Air - NCERT

1 Our earth is surrounded by a huge blanket of aircalled atmosphere. All living beings on this earthdepend on the atmosphere for their survival. Itprovides us the air we breathe and protects us fromthe harmful effects of the sun s rays. Without thisblanket of protection, we would be baked alive by theheat of the sun during day and get frozen duringnight. So it is this mass of air that has made thetemperature on the earth OF THE ATMOSPHEREDo you know that the air we take in while breathing isactually a mixture of many gases? Nitrogen and oxygenare two gases which make up the bulk of theatmosphere. Carbondioxide, helium,ozone, argon andhydrogen are foundin lesser from thesegases, tiny dustparticles are alsopresent in the air.

2 Thepie chart gives you thepercentage of differentconstituents of air (Fig. ).Nitrogen is the mostplentiful gas in the air. Whenwe inhale, we take someamount of nitrogen into ourlungs and exhale it. Butplants need nitrogen for theirsurvival. They can not take4 AirFig. : Constituents of AirDo you know?Carbon dioxidereleased in theatmosphere creates agreen house effect bytrapping the heatradiated from theearth. It is thereforecalled a greenhousegas and without it theearth would have beentoo cold to live , when itslevel in theatmosphere increasesdue to factory smokeor car fumes, the heatretained increases thetemperature of theearth.

3 This is calledglobal warming. Thisrise in temperaturecauses the snow incoldest parts of theworld to melt. As aresult the sea levelrises, causing floods inthe coastal may be drasticchanges in the climateof a place leading toextinction of someplants and animals inthe long (12-01-15)nitrogen directly from the air. Bacteria, that live in thesoil and roots of some plants, take nitrogen from theair and change its form so that plants can use is the second most plentiful gas in the and animals take oxygen from the air as theybreathe. Green plants produce oxygen duringphotosynthesis. In this way oxygen content in the airremains constant. If we cut trees then this balancegets dioxide is another important gas.

4 Greenplants use carbon dioxide to make their food andrelease oxygen. Humans or animals release carbondioxide. The amount of carbon dioxide released byhumans or animals seems to be equal to the amountused by the plants which make a perfect , the balance is upset by burning of fuels, suchas coal and oil. They add billions of tons of carbondioxide into the atmosphere each year. As a result, theincreased volume of carbon dioxide is affecting theearth s weather and and Ponder: Is global warming a serious issue in today s world?Do you know?When air is heated, itexpands, becomeslighter and goes air is denser andheavy. That is why ittends to sink hot air rises,cold air fromsurrounding arearushes there to fill inthe gap.

5 That is howair circulation (12-01-15)22 OUR ENVIRONMENTSTRUCTURE OF THE ATMOSPHEREOur atmosphere is divided into five layers starting fromthe earth s surface. These are Tr oposphere,Stratosphere, Mesosphere, Thermosphere andExosphere (Fig. ).Troposphere: This layer is the most important layer ofthe atmosphere. Its average height is 13 km. The airwe breathe exists all the weatherphenomena like rainfall,fog and hailstorm occur inthis : Above thetroposphere lies thestratosphere. It extends upto a height of 50 km. Thislayer is almost free fromclouds and associatedweather phenomenon,making conditions mostideal for flying important feature ofstratosphere is that itcontains a layer of ozonegas.

6 We have just learnthow it protects us fromthe harmful effect of thesun : This is thethird layer of theatmosphere. It lies abovethe stratosphere. It extendsup to the height of 80 burn up inthis layer on entering fromthe : Inthermosphere temperaturerises very rapidly withincreasing is a part of thislayer. It extends betweenFig. : Layers of the Atmosphere2015-16 (12-01-15)AIR23Do you know?You will be surprisedto know that theearth receives only 1in 2,000,000,000parts of the sun km. This layer helps in radio transmission. Infact, radio waves transmitted from the earth are reflectedback to the earth by this : The upper most layer of the atmosphereis known as exosphere.

7 This layer has very thin gases like helium and hydrogen float into thespace from AND CLIMATE Is it going to rain today? Will it be bright and sunnytoday? How many times have we heard this fromanxious cricket fans speculating the fate of a One Daymatch? If we imagine our body to be a radio and themind its speaker, weather is something that fiddleswith its control knobs. Weather is this hour-to-hour,day to day condition of the atmosphere. A hot or humidweather may make one irritable. A pleasant, breezyweather may make one cheerful and even plan for anouting. Weather can change dramatically from day today. However, the average weather condition of a placefor a longer period of time represents the climate of aplace.

8 Now do you understand why we have dailyweather temperature you feel everydayis the temperataure of theatmosphere. The degree of hotnessand coldness of the air is knownas temperature of theatmosphere changes not onlybetween day and night but alsofrom season to season. Summersare hotter than important factor thatinfluences the distribution oftemperature is is the incoming solarenergy intercepted by the amount of insolationdecreases from the equatortowards the poles. Therefore, theLet s doFor ten days notedown weather reportfrom a localnewspaper andobserve the changesoccurring in Vane: Shows thedirection of the windRain Gauge: Measuresthe amount of rainfallThermometer:Measuresthe temperatureBarometre: Measuresatmospheric pressureFig.

9 : Weather Instruments2015-16 (12-01-15)24 OUR ENVIRONMENT temperature decreases in the same manner. Now doyou understand why poles are covered with snow? Ifthe earth s temperature rises too high, it would becometoo warm for some crops to grow. Temperature in citiesis much higher than that of villages. The concrete andmetals in buildings and the asaphalt of roads getheated up during the day. This heat is released duringthe , the crowded high rise buildings of the citiestrap the warm air and thus raise the temperature ofthe PressureYou will be surprised to know that air above us pressesus with a great force on our bodies. However, we don teven feel it. This is because the air presses us from alldirections and our body exerts a counter pressure is defined as the pressure exerted by theweight of air on the earth s surface.

10 As we go up thelayers of atmosphere, the pressure falls rapidly. The airpressure is highest at sea level and decreases with the distribution of air pressure is influencedby temperature of air at a given place. In areas wheretemperature is high the air gets heated and rises. Thiscreates a low-pressure area. Low pressure is associatedwith cloudy skies and wet areas having lower temperature, the air is cold. Itis therefore heavy. Heavy air sinks and creates a highpressure area. High pressure is associated with clear andsunny air always moves from high pressure areas to lowpressure movement of air from high pressure area to lowpressure areas is called wind. You can see wind at workas it blows dry leaves down the pavement or uprootstrees during a storm.