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Summary
The air we breathe is part of a large blanket of gases that surrounds our planet. This blanket is called the atmosphere. It is held close to the Earth by gravity. The atmosphere is mostly made of two gases: nitrogen and oxygen. It also has small amounts of other gases, water vapor, and tiny dust particles. This air is very important because it gives us oxygen to survive, protects us from the sun’s harmful rays, and creates our weather. Without the atmosphere, life on Earth would not be possible.
The atmosphere has different layers, much like an onion. The lowest layer, where we live, is the troposphere. This is where all our weather, like clouds and rain, happens. As you go higher in this layer, the air gets colder. Above it is the stratosphere, which contains a special layer of ozone gas that shields us from the sun’s dangerous ultraviolet radiation. Airplanes often fly in this calm layer. Higher still are the mesosphere, the very hot thermosphere, and the exosphere, which is the final layer before outer space.
Air has weight, and the force of this weight pressing down on the Earth’s surface is called atmospheric pressure. Cold, heavy air creates high pressure, while warm, light air creates low pressure. This difference in pressure causes air to move. The movement of air from a high-pressure area to a low-pressure area is what we call wind. The greater the pressure difference, the stronger the wind blows.
Around the world, there are large areas of high and low pressure that form distinct belts. Near the equator, there is a low-pressure belt known as the Doldrums, where the air is often very still. In other regions, there are high-pressure belts called the Horse Latitudes. This name comes from old stories of sailing ships getting stuck in these calm winds, forcing sailors to lighten their load, sometimes by putting horses into the sea. These pressure belts create the world’s main wind patterns, such as the Trade Winds and the Westerlies. The spinning of the Earth causes these winds to curve instead of moving in a straight line.
Besides these large global winds, there are also smaller wind systems. Cyclones are powerful, rotating storms that form around a center of very low pressure. Monsoons are seasonal winds that change direction and often bring heavy rain to certain parts of the world. There are also local winds that affect smaller areas. For example, along the coast, a cool sea breeze blows from the sea to the land during the day, while at night, a land breeze blows from the land out to the sea. This happens because land heats up and cools down faster than water.
Exercise
1. What is atmosphere? Write in brief with data about its structure.
Answer: The gaseous uninterrupted envelope of air surrounding the earth from its surface up to some height is known as the Atmosphere. It is the ocean of air surrounding the earth. The atmosphere extends vertically up to an altitude of around 10,000 km from the earth’s surface and remains attached to the earth due to the earth’s gravitational force.
The structure of the atmosphere is composed of various colourless gases, water vapour, and dust particles.
- Gaseous Composition: Of the total volume of atmospheric gases, nitrogen constitutes 78.08 per cent and oxygen 20.94 per cent. Other gases include argon (0.93 per cent), carbon dioxide (0.036 per cent), and neon, helium, methane, krypton, hydrogen, ozone, xenon, etc. (0.014 per cent).
- Layers by Chemical Composition: Based on chemical composition, the atmosphere is broadly divided into two major layers: Homosphere and Heterosphere. The Homosphere extends up to an altitude of about 80 km, and the chemical composition of gases is almost the same throughout this layer. The Heterosphere is the layer above 80 km, where there are considerable chemical changes in the atmospheric gases.
- Layers by Temperature: The atmosphere is also divided into layers based on temperature variations with altitude. These are the Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere.
2. What are the main layers of the atmosphere based on chemical composition of the gases? Write briefly about the characteristics of these layers.
Answer: Based on the chemical composition of gases, modern climatologists have broadly divided the atmosphere into two major layers: Homosphere and Heterosphere.
- Homosphere: This is the lower atmospheric layer where the chemical composition of atmospheric gases is almost the same up to an altitude of about 80 km from the earth’s surface. About 99 per cent mass of all the atmospheric elements is confined to an altitude of 32 km within this layer. The Homosphere is further sub-divided serially from the earth’s surface as Troposphere, Stratosphere, and Mesosphere.
- Heterosphere: This is the layer of the upper atmosphere above 80 km altitude, which has a quite different condition due to considerable chemical changes in the atmospheric gases. This layer has created four sub-layers with respect to the atomic weight of gases: the nitrogen layer (from 80 km to 200 km), the oxygen layer (up to 1125 km), the helium layer (up to 3540 km), and the hydrogen layer (up to about 10,000 km). These layers fall under the Thermosphere and Exosphere, and the air density is too negligible.
3. Write with diagram about the layers of the atmosphere based on the variations in altitude and temperature and write the characteristics of each layer in brief.
Answer: Generally, the structure and characteristics of the atmosphere are studied by dividing it basically into five layers based on variations in altitude and temperature: troposphere, stratosphere, mesosphere, thermosphere, and exosphere.
- Troposphere: This is the lowermost layer touching the earth’s surface, with an average height of 12 km. Its height is 16 km in the equatorial region and 8 km in the polar region. This layer contains about three-fourths of total atmospheric gases and almost the entire water vapour and dust particles. The temperature gradually decreases with an increase in height at a rate of 6.5°C per km, which is called the Lapse Rate. Almost all weather phenomena like clouds, rains, and storms occur here. The highest limit is the Tropopause, where the temperature reaches -60°C.
- Stratosphere: This layer is immediately above the troposphere. No weather phenomena, including cloud formation, are visible in this layer. The air temperature gradually increases with height, from about -60°C in the lower part to about 0°C in the upper part. A layer of ozone gas is present here, which absorbs the sun’s harmful ultraviolet radiations, causing the temperature to rise. It extends for about 40 km from the tropopause, and its topmost boundary is the Stratopause.
- Mesosphere: This layer is just above the stratopause and extends for a height of 30 km. Its main characteristic is the decrease of air temperature with an increase in altitude. The temperature reaches a minimum of -100°C at its uppermost boundary, the Mesopause, at about an altitude of 80 km.
- Thermosphere: This layer is just above the mesopause and extends from 80 km up to 400 km. The air temperature again increases with altitude, rising to about 1650°C, while the air density decreases to about zero. This layer contains electrically charged ion particles and is also known as the Ionosphere. It reflects radio waves sent from the earth’s surface.
- Exosphere: This is the topmost layer, immediately above the thermosphere, extending from a height of 400 km up to 10,000 km. The air in this layer is very thin due to the dominance of only hydrogen and helium gases. The air temperature at this level rises to about 5550°C.
4. Discuss with examples the factors responsible for variation in atmospheric pressure.
Answer: The factors of variation in atmospheric pressure are mainly two: air temperature and the earth’s surface height.
- Air Temperature: The distribution of heat on the earth’s surface is not the same everywhere. With a rise in temperature, air expands, its density decreases, and it becomes light, resulting in low air pressure. For example, in the equatorial region, the temperature is high, so the air pressure is low. Conversely, cold air has low moisture absorption capability, and dry air with less moisture has high air pressure. For instance, in the cold polar regions, the air pressure is quite high. That is why air pressure increases away from the equatorial region towards both the polar regions.
- Altitude from the Earth’s Surface: As we go higher up from the earth’s surface, the atmospheric depth, or the amount of gas present, decreases. With the decline in the depth of the atmosphere, its weight declines, and as a result, air pressure also becomes low. This means atmospheric pressure decreases with an increase in altitude. For example, places with low altitude have relatively higher atmospheric pressure than places at high altitudes. With a rise in altitude of every 900 feet, there is a consequent decrease in air pressure by 1 inch or 34 millibars.
5. With the help of diagram discuss the characteristics of the major pressure belts of the world.
Answer: On the basis of relative difference in atmospheric pressure, mainly four pressure belts are identified in the world.
- (a) Equatorial Low Pressure Belt: This belt lies between 10° North and 10° South latitude. Here, the sun’s rays fall vertically almost throughout the year, making the air warm, light, and low in density, which results in low pressure. The air in this region also contains more water vapour, which keeps the air pressure low. In this belt, the heated air moves up, and there is no horizontal movement of air, so the air remains in a calm condition. This region is known as the Equatorial Doldrum.
- (b) Sub-Tropical High Pressure Belt: This belt is located in two zones: 25°-35° North latitudes and 25°-35° South latitudes. The rising moist and light air from the equatorial belt moves north and south, becomes cold and heavy, and descends in these zones. Cold and heavy air from the polar regions also moves to these belts. This mixing of cold and heavy air forms high-pressure belts. The air movement is mainly downward, so wind is not felt here. This region is known as the Horse Latitude.
- (c) Sub-Polar Low Pressure Belt: This belt is located roughly in 60°-70° North and 60°-70° South latitudes. The earth’s rotational movement is more in these belts than at the poles. As a result, the air of the sub-polar regions deflects towards sub-tropical regions, and low pressure is formed there following a decline in the volume of air.
- (d) Polar High Pressure Belt: Due to a lack of direct sunlight, both polar regions are snow-covered and very cold. The air is too cold and contains almost no water vapour. As a result, air pressure remains permanently high in both the polar regions. This part of the earth is known as the Polar High Pressure Belt.
6. Write the importance of atmosphere towards creation of a favourable physical environment on the earth.
Answer: The atmosphere is a very important element of the earth’s physical environment. Along with land and water, it has made it possible to create a favourable environment for the origin, growth, and sustenance of living creatures on earth, which is known as the Biosphere.
The atmosphere provides gases essential for humans, plants, and animals. It helps in the distribution and circulation of heat and moisture on the earth’s surface. The maintenance of a well-balanced proportion of gases in the atmosphere helps a congenial temperature to prevail. The movement of air helps many places get rid of excessive cold and hot conditions. The atmosphere obstructs the sun’s very harmful ultraviolet radiations from reaching the earth. The ozone gas present at an altitude of about 40-50 km saves the entire living world by absorbing these dangerous radiations.
The formation and change of weather and climate also take place in the atmosphere. Because of the atmosphere, rainfall occurs, and plants and animals thrive. Water vapour in the lower atmosphere absorbs solar radiation and maintains a balanced temperature, so it neither becomes too cold nor too hot. Dust particles floating in the lower atmosphere are also important, as water vapour condenses on these tiny particles to form clouds and give rain. The existence of life on earth cannot be imagined without the atmosphere.
7. What is wind? Discuss the factors of origin of wind.
Answer: The air when it moves is called wind. It means the state of air in motion is wind. Generally, this air movement takes place parallel to the earth’s surface. As per the definition of Meteorology, when air moves parallel to the earth’s surface, it is known as wind.
The origin of wind basically depends on the temperature difference of the atmosphere and its closely associated atmospheric pressure. The earth’s surface is not heated uniformly by the sun’s rays. When the air temperature of a place increases, the air becomes lighter and moves up, and a low pressure is formed at that place. As a result of this, air movement takes place from a high-pressure region, characterized by relatively low temperature, to this low-pressure region. This means that the pressure gradient resulting from the pressure difference starts the movement of air. This air movement always takes place from a relatively high-pressure region to a low-pressure region.
8. What is the most important reason for the origin of wind? Briefly discuss the factors determining velocity and direction of wind.
Answer: The most important reason for the origin of wind is the temperature difference of the atmosphere and its closely associated atmospheric pressure difference. Wind is formed mainly due to this difference and moves from a high-pressure region to a low-pressure region.
The principal forces or factors that determine the direction and velocity of wind are:
- Pressure Gradient Force: This force is generated from the atmospheric pressure difference on the earth’s surface and it basically results in the movement of air. The pressure gradient force determines the wind’s direction, as air always moves from a high-pressure region to a low-pressure region. It also determines the wind’s velocity; the velocity becomes greater if the pressure difference between two places is more, or if the pressure gradient is steeper.
- Gravitational Force: The earth’s gravitation acts as a driving force in air movement. It pulls the envelope of the atmosphere, and the weight or pressure of air depends on this force. It causes a vertical movement of air from high pressure to low pressure as altitude increases. Additionally, due to the gravitational force, the movement of air from one place to another becomes curved instead of a straight line.
- Centrifugal Force: Due to the earth’s rotation on its axis, an outward force is generated from its centre, known as Centrifugal Force or Coriolis Force. This force causes a slight deflection in the wind’s direction. It deflects the wind towards the right in a clockwise direction in the northern hemisphere and towards the left in an anticlockwise direction in the southern hemisphere.
- Frictional Force: This force acts negatively in determining wind velocity. The amount of frictional force depends on the nature of the surface over which the wind blows. A rugged terrain increases friction and lowers wind velocity. Frictional force not only slows down the wind velocity but also changes the wind direction to some extent.
9. What do you mean by ‘Coriolis Force’? With the help of diagram briefly describe its contribution in determining the direction of wind.
Answer: Due to the earth’s rotation on its axis, an outward force is generated from its centre. This is called Centrifugal Force. This phenomenon was first discovered by a French mathematician Gasperd de Coriolis in 1844, and accordingly, this force is known as ‘Coriolis Force’.
When wind blows from a high-pressure region to a low-pressure region, the Coriolis force slightly changes the wind direction. It deflects the wind towards the right in a clockwise direction in the northern hemisphere and towards the left in an anticlockwise direction in the southern hemisphere. This phenomenon is known as Ferrell’s Law. The amount of the force is zero on the equator and gradually increases towards the poles, where it becomes the highest. This means that, except for the equatorial region, the earth’s rotation influences the air movement of any place on the earth.
10. What is meant by ‘Pressure Gradient Force’? What is its contribution in air movement.
Answer: The force generated from the atmospheric pressure difference on the earth’s surface that basically results in the movement of air is known as the Pressure Gradient Force. The rate of atmospheric pressure difference between two places is called the pressure gradient.
The contribution of the Pressure Gradient Force in air movement is that it starts the movement of air. This air movement always takes place from a relatively high-pressure region to a low-pressure region, which is in the direction of the pressure gradient. The pressure gradient force also determines the wind’s velocity; the velocity of the wind becomes more if the pressure difference between the two places is greater or the pressure gradient is steeper. The pressure gradient between two places also determines the wind direction.
11. Briefly discuss the contribution of the earth’s gravitational force in air movement.
Answer: The earth’s gravitational force acts as a driving force in air movement. The envelope of the atmosphere surrounding the earth is pulled by the earth’s gravitation. The weight or pressure of air also basically depends on this gravitational force. As the gravitational force pulling the air decreases with increasing altitude, the air becomes lighter and air pressure falls. This causes a vertical movement of air in the atmosphere from high pressure to low pressure. Furthermore, it is due to the gravitational force that the movement of air from one place to another becomes curved instead of a straight line.
12. What do you mean by naming of wind? How the wind blowing over a place is named?
Answer: The naming of wind is done based on wind direction. The name of a wind is given according to the direction from which the wind blows. For instance, the wind that blows from the west direction is called a westerly wind, and a wind from the north-east is a north-easterly wind. On the earth’s surface, the direction from which wind comes is called Windward, and the direction to which it blows is called Leeward.
13. How the velocity of wind is determined? What are the units of wind velocity?
Answer: The velocity of wind is measured with the help of an instrument called an Anemometer. Nowadays, with an instrument called an Anemograph, the direction and velocity of wind are automatically recorded. A British Scientist, Sir Francis Beaufort, also developed a 0-12 Number Scale of wind velocity in 1805 to understand the nature and impact of different types of wind, which is known as the Beaufort Scale.
The velocity of wind is expressed in Knot. The wind velocity of 1 Knot means 1 nautical mile per hour, which is 1.854 Kilometre per hour or 30.9 meters per minute.
14. How the classification of wind is done? Briefly discuss with examples.
Answer: Mainly based on the spatial extent, the winds can be broadly divided into three groups: (1) Primary Circulation; (2) Secondary Circulation; and (3) Tertiary Circulation or Local Winds.
Primary Circulation is the air movement covering the whole earth based on the distribution of permanent pressure belts on the earth’s surface. Examples of this circulation include trade winds, westerlies, and polar easterly winds.
Secondary Circulation is the wind circulation that results from variations in landforms, the distribution of land and water bodies, and regional and seasonal variations in temperature and pressure. Examples of secondary circulations are cyclones, anticyclones, air masses, fronts, and monsoons.
Tertiary Circulation, or Local Wind, is the wind circulation that takes place in a limited area depending on purely local factors like topographic and altitude variations. Examples of local winds include sea breeze, land breeze, mountain wind, and valley wind.
15. What do you mean by primary circulation of wind? Show distribution of primary circulations in a diagram. Mention its important characteristics.
Answer: The air movement covering the whole earth based on the distribution of permanent pressure belts on the earth’s surface is called Primary Wind Circulation. It is also known as Permanent Wind or Planetary Wind.
The primary circulation pattern prepares the necessary environment for other lower-order circulation patterns. Such permanent planetary circulations continue uninterruptedly from the sub-tropical and polar high-pressure belts to the equatorial and sub-polar low-pressure belts in both hemispheres. The primary wind system includes trade winds, westerlies, and polar easterly winds.
Trade winds are surface winds that blow from the sub-tropical high-pressure belts towards the equatorial low-pressure belt. Due to the Coriolis force, this wind blows from the north-east to the south-west in the northern hemisphere (North-East Trade Wind) and from the south-east to the north-west in the southern hemisphere (South-East Trade Wind). Where these winds meet in the equatorial region is known as the Inter-Tropical Convergence Zone, which is also called a calm zone or Doldrum because the air movement is weak and still.
Westerlies are permanent winds that blow from the sub-tropical high-pressure belt towards the high-latitude low-pressure belt in the sub-polar region. The Coriolis force deflects this wind from the south-west to the east and north-east in the northern hemisphere, and from the north-west to the east and south-east in the southern hemisphere. Compared to trade winds, westerlies are more variable and intense.
Polar Wind is the normal air circulation from the polar high-pressure region to the sub-polar low-pressure region. As this wind blows from the east in both hemispheres, it is also known as polar easterlies.
16. What is secondary circulation of wind? Discuss its contribution in determining the climate of a place with examples.
Answer: The wind circulation that results in different parts of the earth due to variation in the nature of landforms, differences in the distribution of land and water bodies, and regional and seasonal variation in temperature-pressure is known as Secondary Circulation. These winds bring about significant weather changes in an area and sometimes make the atmospheric condition very unstable. Examples include cyclones, anticyclones, air masses, fronts, and monsoons.
An air mass determines the weather of the area through which it passes. When a divergent air mass moves, it not only changes the weather of the area but also experiences changes in its own temperature and humidity.
Fronts are formed when two different air masses meet, and weather changes take place in such fronts. The progress of a front brings about changes in air temperature and humidity and results in the formation of clouds.
Cyclones cause significant weather events. Tropical cyclones are violent and destructive, causing huge loss of lives and property. Extra-tropical cyclones, found in temperate regions, often cause dense clouds, heavy rainfall, hail, and thunderstorms, making the weather unstable.
Anticyclones are generally associated with fair weather. However, high-latitude cold anticyclones can result in the movement of extremely cold wind, known as a ‘cold wave’. Winds from sub-tropical anticyclones can result in hot waves.
Monsoon winds cause seasonal changes in climate. In summer, moist winds from the ocean bring heavy rainfall to land surfaces. In winter, dry and cold winds blow from the land to the ocean, and rainfall generally does not occur from such dry winds.
17. What is local wind? Briefly discuss with examples how local wind determines the weather condition of a place.
Answer: The wind circulation that takes place in a limited area depending on purely local factors, such as topographic variation and altitude variation, is known as Local Wind or Tertiary Circulation. It is also called temporary air movement. The impact of this local wind is quite notable in determining the weather of an area, though its zone of influence is very limited.
Examples of local winds and their effects on weather are:
Land Breeze and Sea Breeze: In coastal areas, the wind that blows from the sea to the land during the day is the Sea Breeze, and the wind that blows from the land to the sea at night is the Land Breeze. These create daily variations in local weather.
Mountain and Valley Wind: In foothill areas, the wind that blows from the valley up the mountain slopes during the day is the Valley Wind. At night, the wind moves down from the highland to the valley, which is called the Mountain Wind. Because of these winds, fogs are generally not seen in the morning and evening, even during winter, in these regions.
Other local winds bring marked changes in weather conditions locally. For example, the Chinook wind in the U.S.A. is a warm, dry wind that causes snow-covered plains to become free of ice due to melting. The Sirocco is a hot, dry southerly wind from the Sahara desert that badly affects human skin, soil, and plants, and often creates dust storms. The Mistral is a dense, cold wind in south-west Europe that moves towards the Mediterranean Sea in winter.
18. What is a cyclone? What are its types? Mention briefly how it influences the climate of a place?
Answer: A cyclone is a state of atmospheric circulation in which high-velocity wind takes a cyclic path around a low-pressure centre. As the atmospheric pressure is much higher in the areas around the low-pressure centre, the wind velocity becomes very high.
Mainly, the cyclone is of two types: (1) Tropical Cyclone and (2) Mid-Latitude or Extra-Tropical Cyclone.
Cyclones influence the climate of a place significantly. Tropical cyclones are violent, destructive, and harmful. They release a very high amount of energy and remain active for a long period, covering a large area. They cause huge loss of lives and properties in regions like the south-eastern part of North America, the eastern part of Japan, and the east coastal region of India.
Extra-Tropical Cyclones cause the temperate region to often witness changes in weather with unstable atmospheric conditions. These cyclones can form at any time of the year and often cause dense clouds and heavy rainfall. When the air in the higher layer of such cyclones becomes very cold, it often results in hail and thunderstorms. After some time, the sky becomes clear, but due to mild air movement, it remains somewhat cold.
19. What is an air mass? How is it classified? What is its contribution in determining the climate of a place?
Answer: An immense body of air having homogenous temperature and humidity conditions is called an air mass. When the air in a wide area remains stable for a considerably long period of time, its lower layer is influenced by the underlying land surface, and in the process, this air absorbs the prevailing temperature and humidity character of the surface layer.
Based on the source region and associated characteristics, an air mass can be mainly classified into four types:
- Tropical Continental Air mass (cT)
- Tropical Maritime Air mass (mT)
- Polar Continental Air mass (cP)
- Polar Maritime Air mass (mP)
These air masses can be further divided depending on their temperature and degree of dynamism.
A diverging air mass determines the weather of the area through which it passes. The movement of a dynamic air mass causes the conduction of heat from a warm region to a cold region, thereby maintaining an equilibrium in atmospheric temperature. When a divergent air mass moves, it not only changes the weather of the area but also experiences a change in its own temperature and humidity conditions.
20. What do you mean by ‘Front’? How it is formed? Briefly discuss the relationship between climate and front.
Answer: When two different air masses with sharp contrasts in temperature, humidity, pressure, and density come closer from opposite directions, they do not mix readily and form an air surface discontinuity or line of discontinuity between them. This is called a Front.
The formation of a front requires two special conditions of the two air masses:
- Between the two air masses, one should be colder and heavier than the other.
- The air movement should be convergent so that the two air masses from opposite directions move closer to one another.
As a result of the meeting of two characteristically contrasting air masses, the comparatively warm, lighter air comes over the cold, heavier air and maintains a state of equilibrium.
Weather changes take place in such fronts. The progress of a front brings about changes in air temperature and humidity and results in the formation of clouds in the frontal region.
21. What do you mean by ‘Monsoon Wind’? How it is formed? In which areas of the earth its impact is quite distinct?
Answer: The wind that blows periodically depending on the change in the season is called Monsoon Wind. It is a kind of surface wind whose direction of movement in summer is completely opposite of winter. It is basically a regional air circulation of the tropical and sub-tropical regions.
The change in season leads to differences in temperature and pressure, which result in a change in the direction of the monsoon wind. This is due to the differential capacity of land and water surfaces to absorb and release heat, causing a pressure difference between them. In summer, the land surface temperature rises, leading to low pressure on land and high pressure over the water surface. Consequently, cold and moist air from the oceanic high-pressure region moves towards the land. In winter, the direction changes because the water surface has a higher temperature than the land surface, creating low pressure over the water and high pressure on land. This causes dry and somewhat cold wind to blow from the continental high-pressure region to the water surface.
The impact of monsoon circulation is quite distinct and forceful in Asia, particularly in its south and south-eastern parts. The areas of the earth which are surrounded by oceans, such as South Asia, South-East Asia, and South-East China, largely witness monsoon circulation. It is also prevalent in other parts of the world, including South Africa, the south-east and south-west coasts of the USA, the Gulf region of Mexico, and north-east Australia.
22. Write with reasons:
(a) Why do all weather phenomena occur mainly in troposphere?
Answer: All weather phenomena, such as clouds, rains, cyclones, and storms, occur mainly in the troposphere because this layer alone contains about three-fourths of the total atmospheric gases and almost the entire water vapour and dust particles.
(b) What would be the consequence of the increase in the amount of carbon dioxide?
Answer: Due to the increased use of fossil fuels and large-scale deforestation, the amount of atmospheric carbon dioxide is gradually increasing. As a consequence, the atmospheric temperature is also rising. This phenomenon is known as Global Warming.
(c) What is the reason behind the prevalence of high pressure of dry air than moist air?
Answer: The pressure of moist air is low because warm air, which has a higher capability for moisture absorption, becomes moist and light. In contrast, cold air has a low moisture absorption capability, and the dry air with less moisture has high air pressure.
(d) How does the ozone layer help the living-beings?
Answer: The ozone gas present in the atmosphere at an altitude of about 40-50 km saves the entire living world by absorbing the sun’s ultraviolet radiations, which are dangerous to both plants and animals.
(e) Why is the air pressure highest at the sea level?
Answer: As one goes higher up from the earth’s surface, the atmospheric depth, which is the amount of gas present in the atmosphere, decreases. With the decline in the depth of the atmosphere, its weight declines, and as a result, air pressure also becomes low. This means atmospheric pressure decreases with an increase in altitude. Hence, places with low altitude, like the mean sea level, have relatively higher atmospheric pressure.
(f) What is the reason behind variation in vertical extent of the troposphere in the polar region and equatorial region?
Answer: The vertical extent of the troposphere in the equatorial region is 16 km and in the polar region it is 8 km. It varies due to the variation in latitudinal extensions of the equatorial region and the polar region.
(g) Why is horizontal movement of wind parallel to the earth’s surface not felt in the equatorial low pressure belt?
Answer: In the equatorial low pressure belt, the air, after getting heated, moves up. Under this circumstance, no horizontal movement of air parallel to the earth’s surface is visible. Instead, the air in this belt remains in a calm condition, which is why this region is known as the Equatorial Doldrum.
23. Give short answer to the following questions:
(a) What is the vertical extent of the atmosphere?
(b) Up to what altitude from the earth’s surface does chemical composition of gases remain almost same?
(c) What is the name of the boundary line between homosphere and heterosphere?
(d) What is Ferrell’s Law?
(e) What is Beaufort Scale? Write briefly about the utility of this scale withexample.
(f) Write in brief about Lapse Rate.
Answer: (a) The vertical extent of the atmosphere is 10,000 km.
(b) The chemical composition of atmospheric gases is almost same up to an altitude of about 80 km from the earth’s surface.
(c)The name of the boundary line between the Homosphere and the Heterosphere is the Ionosphere.
(d) The force generated due to the earth’s rotation deflects the wind towards right in clockwise direction in the northern hemisphere and towards left in the
anticlockwise direction in the southern hemisphere. This phenomenon is known as Ferrell’s Law.
(e) A British Scientist Sir Francis Beaufort developed a 0-12 Number Scale of wind velocity in 1805 to understand the nature and impact of different types
of wind. This is known as Beaufort Scale.
(f)The temperature in the lower part of troposphere is more and it gradually decreases with increase in height. Generally, this air temperature decrease takes
place at the rate of 6.5°C per km ascent. This is called Lapse Rate.
24. Write short note :
Answer: (a) Homosphere: The lower atmospheric layer extending up to an altitude of about 80 km from the earth’s surface is called the Homosphere. This name is given because the chemical composition of atmospheric gases is almost the same throughout this layer. About 99 per cent of the mass of all atmospheric elements is confined to an altitude of 32 km within the Homosphere. Based on characteristic variations with altitude, the Homosphere is further sub-divided into three layers, starting from the earth’s surface: the Troposphere, Stratosphere, and Mesosphere.
(b) Heterosphere: The layer of the upper atmosphere above 80 km altitude is known as the Heterosphere. It is characterized by considerable chemical changes in atmospheric gases and conditions that are quite different from the lower atmosphere. This layer is composed of four sub-layers based on the atomic weight of the gases present. These are the nitrogen layer (from 80 km to 200 km), the oxygen layer (up to 1125 km), the helium layer (up to 3540 km), and the hydrogen layer (up to about 10,000 km). These layers fall under the broader categories of the Thermosphere and Exosphere. The air density in the Heterosphere is too negligible.
(c) Air temperature, earth’s surface height and air pressure relationship: The main factors causing variation in atmospheric pressure are air temperature and the height of the earth’s surface. When air temperature rises, the air expands, and its density decreases, making it lighter. The pressure of such light air is also low. Therefore, a rise in air temperature results in a decline in air pressure. As we go higher up from the earth’s surface, the depth of the atmosphere, or the amount of gas present, decreases. This decline in atmospheric depth leads to a decline in its weight, and as a result, air pressure also becomes low. This means atmospheric pressure decreases with an increase in altitude. Consequently, places at low altitudes have relatively higher atmospheric pressure. For every 900 feet rise in altitude, there is a decrease in air pressure by 1 inch or 34 millibars.
(d) Polar High Pressure Belt: Due to the lack of direct sunlight, both polar regions of the earth are snow-covered and very cold. The air in these regions is also too cold. Because of the extreme cold, the water in this region remains deposited as ice, and the air contains almost no water vapour. As a result, air pressure remains permanently high in both polar regions. This part of the earth is known as the Polar High Pressure Belt.
(e) Wind system: Wind is an important medium that helps transfer heat, moisture, and other physical characteristics of the atmosphere from one place to another. In a broad sense, this is known as the wind system. The nature and dynamics of global wind systems are dependent on the earth’s pressure belts. The wind system is broadly divided into three categories:
- Primary Circulation: This includes permanent or planetary winds like trade winds, westerlies, and polar easterlies, which cover the whole earth based on permanent pressure belts.
- Secondary Circulation: This circulation results from variations in landforms, the distribution of land and water, and regional temperature-pressure changes. It includes phenomena like cyclones, anticyclones, air masses, fronts, and monsoons.
- Tertiary Circulation: Also known as local winds, this circulation occurs in a limited area due to purely local factors. It includes sea breeze, land breeze, mountain wind, and valley wind.
(f) Pressure Gradient: The rate of atmospheric pressure difference between two places is called the pressure gradient. The force generated from this pressure difference on the earth’s surface results in the movement of air and is known as the Pressure Gradient Force. The pressure gradient determines both the direction and velocity of the wind. Air always moves from a high-pressure region to a low-pressure region, which is the direction of the pressure gradient. The velocity of the wind is higher if the pressure gradient is steep (meaning a large pressure difference over a short distance). Conversely, wind velocity decreases as the distance between the places increases. In an area where the pressure gradient is very low, wind movement is almost zero.
(g) Trade wind: The surface wind that blows from the sub-tropical high-pressure belts, located around 30° North and 30° South latitudes, towards the equatorial low-pressure belt is known as the Trade Wind. Due to the Coriolis force, this wind blows from the north-east to the south-west in the northern hemisphere (called the North-East Trade Wind) and from the south-east to the north-west in the southern hemisphere (called the South-East Trade Wind). These two trade winds meet in the equatorial region, forming a divide known as the Inter-Tropical Convergence Zone. The air movement in this zone is very weak and still, which is why it is also called the calm zone or Doldrum.
(h) Horse latitude: The high-pressure belt of the sub-tropical region, which lies within 30° to 35° latitudes in both hemispheres, is known as the Horse Latitude. In this belt, the air movement is mainly downward, so wind is not felt, and the air movement is so low that it can be difficult for ships to sail. In the middle ages, European navigators found it very difficult to sail ships in this belt. To reduce the load on the ships, navigators used to throw horses into the sea. It is for this reason that the region is known as Horse Latitude.
(i) Roaring Forties: In the southern hemisphere, the area between 40° and 60° latitudes is mostly covered by ocean. Here, the westerlies are very intense. Because of this, navigators often call the latitude belts by specific names. The belt between 40° and 50° latitude is called the Roaring Forties. Similarly, the fifties (50°-60°) are called the Furious Fifties, and the sixties (60°-70°) are called the Screaming Sixties.
(j) Anticyclone: An Anticyclone is an atmospheric condition where a divergent wind from the centre of a high-pressure belt takes a cyclic path. Since the atmospheric pressure is highest at its centre, an anticyclone is also known as a High-Pressure Centre. It is the complete opposite of a cyclone. Due to the earth’s rotation, the wind in an anticyclone moves in a clockwise direction in the northern hemisphere and an anticlockwise direction in the southern hemisphere. Anticyclones are generally associated with fair weather. They are of two types: the Sub-Tropical Warm Core Anticyclone and the High-Latitude Cold Core Anticyclone.
(k) Jet Stream: A very high-velocity wind, with speeds around 640 kilometers per hour, that blows towards the east at a high altitude in the upper layer of the troposphere in the tropical region is known as a Jet Stream. This Jet Stream brings about significant changes in the weather and climate of any area it affects.
(l) Isobar: Isobars are lines on a map that join places having equal barometric pressure. In an area where the pressure gradient is very low and the atmospheric condition is calm, the isobars can be hardly seen on a map of pressure distribution.
(m) Tropical Cyclone: A tropical cyclone is formed when a high-velocity wind from a high-pressure region takes a cyclic path of a violent nature around a lower-pressure centre in the tropical region. These cyclones form only during the summer season over oceans. They are violent, destructive, and harmful in nature, with wind velocities varying from 120 to 280 kilometers per hour. Due to the earth’s rotation, the cyclonic wind blows in an anticlockwise direction in the northern hemisphere and a clockwise direction in the southern hemisphere. Tropical cyclones are known by different names in different parts of the world, such as Typhoon in the Pacific, Hurricane in the West Indies, and Cyclone in the Indian Ocean.
25. Write the differences:
Answer: (a) Horizontal wind and Vertical wind: When air moves parallel to the earth’s surface, it is known as horizontal wind. This movement is always horizontal. On the other hand, when air moves vertically up, it is called vertical wind or current. The horizontal movement of air is more significant because the volume of air involved in it is many times more than that in the vertical movement.
(b) Wind and Air mass: Wind is the state of air in motion, which generally takes place parallel to the earth’s surface. In contrast, an air mass is an immense body of air with homogenous temperature and humidity conditions that remains stable over a wide area for a long period. While wind is defined by its movement, an air mass is defined by its uniform characteristics in its source region before it starts to move and diverge.
(c) Tropical cyclone and Extra-tropical cyclone: The differences between a tropical cyclone and an extra-tropical cyclone are:
- Location: Tropical cyclones form in the tropical region, while extra-tropical cyclones form in the temperate and high-latitude regions between 30° and 65° latitudes.
- Formation: Tropical cyclones form over oceans during the summer. Extra-tropical cyclones, also called temperate or wave cyclones, form at any time of the year in a frontal region where cold polar wind meets warm tropical wind.
- Characteristics: Tropical cyclones are violent and destructive, with wind speeds of 120-280 km/h. Extra-tropical cyclones have a larger diameter (300-1500 km) but a smaller pressure difference between the centre and periphery (10-35 millibars), making them generally less violent.
- Shape: The shape of an extra-tropical cyclone can be circular, elliptical, or sometimes ‘V’-shaped.
(d) Sea breeze and Land breeze: The difference between sea breeze and land breeze is based on the time of day and the direction of wind flow between land and sea.
- Sea Breeze: During the daytime, the land heats up faster than the sea, creating low pressure over the land and high pressure over the sea. The wind that blows from the sea surface to the land surface during the day is called a Sea Breeze.
- Land Breeze: At night, the land cools down faster than the sea, resulting in high pressure over the land and low pressure over the sea. The air movement from the land surface to the water surface during the night is called a Land Breeze.
(e) Mountain wind and Valley wind: The difference between mountain wind and valley wind is based on the direction of airflow in mountainous regions during the day and night.
- Valley Wind: During the daytime, the air in the valley gets heated, becomes lighter, and moves upward along the mountain slopes. This wind that blows from the river valley or plain upward along the slopes is known as Valley Wind.
- Mountain Wind: At night, the air on the mountain peaks becomes cold and heavy compared to the valley air. As a result, this cold, dense wind moves down from the highland towards the valley along the mountain slopes. This is called Mountain Wind.
(f) Cyclone and Anticyclone: The differences between a cyclone and an anticyclone are:
- Pressure Centre: A cyclone is a low-pressure centre with pressure increasing outwards. An anticyclone is a high-pressure centre with pressure decreasing outwards.
- Wind Direction: In a cyclone, winds converge towards the low-pressure centre. In an anticyclone, winds diverge from the high-pressure centre.
- Rotation: In the Northern Hemisphere, winds in a cyclone rotate anticlockwise, while in an anticyclone, they rotate clockwise. The opposite occurs in the Southern Hemisphere.
- Weather: Cyclones are associated with unstable weather, clouds, and rain. Anticyclones are generally associated with fair and calm weather.
(g) Cold Front and Warm Front: A front is a line of discontinuity formed when two different air masses meet. The difference between a cold front and a warm front depends on which air mass is advancing.
- Cold Front: A cold front is a front in which a cold air mass advances and shifts a warm air mass.
- Warm Front: A warm front is a front in which an advancing warm air mass moves over a cold air mass and shifts its position.
(h) Cold wave and Hot Wave: Both cold waves and hot waves are associated with anticyclones but originate in different regions.
- Cold Wave: The movement of extremely cold wind, particularly during winter, resulting from a high-latitude cold anticyclone is known as a ‘cold wave’.
- Hot Wave: The considerably warm wind that moves towards the east from the semi-permanent anticyclones of the sub-tropical region results in hot waves, which create an unnaturally warm and dry weather condition.
(i) Troposphere and Stratosphere: The differences between the Troposphere and the Stratosphere are:
- Position: The Troposphere is the lowermost layer of the atmosphere, touching the earth’s surface. The Stratosphere is the layer immediately above the Troposphere.
- Temperature: In the Troposphere, temperature decreases with an increase in height (Lapse Rate). In the Stratosphere, temperature gradually increases with height, from about -60°C to 0°C.
- Weather: Almost all weather phenomena, like clouds, rain, and storms, occur in the Troposphere. The Stratosphere has no weather phenomena and is almost free of clouds.
- Composition: The Troposphere contains about three-fourths of all atmospheric gases and almost all water vapour and dust. The Stratosphere contains the ozone layer, which absorbs harmful ultraviolet radiation from the sun.
26. Find out the correct answer:
(a) Where is Ozone layer located?
(1) Troposphere
(2) Stratosphere
(3) Mesosphere
(4) Thermosphere
Answer: (2) Stratosphere
(b) What is amount of oxygen in the atmosphere in terms of volume?
(1) 20.94%
(2) 29.01%
(3) 32.47%
(4) 78.08%
Answer: (1) 20.94%
(c) The most important reason for air movement is
(1) Humidity difference
(2) Pressure difference
(3) Gravitational Force
(4) Centrifugal Force
Answer: (2) Pressure difference
(d) The instrument used for determination of wind velocity is
(1) Wind vane
(2) Anemometer
(3) Beaufort scale
(4) Hydrometer
Answer: (2) Anemometer
(e) The unit of wind velocity is
(1) Knot
(2) Milibar
(3) Percentage
(4) Degree
Answer: (1) Knot
(f) Monsoon wind belongs to which of the following class?
(1) Local winds
(2) Primary circulation
(3) Secondary circulation
(4) Permanent wind
Answer: (3) Secondary circulation
(g) The name of the cyclone formed in the coastal region of the Pacific Ocean in the east is
(1) Cyclone
(2) Hurricane
(3) Willy-Willy
(4) Typhoon
Answer: (4) Typhoon
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