitudes. This leads to formation of high pressure belt along the subtropical region. It is said that to avoid the slowing down of ship due to high pressure, the horses were thrown into the sea. So this belt is called as horse latitude. The sinking air bifurcates into two branches towards the equator and poles. They are called as trade winds and westerlies respectively. Westerlies flow towards the pole from subtropics and turn towards right and left in northern hemisphere and southern hemisphere respectively.
Sub-polar low pressure belt: (50˚ to 60 ˚ North and South)
The warm westerlies went from subtropical region moves towards the pole and collide with the cold polar easterly winds from polar high pressure region and rises up to form sub polar low pressure belt.
Polar high pressure belt (80˚ North and South Pole)
The constant low temperature at the poles due to inclined solar radiation and reduced insolation leads to the formation of polar high pressure belt on the poles.
The high pressure on the surface always coincide with the low pressure at higher altitude, while the low pressure on the surface always coincide with higher pressure on the higher altitude. High pressure always has divergence of air masses from the centre but low pressure has convergence of air.
Basis of formation of pressure belts:
There are two important bases on which the pressure belts are formed. They are:
Temperature: The Equatorial low pressure and polar high pressure belts are formed due to high and low temperature respectively, so they are called thermally formed pressure belts. Dynamism: The subtropical high and sub polar low pressure belts are formed due to the movement and collision of wind system. So they are called a dynamically formed pressure belt system.
Meridional cell system: The cell along the trade winds Equatorial low and subtropical high pressure belts is called a Hadley cell. Meanwhile, the cell found by westerly wind along the subtropical high and Sub-polar low pressure belt is called Ferral cell. The cell at pole formed by polar easterlies with polar high and sub polar low is called a Polar cell.
ITCZ Inter-Tropical Convergence Zone:
The region where both trade winds meet is known as Inter Tropical Convergence Zone.
Shifting of pressure belts and primary wind system
These pressure belts and primary wind system are dynamic in character as they shift 5˚ N and 5˚ S from their position along with the apparent movement of the sun.
- Define secondary wind system:
Ans. Monsoon and cyclones are considered as secondary or regional wind systems.
The word monsoon is derived from the Arabic world museum, which means changing season. Monsoon winds are seasonal winds which reverse their direction due to unequal heating and cooling of the land and the water.
Mechanism of monsoon:
The land absorbs more heat energy during summer, which leads to the formation of low pressure over continent. But the ocean will have relatively lower temperature than the continent, leading to the formation of high pressure system over ocean. So the wind blows from sea to land during summer season.
Meanwhile, the land reradiates more heat energy to space during winter, leading to the formation of high pressure above the continent. But the ocean will have relatively higher temperature than the continent, leading to formation of low pressure system over ocean. So wind blows from land to sea during the winter season. This mechanism has an important effect on rainfall received over the region.
Nature of monsoon system: There are three distinct characteristics related to monsoon wind system which differentiate it from other wind systems. They are:
Reversal of wind direction between seasons:
They affect a large part of the continents and oceans
The formation of low and high pressure system over land and water and their interchange between the seasons
Monsoon system is classified into two groups based on the location. They are:
a) Asian monsoon
b) South Asian Monsoon
a) Asian Monsoon: The Asian monsoon system is divided into two components based on season it flows. The presence of high temperature with low pressure in the Lake Baikal region and low temperature with high pressure in the Ellucian Highlands region leading to flow of wind from Pacific Ocean to interior part of Asia during summer is called summer monsoon of Asia. This leads to rainfall in the East Coast of Asia.
Meanwhile, in winter, the low temperature and high pressure in the Lake Baikal region and high temperature and low pressure in the Ellusion Highlands region leading to flow of wind from Central Asia to Pacific Ocean is known as winter monsoon of Asia. As the wind system flows offshore, the rainfall does not occur in the continent of Asia accept western coast of Japan.
b) South Asian monsoon: South Asian monsoon includes the countries in the southern part of Himalayas, in India, Pakistan, Bangladesh, Sri Lanka, Maldives, Nepal and Bhutan. This monsoon system has been classified into two groups based on the direction of origin of wind, namely Southwest Monsoon and North East Monsoon.
Southwest monsoon
During summer, the Indian peninsula is heated more than the sea around it. Intense low pressure is formed in the region of Peshawar of Pakistan. At the same time, the Indian Ocean has higher pressure due to relatively low temperature. So the wind blows from Indian Ocean towards South Asia as southeast winds. The wind turns towards right, after crossing the equator, due to Coriolis force and blows as southwest winds. This brings heavy rains around four months of the year. This is known as Southwest monsoon in Indian Subcontinent. This wind system, because of the peninsular shape of India, bifurcates into two branches as Arabian Sea branch and Bay of Bengal branch.
Arabian Sea Branch: The Arabian Sea branch strikes the Western Ghats and rises over it. The orographic effect of the Western Ghats results in heavy rainfall in the west coast, windward side and low rainfall in the leeward side. So the west coast of India receives high rainfall when compared to the eastern side of the Western Ghats. Kerala is the first state to receive rainfall from South West monsoon in India which occurs during first week of June. Then the wind gradually moves towards the north of the western coast and leads to gradual development of the monsoon in parts of Karnataka, Goa, Maharashtra, Gujarat and Rajasthan. The wind further advances towards foothills of the Himalayas and creates orographic rainfall in the Himalayan states, Punjab and Haryana. The other part of the Arabian Sea branch moves towards the east and results in onset of monsoon in Uttar Pradesh and Bihar. Here it unites with the Bay of Bengal branch and leads to heavy rainfall and flood.
Bay of Bengal branch: Bay of Bengal branch flows from southwest which results in orographic rainfall in Sri Lanka and reaches Andaman and Nicobar Islands and results in orographic rainfall. Indira Point in the Great Nicobar is the first place which receives rainfall during the southwest monsoon in India during middle of the May. The wind flows parallel to the East Coast of India and Eastern Ghats. So Coromandel Coast of India does not get enough rainfall during southwest monsoon. The wind strikes Arakanyoma Mountain in Myanmar and results in heavy rainfall in western coast of Myanmar. The winds funnel towards north eastern part of India after deflected by the Arakanyoma Mountain in Myanmar. This wind strikes Meghalaya plateau which leads to heavy rainfall in Bangladesh and north eastern part of India. Mawsynram, the wettest place, (highest annual rainfall) in the world is located in the windward side of Meghalaya Plateau east of Khasi Hills.
The wind further advances towards the Himalayas where it creates heavy rainfall in the southern slopes. This leads to flood in river Brahmaputra. The wind gradually moves towards the west and results in onset of monsoon in Bhutan, Sikkim, West Bengal, Nepal and Bihar. It joins with Arabian Sea branch in Bihar and results in heavy rainfall and flood.
The southwest monsoon gradually withdraws from South Asian continent due to apparent movement of the sun towards the Southern Hemisphere. This is called a withdrawal of southwest monsoon.
Northeast monsoon
During winter, the Indian subcontinent becomes colder than the Indian Ocean. As a result, the wind blows from northeast to southwest direction. This is dry wind system and it does not produce rainfall in the coastal regions of South East Asia. Accept the Coromandel Coast of India and Sri Lanka.
- Define tertiary winds.
Ans. The tertiary winds are formed due to pressure gradient, which may develop on a local scale because of differences in the heating and cooling of the Earth’s surface. Eg land and sea breeze and mountain and valley breeze.
Local Winds
Local wind system influences the weather pattern wherever they blow. Some important local winds are:
Borra: North easterly from Eastern Europe to north eastern Italy.
Chinook: Warm, dry westerly of the Rocky Mountains.
Fohn: Warm dry southerly off the northern side of the Alps and Switzerland.
Harmattan: Dry northerly wind across central Africa.
Karaburan: ‘Black Storm’ A spring and summer katabatic wind of Central Asia.
Khamsin: South easterly from North Africa to the Eastern Mediterranean.
Loo: Hot and dry wind which blows over plains of India and Pakistan.
Mistral: Cold northerly from central France and the Alps of Mediterranean.
Nor’easter: Strong winds from the North East in the south eastern United States, especially New England.
Nor’wester: Wind that brings rain to the West Coast and warm dry winds to the East Coast of New Zealand, South Island caused by the moist prevailing winds being uplifted over the Southern Alps, often accompanied by distinctive arched cloud pattern.
Pampero: Argentina, very strong wind which blows in the Pampas.
Simoon: Strong drive desert wind that blows in the Sahara, Israel, Jordan, Syria and the desert of Arabia.
Sirocco: Southerly from North Africa to Sahara and southern Europe.
Zonda wind: On the eastern slope of the Andes in Argentina.
- What are jet streams? Describe their impact.
Ans. Jet streams are high altitude westerly wind system blows at a height of 6 to 14 km with a very high speed of up to 450 km per hour. In wavy form in both the hemisphere as they encircle the poles, they are called as circumpolar wind system. Although the jet streams flow at higher altitude, they also influence the surface weather pattern of the Earth.
Major impacts of jet stream:
Creation of polar vortex: Polar westerly jet stream will carry cold polar air masses towards temperate region, which creates severe cold waves in North America and Eurasia during winter.
Sudden burst of southwest monsoon: Southern withdrawal of polar westerly jet stream from Indian subcontinent to north part of Pamir leads to sudden burst of southwest monsoon into Indian subcontinent.
Late and early monsoon in South Asia: Rate of withdrawal of polar westerly jet stream decides the onset of southwest monsoon. Slower and faster rate of wind withdrawal leads to late and early onset of southwest monsoon.
Intensity of monsoon rainfall: The arrival of tropical easterly jet stream influences the intensity of South West monsoon. This leads to increasing intensity of rainfall during South West monsoon.
Bringing rainfall to India by Western disturbances: Polar westerly jet stream carries rainy clouds from cyclones formed over Mediterranean Sea during winter towards India. These clouds pile up on the Himalayas and results in rainfall over the states of Punjab and Haryana. This also assists in the cultivation of wheat in India.
Development of super cyclone: The condition at which the speed of the jet stream is transferred to tropical cyclones that leads to development of super cyclone.