Source: VII NCERT Geography Chapter 4
- Our earth is surrounded by a huge blanket of air called atmosphere.
- Nitrogen and oxygen are two gases which make up the bulk of the atmosphere. Carbon dioxide, helium, ozone, argon and hydrogen are found in lesser quantities. Nitrogen is the most plentiful gas in the air .
- When we inhale, we take some amount of nitrogen into our lungs and exhale it.
- But plants need nitrogen for their survival.
- Oxygen is the second most plentiful gas in the air .
STRUCTURE OF THE ATMOSPHERE:
Our
atmosphere is divided into five layers starting from the earth’s sur face.
These are Troposphere , Stratosphere, Mesosphere,
Thermosphere and Exosphere.
Troposphere:
- This layer is the most important layer of the atmosphere.
- Its average height is 13 km.
- The air we breathe exists here.
- Almost all the weather phenomena like rainfall, fog and hailstorm occur in this layer.
Stratosphere:
- Above the troposphere lies the stratosphere.
- It extends up to a height of 50 km.
- This layer is almost free from clouds and associated weather phenomenon, making conditions most ideal for flying aeroplanes.
- One important feature of stratosphere is that it contains a layer of ozone gas, it protects us from the harmful effect of the sun rays.
Mesosphere:
- This is the third layer of the atmosphere.
- It lies above the stratosphere.
- It extends up to the height of 80 km.
- Meteorites burn up in this layer on entering from the space.
Thermosphere:
- In thermosphere temperature rises very rapidly with increasing height.
- Ionosphere is a part of this layer.
- It extends between 80-400 km.
- This layer helps in radio transmission.
- In fact, radio waves transmitted from the earth are reflected back to the earth by this layer.
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.
- Weather is the hour -to-hour , day to day condition of the atmosphere.
- The average weather condition of a place for a longer period of time represents the climate of a place.
- The degree of hotness and coldness of the air is known as temperature.
- An important factor that influences the distribution of temperature is insolation . Insolation is the incoming solar energy intercepted by the earth.
- The amount of insolation decreases from the equator towards the poles. Therefore, the temperature decreases in the same manner.
- Air pressure is defined as the pressure exerted by the weight of air on the 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 temperature of air at a given place.
- 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.
- The air always moves from high pressure areas to low pressure areas.
- The movement of air from high pressure area to low pressure areas is called wind. Winds can be broadly divided into three types.
1. Permanent winds– The trade winds, westerlies and
easterlies are the permanent winds. These blow constantly throughout the year
in a particular direction.
2. Seasonal winds– These winds change their
direction in different seasons. For example monsoons in India.
3. Local
winds – These blow only during a
particular period of the day or year in a small area. For example, land and sea
breeze.
- When water evaporates from land and different water bodies, it becomes water vapour.
- Moisture in the air at any time, is known as humidity. When the water vapour rises, it starts cooling. The water vapour condenses causing formation of droplets of water .
- Clouds are just masses of such water droplets. When these droplets of water become too heavy to float in air , then they come down as precipitation. Jet planes flying in the sky leave a white trail behind them.
- The moisture from their engines condenses.
- Precipitation that comes down to the earth in liquid form is called rain.
- Most of the ground water comes from rainwater .
- On the basis of mechanism, there are three types of rainfall: the convectional rainfall, the orographic rainfall and the cyclonic rainfall.
- Carbon dioxide released in the atmosphere creates a green house 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.
- The standard unit of measuring temperature is degree Celsius. It was invented by Anders Celsius. On the Celsius scale the water freezes at 0°C and boils at 100°C.
- On the moon there is no air and hence no air pressure.
- A wind is named after the direction from which it blows, e.g. the wind blowing from the west is called westerly.
- Other forms of precipitation are snow, sleet, hail.