Welcome dear students! Today we are going to learn about Atmosphere from Class 8 Social_Science. After studying this chapter, you will learn the meaning, importance, composition and formation of atmosphere. You will also learn about the components of atmosphere, temperature, pressure, winds, humidity, clouds and their functions and effects. Finally, you will be able to analyze the differences between weather and climate. Let us begin our journey into the envelope of air that surrounds our planet.
Do you know how we are protected and helped by the envelope of air? The atmosphere is a thin layer of gases, dust particles and water vapour surrounding the Earth. This layer of gases forms a protective boundary between the outer space and the Earth surface. The thickness of the atmosphere is around one thousand kilometers. Atmosphere is very important for all forms of life on the earth. Different gases of atmosphere help plants, animals and human beings to prepare their food. Atmosphere traps heat and makes Earth a planet for all the living organisms.
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Now let us look at the composition of atmosphere. The atmosphere is a mixture of different gases, dust particles and water vapour. The important gases in the atmosphere are Nitrogen at seventy eight point zero eight percent, Oxygen at twenty point nine four percent, and the remaining one percent consists of Argon at zero point nine three percent, Carbon dioxide at zero point zero three percent, Ozone at zero point zero zero zero zero zero five percent, and other trace gases. The atmosphere also contains dust particles which help us in the formation of water droplets. The water vapour in the atmosphere is the source of clouds and precipitation. The atmosphere traps heat and energy and has influence on the weather conditions of a place.
Next, we will study the layers of atmosphere. The atmosphere may be divided into five important layers on the basis of its characteristic features. These are Troposphere, Stratosphere, Mesosphere, Thermosphere which is also called Ionosphere, and Exosphere. Let us examine each one. Troposphere is the lowest layer of the atmosphere. It extends up to eighteen kilometers at the equator and eight kilometers near the poles. This layer has all the atmospheric elements such as temperature, pressure, winds, clouds, rainfall. All weather changes occur in the troposphere. Temperature and pressure decrease with the increase of altitude.
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Stratosphere is the second layer of the atmosphere. This layer extends up to fifty kilometers from the surface. It lies between troposphere and mesosphere. In this layer Ozone is the most important gas which absorbs ultraviolet rays of the sun and protect all forms of life on the Earth. This layer is free from clouds and other important weather phenomenon and provides ideal flying conditions for jet aircrafts. Mesosphere is the third layer in the Earth atmosphere. It extends up to eighty kilometers from the surface and lies above stratosphere. In this layer temperature decreases with the increase in altitude. This layer has the coldest temperature in the atmosphere. Thermosphere layer lies next to Mesosphere. In this layer temperature rises drastically. It is also called Ionosphere as the gaseous atoms are ionized due to very high temperature. The ions found in this layer help in reflecting radio waves. Exosphere is the top most layer of the atmosphere. In this layer elements of atmosphere are rare and pressure is extremely low.
Now we move to the elements of weather. Weather condition of a place is influenced by various elements such as temperature, pressure, wind, humidity, clouds, rainfall. Atmospheric condition of a place at a given time is called Weather. In contrast the average weather condition of an area over a long period of time is called Climate.
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Let us discuss temperature. The Sun is the main source of energy to the Earth which supplies heat through insolation. Insolation means incoming solar radiation from the sun to the Earth. Temperature is recorded by an instrument called thermometer. Centigrade and Fahrenheit are the important thermometers used to measure atmospheric temperature. The important factors that influence atmospheric temperature are latitude, altitude or height, distance from the sea, wind, ocean currents, relief, clouds, rainfall. Normal lapse rate is the decreasing rate of temperature with the increase of altitude. The rate of decrease is one degree Celsius for every one hundred sixty five meters or six point four degrees Celsius for every one thousand meters of height. Inversion of temperature occurs when in some situations temperature also increases with increasing height. This takes place in mountain valleys during long winter nights with clear sky, dry air, no wind and snow covered surface.
Temperature Zones are based on the distribution of temperature which is not uniform on the Earth surface. On the basis of insolation, the globe is divided into three temperature zones. They are Torrid zone, Temperate zone, and Frigid zone. Torrid zone is the zone of high temperature. This region is found between zero degrees or Equator and Tropic of Cancer in the north at twenty three point three degrees North and the Equator and Tropic of Capricorn at twenty three point three degrees South in the south. This region receives direct rays of the Sun.
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Temperate zone is the region where the temperature is neither very hot nor cold. This region lies between twenty three point three degrees North to sixty six point three degrees North, which is Tropic of Cancer to Arctic circle, and twenty three point three degrees South to sixty six point three degrees South, which is Tropic of Capricorn to Antarctic circle. Frigid zone is the coldest region. This zone is found between sixty six point three degrees North to ninety degrees North, which is Arctic circle to North pole, and sixty six point three degrees South to ninety degrees South, which is Antarctic circle to South pole. The temperature in this zone is very low due to slanting rays of the sun. In summer the temperature is slightly high and in winter the temperature is low. Isotherms are lines drawn on the map or globe connecting places having the same temperature.
Let us look at the facts file for places of highest and lowest temperature. First, Al Aziziya of Libya in Africa has recorded the highest temperature average plus fifty eight degrees Celsius, whereas Verkhoyansk of Siberia has recorded the lowest temperature average minus twenty four degrees Celsius. Second, in India Ganganagar of Rajasthan recorded the highest temperature in summer average plus fifty four degrees Celsius and Leh of Ladakh has recorded the lowest temperature average minus ten degrees Celsius in winter. Third, Vostok of Antarctica is considered as the coldest place on the Earth with an average temperature of minus eighty nine degrees Celsius.
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Now let us understand atmospheric pressure. Air has weight and it exerts pressure. This is called atmospheric pressure. Air pressure is measured by an instrument called Barometer. The unit used to show the pressure is millibar. The average air pressure of the atmosphere at the sea level is one thousand thirteen point two five millibar. The atmospheric pressure is affected by many factors such as temperature, rotation of the Earth, altitude, water vapour. Temperature is the most important factor that influences atmospheric pressure. The regions which have high temperature record low pressure due to the expansion of air, while the regions of low temperature have high pressure due to the contraction of air. Thus temperature and pressure are inversely related. The atmospheric pressure decreases with the increase in altitude. The amount of decrease is about thirty four millibar per every three hundred meters altitude.
The pressure on the earth is distributed in the form of belts. There are seven pressure belts. They are the Equatorial low pressure belt, North Sub tropical high pressure belt, South Sub tropical high pressure belt, North Sub Polar low pressure belt, South Sub polar low pressure belt, North Polar high pressure belt, and South Polar High pressure belt. The equatorial low pressure belt is a zone of high temperature and low pressure. It lies between zero degrees to five degrees North and South of the equator. This region gets direct rays of the Sun almost throughout the year. Hence air is always very warm and hot. This is a calm region with very little wind. So it is known as Doldrum which means belt of calm or equatorial calm. This region is also called Inter Tropical Convergence Zone where the trade winds converge.
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Sub tropical high pressure belts are found between thirty degrees and thirty five degrees north and south of the equator. There are two sub tropical high pressure belts. The North sub tropical high pressure belt is found between thirty degrees to thirty five degrees north latitudes. This region is also popularly called Horse latitudes. The South sub tropical high pressure belt is found between thirty degrees to thirty five degrees of south latitudes. Sub polar low pressure region is found between sixty degrees to sixty five degrees north and south of the equator. There are two sub polar low pressure belts. The North sub polar low pressure belt is from sixty degrees north to sixty five degrees north. The South sub polar low pressure belt is from sixty degrees south to sixty five degrees south. The winds are stormy especially in winter in this belt. Polar high pressure belts region is found between eighty degrees to ninety degrees north and south latitudes in both hemisphere. These are extremely cold regions with very high pressure throughout the year. Isobars are imaginary lines drawn on the map or globe connecting places having the same pressure. They are used to show the distribution of pressure on the map.
Next, we will learn about wind. Wind is the horizontal movement of the air on the surface of the Earth. Wind blows on the Earth due to rotation of the Earth and difference in pressure. The direction of the wind is shown by an instrument called wind vane or weather cock. Anemometer is used to measure the speed of the wind. Winds are classified into four major types. They are Planetary winds, Seasonal winds, Local winds, and Cyclones and Anti cyclones.
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Planetary winds are also called Permanent winds or Prevailing winds or Regular winds which blow more or less in the same direction through out the year. These winds play a major role in climate change, desert formation, guide navigation routes. There are three types of planetary winds: Trade winds, Anti trade winds, and Polar winds. Trade winds blow from sub tropical high pressure belts to equatorial low pressure region. The trade winds in the northern hemisphere blow from North East to South West direction, which are North East trade winds, and in the southern hemisphere they blow from South East to North West direction, which are South East trade winds. Westerlies or Anti trade winds blow from sub tropical high pressure belts to sub polar low pressure belts. These are from south west to north east in the northern hemisphere and north west to south east in the southern hemisphere. The westerlies of southern hemisphere are very strong over the oceans. Hence they are called Roaring Forties at forty degrees south latitude, Furious Fifties at fifty degrees south latitude, and Shrieking Sixties at sixty degrees south latitude. Polar winds are also called Easterlies. These winds blow from polar high pressure belts to sub polar low pressure belts. They blow from North East to South West in the northern hemisphere and South East to North West in the southern hemisphere. These are the cold dry winds blowing from the polar ice caps.
Seasonal winds are also called periodic winds. These winds change their direction periodically or seasonally. The monsoon winds of India are the typical periodic winds. In India South west monsoon winds blow from South West to North East direction during June to September and North east monsoon winds blow from North East to South West direction from late September to middle of December.
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Local winds also known as periodic winds are the result of variation in local temperature, pressure, humidity which in turn are attributed to the formation of air currents, crossing mountain ranges, valleys and other relief barriers. The important periodic and local winds are Land breeze, Sea breeze, Mountain breeze also called Katabatic winds, Valley breeze also called Anabatic winds, and other few winds are Loo in India, Chinook or Snow eater in USA, Fohn in Alps region, Mistral in France, Sirocco in Sahara region, Brick Fielder in Australia, and Blizzard in Antarctica.
Cyclones and Anti cyclones blow due to great variation in pressure. These are temporary but sometimes dangerous and destructive. A cyclone is a small low pressure area in the centre surrounded by high pressure. The winds blow spirally towards the low pressure area. In the northern hemisphere the direction of cyclonic winds is anti clockwise or counter clockwise and in the southern hemisphere it is clockwise. Cyclones are of two types. Tropical cyclones are found in the tropical region. They are very powerful and disastrous. Temperate cyclones are found in the temperate region. They are comparatively less strong and less dangerous. Cyclones are called by different names in different regions. In Japan and China they are called Typhoon. In USA and Mexico they are called Hurricane. In Australia they are called Willy willies. In India they are called Cyclone. In Russia they are called Whirlpool. In the last few years Indian coasts has experienced few disastrous cyclones. Some of them are Bola, Nargis, Nisha, Aila, Laila, Bijli, Jal, Tej, Biparjoy, Mocha, and Michaung. An anti cyclone is a high pressure area in the centre with winds blowing outwards, towards the low pressure areas. In the northern hemisphere the direction of anti cyclone wind is clockwise and in the southern hemisphere it is anti clockwise.
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Let us now study humidity. Humidity is the amount of water vapour or moisture present in the air. Humidity is measured by using an instrument called Hygrometer or Psychrometer. Humidity is expressed in different ways. Important among them are Absolute humidity, Relative humidity and Specific humidity. Absolute humidity refers to total amount of water vapour present in a given volume of air. It does not take temperature into consideration. Relative humidity is the ratio between the actual amount of water vapour present in the air and the amount of water vapour the air can hold at that temperature. It is expressed in percentage. Relative humidity equals the actual amount of water vapour in the air divided by the maximum amount of water vapour the air can hold, multiplied by one hundred. Specific humidity is the actual amount of water vapour present in a given mass of air.
Next, we will learn about clouds. Cloud is a mass of small water drops or ice crystals, formed by the condensation of the water in the atmosphere, usually at a considerable height above the Earth surface. Clouds are classified into four types on the basis of their shapes and heights at which they occur. The important types of clouds are the Stratus, the Cumulus, the Cirrus and the Nimbus clouds. Stratus clouds are usually low clouds occurring at less than two kilometers of height. They appear as thin sheets or layers of large extent. Stratus clouds are associated with fair weather.
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Cumulus clouds are cauliflower shaped clouds of great vertical extent. The base of cumulus clouds is nearly horizontal, while the top has a dome shaped appearance. These are popularly called wool packs. They come out with black and white in colours. Cumulus clouds are rain bearing clouds. Cirrus clouds are the highest clouds in the atmosphere about six kilometers high. These clouds resemble curls of hair, have a feathery or fibrous appearance. They indicate fair weather and often given a brilliant sunset. They look like patches of cotton fibres floating in the air. Their popular name is mare tail or witch broom. Nimbus clouds are rain clouds which occur at low levels. These clouds have the shape of stratus or cumulus. They are dark grey or black in appearance. They cause heavy rainfall or snowfall. The Nimbo stratus are thick, heavy, rain bearing clouds.
Now let us discuss rainfall. Precipitation in the form of water droplets is called rainfall. The total amount of rain received on a given area during a given time as measured by a rain gauge in millimeters or inches. Rainfall is of three types. They are Convectional rainfall, Orographic rainfall and Cyclonic rainfall. Convectional rainfall is the rain caused by the process of convection. In areas of high temperature, air rises up due to heating. This rising air cools, gets saturated, condensation takes place and later rainfall occurs. Convectional rainfall is very common in the equatorial region and also in the tropical regions in summer. In the equatorial region, convectional rainfall is called afternoon rain, as it occurs mostly in the afternoon. It is accompanied by thunder and lightning.
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Orographic rainfall is also called Mountain rainfall or Relief rainfall. When moisture laden air is obstructed by a mountain barrier, the air is forced to rise up. As the air rises upwards, it cools and gets saturated. Further cooling of this air causes condensation resulting in orographic or mountain rainfall. The windward side of the mountain receives more rainfall, while the leeward side receives less. The leeward side is generally called the rain shadow region. For example, Mangaluru is the windward side and Hassan the leeward side or rain shadow region. Cyclonic rainfall occurs in a cyclone where the air blows spirally inwards. In the tropical cyclones the air rises upwards in circular movement. So condensation takes place to cause heavy rainfall. The temperate cyclonic rain is also called frontal rain. When the warm air mass and the cold air mass meet, the warm air being lighter is forced to rise over the heavier cold air. The warm air after rising, cools and condenses, resulting in rainfall.
The distribution of rainfall on the Earth depends on location and the climatic condition of that region. The most important heavy rainfall areas in the world are the equatorial region, the eastern margins of sub tropical belts, the western margins between forty degrees and sixty degrees North and South latitudes. Scanty rainfall regions are the polar areas, the western margins of sub tropical regions and the tropical and temperate deserts.
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Here is a special section for you. Do you know that Mawsynram of Meghalaya in India has recorded one thousand one hundred forty centimeters of rainfall per year. It is considered as the wettest or rainiest region on the Earth. Agumbe of Karnataka is called Cheerapunji of South India. Royli of Rajasthan in India receives lowest rainfall at eight point three centimeters per year. Nayakanahatti of Chitradurga is considered the driest place in Karnataka. Atacama desert of Chile is the driest region on the Earth. It has not received rainfall for the last two hundred years.
Let us clearly distinguish weather and climate. The atmospheric conditions of a smaller area at a particular time is called weather. For example cloudy, sunny, fine weather. The scientific study of weather is called Meteorology. Climate is the average condition of the atmosphere over a long period of time of a larger area. Examples include Equatorial climate, Tropical monsoon climate, Desert climate, Mediterranean climate, and Tundra climate. The scientific study of climate is called Climatology. Factors affecting the climate of a place are Latitude, altitude, winds, distance from the sea, distribution of land and water bodies, ocean currents.
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Now, let us move to the exercises to test your understanding. I will read each question and provide the complete answer. Exercise One: Fill in the blanks with suitable words. Question one: The two major gases of the atmosphere are Nitrogen and Oxygen. Question two: The lowest layer of the atmosphere is Troposphere. Question three: The average air pressure of the atmosphere at the sea level is one thousand thirteen point two five millibar. Question four: Westerlies is also known as Anti trade winds. Question five: The scientific study of weather is Meteorology.
Exercise Two: Answer the following questions briefly. Question six: What is atmosphere? Answer: Atmosphere is a thin layer of gases, dust particles and water vapour surrounding the Earth. This layer of gases forms a protective boundary between the outer space and the Earth surface. Question seven: Name the important layers of the atmosphere. Answer: The important layers of the atmosphere are Troposphere, Stratosphere, Mesosphere, Thermosphere which is also called Ionosphere, and Exosphere. Question eight: What is the significance of ozone layer? Answer: In the stratosphere, Ozone is the most important gas which absorbs ultraviolet rays of the sun and protects all forms of life on the Earth. Question nine: What is Doldrum? Where is it found? Answer: Doldrum means belt of calm or equatorial calm. It is found in the equatorial low pressure belt between zero degrees to five degrees North and South of the equator. It is a calm region with very little wind. Question ten: Name the different types of planetary winds. Answer: The different types of planetary winds are Trade winds, Anti trade winds or Westerlies, and Polar winds. Question eleven: What are local winds? Give any two examples. Answer: Local winds are the result of variation in local temperature, pressure, and humidity, which lead to the formation of air currents crossing mountain ranges, valleys, and other relief barriers. Two important examples are Land breeze and Sea breeze. Other examples include Loo in India and Chinook or Snow eater in the USA. Question twelve: Mention the different types of clouds. Answer: The different types of clouds are Stratus, Cumulus, Cirrus, and Nimbus clouds. Question thirteen: Distinguish between weather and climate. Answer: Weather is the atmospheric condition of a place at a given time, while climate is the average weather condition of an area over a long period of time.
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Exercise Three: Define the following. Question fourteen: Ionosphere. Answer: The Thermosphere layer is also called the Ionosphere. In this layer, temperature rises drastically, and the gaseous atoms are ionized due to very high temperature. Question fifteen: Normal lapse rate. Answer: Normal lapse rate is the decreasing rate of temperature with the increase of altitude. The rate of decrease is one degree Celsius for every one hundred sixty five meters or six point four degrees Celsius for every one thousand meters of height. Question sixteen: Torrid zone. Answer: Torrid zone is the zone of high temperature. This region is found between zero degrees or Equator and Tropic of Cancer in the north at twenty three point three degrees North and the Equator and Tropic of Capricorn at twenty three point three degrees South in the south. This region receives direct rays of the Sun. Question seventeen: Orographic rainfall. Answer: Orographic rainfall is also called Mountain rainfall or Relief rainfall. When moisture laden air is obstructed by a mountain barrier, the air is forced to rise up. As the air rises upwards, it cools and gets saturated. Further cooling of this air causes condensation resulting in orographic or mountain rainfall. Question eighteen: Climatology. Answer: Climatology is the scientific study of climate.
Exercise Four: Terms to remember. I will briefly explain each as per the text. Insolation means incoming solar radiation from the sun to the Earth. Inversion of temperature occurs when in some situations temperature also increases with increasing height, taking place in mountain valleys during long winter nights with clear sky, dry air, no wind and snow covered surface. Anemometer is an instrument used to measure the speed of the wind. Roaring forties refers to the strong westerlies of the southern hemisphere over the oceans at forty degrees south latitude. Nimbus clouds are rain clouds which occur at low levels, have the shape of stratus or cumulus, are dark grey or black in appearance, and cause heavy rainfall or snowfall. Meteorology is the scientific study of weather.
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Exercise Five: Activity. The activity asks you to draw a diagram showing different temperature zones on the basis of latitudes. To complete this, you should draw a simple globe or a circle representing the Earth. Draw a horizontal line across the middle for the Equator at zero degrees. Mark the Tropic of Cancer at twenty three point three degrees North and the Tropic of Capricorn at twenty three point three degrees South. Shade the area between them and label it Torrid Zone. Next, mark the Arctic Circle at sixty six point three degrees North and the Antarctic Circle at sixty six point three degrees South. Shade the areas between the Tropics and the Circles and label them Temperate Zones. Finally, shade the areas from the Circles to the North Pole at ninety degrees North and the South Pole at ninety degrees South, and label them Frigid Zones. This visual representation will help you remember the latitudinal divisions of temperature.
Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]