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Subject : Geography

Class : 7th

Topic : Air ( Chapter no -4)

Name of the teacher : Krati Tomar

Name of the JNV : JNV South Tripura

AIR

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The mixture of gases that surrounds the earth and that we breathe is called air.

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Composition of the Atmosphere

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Nitrogen

Nitrogen : Nitrogen is the most plentiful gas in the air. Plants need nitrogen for their survival. They cannot take nitrogen directly from the air. Bacteria that live in the soil and roots of some plants , take nitrogen from the air and changes its form so that the plants can use it.

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Carbon dioxide

Carbon dioxide: Green plants use carbon dioxide to make their food and release oxygen. Carbon dioxide released in the atmosphere creates a greenhouse effect by trapping the heat radiated from the earth It is therefore called a greenhouse gas and without it the earth would have been too cold to live in.

Increased volume of carbon dioxide is affecting the earth's weather and climate.

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Oxygen

Oxygen : It is second most plentiful gas in the air. Humans and animals take oxygen from the air as they breathe. Green plants produce oxygen during photosynthesis. In this way oxygen content in the air remains constant. If we cut trees then this balance get Disturbed

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Layers Of Atmosphere

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Structure Of The Atmosphere

Troposphere : All weather phenomenon takes place in this layer. It's average height is 13 km.

Stratosphere : It is clouds free layer .It contains a layer of ozone gas. It extends up to a height of 50 km .

Mesosphere: Meteorites burn up in this layer on entering from the space .It extends up to the height of 80 km.

Thermosphere: In thermosphere temperature rises very rapidly with the increasing height. Ionosphere is a part of this layer . It extends between 80-400 km. This layer helps in radio transmission.

Exosphere: The upper most layer of the atmosphere is known as exosphere. This layer has very thin air .light gases like helium and hydrogen float into the space from here.

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Weather And Climate

Climate : climate is the condition of atmosphere that prevail over a long period , say 30 years . The major elements of climate are temperature, pressure, humidity, precipitation, winds etc.

Weather : weather is the condition of atmosphere prevailing on a limited area over a short period of time. It changes day to day hour to hour in even in minutes. The major elements of weather are clouds , rain, sunshine, etc.

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Temperature

Temperature : The degree of hotness and coldness of air is known as temperature . The temperature you feel every day is the temperature of atmosphere . The temperature of the atmosphere changes not only between day and night but also from season to season summer are hotter than winters.

The standard unit of measuring temperature is degree Celsius.

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Insolation

The full form of insolation is incoming solar radiation means the incoming solar energy intercepted by the earth.
Insolation influences the distribution of temperature.
The amount of insolation decreases from the equator towards the poles. Therefore , the temperature decreases in the same manner.

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Air Pressure

Air pressure is defined as the pressure exerted by the weight of air on earth' s surface.

As we go up the layers of atmosphere, the pressure falls rapidly. The air pressure is highest at sea level and decreases with height.

Horizontally the distribution of air pressure is influenced by the temperature of air at a given place.

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Low pressure and high pressure

Low Pressure: In areas where temperature is high the air gets heated and rises. This creates a low pressure area. Low pressure is associated with cloudy skies and wet weather.

High Pressure: In areas having lower temperature, the air is cold. It is therefore heavy air sinks and creates a high pressure area. High pressure is associated with clear and sunny skies.

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Wind

The movement of air from high pressure to low pressure areas is called wind .Winds can be broadly divided into three types.

1. Permanent winds: These blow constantly throughout the year in a particular direction. The trade winds , westerlies and easterlies are the permanent winds.

2. Seasonal winds: Theses winds change their direction in different seasons . For example monsoons in India.

3. local winds: These blow only during a particular period of day or year in a small area. For example land and sea breeze and dry local wind of northern planes (loo).

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Major Pressure Belts And wind system

Major Pressure Belts & Wind System

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Rainfall

1- When the water evaporates from land and different water bodies, it becomes water vapour .

2- When the water vapour rises , it starts cooling.

3-The water vapour condenses causing formation of droplets of water. Clouds are just masses of such water droplets.

4- When these droplets of water become too heavy to float in air, then they come down as precipitation.

5-Precipitation that comes down to the earth in liquid form is called rain.

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Cyclonic Rainfall

Frontal or cyclonic rain is caused by cyclone activity and it occurs along the fronts of the cyclone. It is formed when two masses of air of different temperature , humidity and density meets . For example meeting of moisture Laden warm topical wind with the polar air mass. The layers separating them is called the front.

On the basis of mechanism, there are three types of rainfall.

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Cyclonic Rainfall

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Relief Rainfall

Relief rain usually occurs along coastal areas where a line of hills run along the coast . When wet onshore wind from the sea meets a mountain, hill or any other sort of barrier, It is forced to rise along the slope and cools . When the air temperature falls to it's dew point water vapour condenses to form clouds.

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