NCERT Class 7 Geography Chapter 4: Air Completes Notes Part 2

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Weather vs. Climate

  • Earth receives only 1 in 2,000,000,0000 parts of the sun’s energy

  • Weather: hour to hour / day to day

What is the different between weather and Climate, we try to understand the amount of sunlight that’s received we see only 1 in 2,000,000,0000 parts is received on to the earth. Now you have the phenomena one that occurs day to day or on Hour to Hour basis and that phenomena is known as weather.

Next comes climate, climate is an average and we define climate as a change or an average that is taken for a duration of nearly 30 years or so. What we consider for a duration of 30 years is defined as climate. Based on that we can say climatic condition of the deserts of Rajasthan means we have taken the average over a period of 30 years and based on that we have decided or we have come to conclusion that these are the arid parts on globe. Climate is a kind of average for nearly more than 30 years which is taken into consideration. Now next is instruments.


  • Thermometer: Temperature

  • Barometer: Pressure

  • Rain Gauge: Amount of Rainfall

  • Wind Vane: Direction of Wind

There are various instruments which we use to understand the atmosphere phenomena to measure temperature we use thermometer, thermometer can be either in degrees so these are the commonly used scales for temperature. Then barometer by means which we measure pressure, you have aneroid barometers and the other barometers. Then you have rain gauge, thar measure the amount of rainfall. You have beaker with the amount of water being filled and you can decide how much rainfall occurs last night and so on. Then you have wind vane which gives you a direction of wind so its rotated be movement based in the wind direction so that is known as wind vane. Now next is temperature.


  • Degree of hotness or coldness of air

  • Temp Change b/w day & night, season to season (Summers are hot)

  • Insolation: Incoming Solar Radiation

  • Equator to Poles

  • High Temp in Cities: Concrete, Cement, Asphalt – Trap Heat

  • Standard unit of measuring temperature is degree Celsius

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    Temperature - Insolation, Heat Transfer, 3 Lapse Rate & 4 Scales in Climatology

Temperature is a kind of interesting concept that we must understand in atmosphere because in simple term if I try to explain what is temperature, it is degree of hotness or coldness of a body or the when we talk about atmosphere I can say of a mass of air so, the amount of hotness or coldness or the degree of hotness or coldness of an air parcel would be known as temperature. Now this temperature can change from day to night, hour to hour, from one season to another season and different sections, now what raised come on to the earth from the sun. So there are rays falling from the sun onto the earth known as the incoming solar radiation.

Now happens to the temperature, temperature decreases as we move from equator to poles is because you have fall in temperature as you move from equator to polls. You have decrease in temperature again the urban area are sure to have kind of high temperature this is because of the cemented road, you have Asphalt that is used in making the road, concrete cement of the buildings, all these trap the heat as a result they lead to formation of a zone which is known as urban heat island.

Now when we talk about the unit for measuring temperature as I said you have degrees are the standard unit of which commonly use is . Ranges from to , is the boiling temperature and is the freezing temperature. Now next is pressure.


  • Pressure exerted by the weight of air on the earth’s surface

  • Pressure with height (Max. at sea level)

  • Distribution – Horizontally

  • Air heats & rise – High Temp. – Creates LP – Cloudy & Wet

  • Low Temp.– Air is cold, heavy, dense, sinks – HP – Clear & Sunny Skies

Image of Pressure

Image of Pressure

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    Understanding Pressure - Vertical, Horizontal Distribution & Measurement in Climatology

Now concept of pressure is interesting what happens when you are standing in an open space, when I standing here, pressure is being exerted on to me from all sides by air. Now obviously this pressure in not visible to me but we can understand pressure changes as we go to moon and as we go below the surface all into the ocean. Now what happens on the moon there is no air as a result what would happen the inside blood pressure is there but there is no pressure to equalize the internal pressure as a result you would have your arteries to burst out and there would be bursting of arteries and that would lead to bleeding, so bleeding of arteries is common as you move to regions where is no air.

Now moving onto pressure we define pressure as the weight exerted and its exerted as a weight of the air on the earth surface as I move up the pressure decrease, so maximum pressure occurs at sea level so at the sea level you have the highest pressure.

Now this was the vertical distribution of pressure there is another distribution which is known as the horizontal distribution of pressure. When I talk about horizontal distribution of pressure, it important to understand concept of air movement so. I have Air mass here now this air mass is heated. So what I am providing to this is heat as this air masses is heated, it would rise when it is rising it would also expand so that means this air mass would have high temperature and since it has a high temperature it would have less pressure. This would be the air mass with low pressure and this low pressure air mass would be kind of region which is more cloudy, which has more weight, which is more disturbed as compared to a low temperature. So once this rises and expands what would happen is it would be formation of cloud that take place, you have kind of low temperature that come into play and this low temperature occurs where you have subsidence. You have cold air mass that moves down, its heavy so it would be dense and it would sink to the surface and the sinking at low temperature leads to creation of high pressure and this zone of high pressure would be the region with clear sunny sky.

Air movement occurs from a region of high pressure to a region of low pressure, since you have a movement of air towards the region of low pressure. This region would be definitely unstable so it would be cloudier, more foggy, more wind movement in the sky to be seen, however seen air is moving out from the region of high pressure this region would be the region of clear sunny sky.