- Air is the invisible gaseous substance surrounding us.
- The blanket of air that surrounds Earth is called the atmosphere. It is made of many gases and it is very important for the existence of life on the Earth.
- Livings beings cannot survive without atmosphere as it provides us with the air we breathe and protects us from the harmful rays of Sun.
- Major changes in the atmosphere can cause floods, cyclones, droughts etc.
Composition of Atmosphere
Air is a mixture of varying amounts of gases, moisture, and other particles.
- Nitrogen and oxygen are the two gases which make up the main part of the environment.
- Nitrogen is the most abundant and makes up around 78% of the total atmosphere
- Plants need nitrogen for their survival but they cannot take it directly from the air. Form of Nitrogen (from air) is changed by the bacteria that live in the soil and roots of some plants so that plants can use it.
- Oxygen makes up around 21% of the total atmosphere
- Humans and animals use oxygen from the air for breathing.
- Green plants produce oxygen during photosynthesis.
- The remaining 1% of the total atmosphere is made up small amounts of gases such as argon, carbon dioxide, ozone, methane, hydrogen, helium, neon and krypton.
- Apart from these gases, tiny dust particles are additionally present in air.
- Carbon dioxide is called a greenhouse gas as it creates a greenhouse effect by trapping the heat radiated from the earth.
- Global warming: is when level of Carbon dioxide in the atmosphere increases due to smoke or fumes emitted by vehicles and factories, more heat is retained hence increasing the temperature of the earth
- Air Circulation: Air when heated, expands, becomes lighter and goes up. Cold air, which is dense and heavy, sinks down. When hot air rises, cold air from surrounding area fill in the gap.
Structure of Atmosphere
- The layers of atmosphere are divided on the basis of the characteristics of the gases found at that height.
- Starting from the Earth's surface, atmosphere is divided into 5 major layers. These are
- Troposphere
- is the primary layer that starts from the surface of the earth and extends up to 8 to 15 km in height
- has around 80% of the mass of the atmosphere
- has almost all the weather phenomena like rainfall, fog and hailstorm occurring in this layer
- Stratosphere
- is the second layer, above the troposphere, that starts from where troposphere ends and extends up to 50 to 60 km from earth's surface
- has no cloud and the air is dry containing very little moisture
- is most ideal for flying airplanes
- also contains a layer of Ozone gas which protects us from the harmful radiations of the sun
- Mesosphere
- is the third layer that starts from where stratosphere ends and extends up to 95 to 120 km from earth's surface
- is where most of the meteorites burn up upon entering
- Thermosphere
- is the fourth layer that starts from where mesosphere ends and extends upto 400 km
- helps in radio transmission as the radio waves transmitted from the earth are reflected back to the earth by this layer
- The air in thermosphere is very thin and the temperatures can get extremely hot here
- In this layer the temperature rises very rapidly with the increase in height
- Ionosphere is a part of this layer
- Exosphere
- is the upper most layer of the atmosphere and has very thin air
- starts after thermosphere and extends till space
- is the last and the thinnest layer of the atmosphere
- Light gases like helium and hydrogen float into the space from this layer.
Weather and Climate
- The weather of a place is the condition of the atmosphere at a particular time with regards to heat, sunshine, dryness, wind, cloudiness, rain etc. in that area.
- The climate is the average weather condition of a place taken over a long period of time.
The weather and climate of a place depend on many geographic factors such as height, temperature, air pressure, wind, and moisture.
Temperature
- is the degree of hotness or coldness of the air
- changes from day to night as well as from season to season
- is highly influenced by insolation which is the amount of solar radiation received by an area
- The amount of insolation decreases from equator towards poles because sunlight falls almost vertically on the equator, while it falls at slanting angle on the poles.
- As the amount of insolation decreases, the temperature decreases in the same manner.
- Temperature in cities is much higher than villages because during the day the concrete and metals in buildings and the asphalt of roads get heated up and this heat is released during the night. Also the high rise buildings in the cities trap the warm air thus raising the temperature of the cities.
- The standard unit of measuring temperature was invented by Anders Celsius and is known as degree Celsius.
- On the Celsius scale the water freezes at 0°C and boils at 100°C.
Air Pressure (atmospheric pressure)
- is defined as the pressure exerted by the number of gases present in the air on the surface of the earth
- falls rapidly as we go up the layers of atmosphere
- is highest at sea level and decreases with height
- Temperature of air at a given place influences the distribution of air pressure.
- In creating
- A low pressure area is created in areas where temperature is high due to the rising of heated air. Low pressure is associated with cloudy sky and wet weather.
- In areas with lower temperature the air is heavy because it is cold and therefore creates a high pressure area. High pressure is associated with clear and sunny sky.
- The air always moves from high pressure areas to low pressure areas.
- On the moon there is no air and therefore no air pressure.
- Barometer is used to measure atmospheric pressure.
Wind
- The movement of air from high pressure area to low pressure areas is called wind.
- A wind is named after the direction from which it blows. For example the wind blowing from the east is called easterly.
- There are mainly three types of winds -
- Permanent Winds
- blow constantly throughout the year in a particular direction
- Example - The trade winds, westerlies and easterlies
- Seasonal Winds
- change their direction in different seasons
- Example - monsoons in India
- Local winds
- blow in a small area during a particular period of day or year
- Example - land and sea breeze
Moisture
- is the tiny drops of water present in the air, on a surface, or in the ground
- in the air at any given time is called humidity
- Water evaporates from sea, lake, land etc. and becomes water vapour.
- As it rises water vapour starts cooling and condenses forming water droplets.
- Masses of water droplets form clouds which come down as precipitation when they become too heavy.
- Rain, snow, sleet and hail are various forms of precipitation.
- Precipitation that comes down to the earth in liquid form is called rain.
- Rainfall is very important for the survival of all plants and animals.
- Based on the mechanism, there are three types of rainfall.
- The Convectional Rainfall:
- occurs mostly in tropics where it is hot
- is when hot air rises and it cools and condenses forming rain.
- If the air is too hot it rises very quickly and can cause thunderstorms.
- The Orographic (Relief) Rainfall:
- occurs when air that has been blown over the sea is forced up over an area of high land
- This causes the air to cool and the moisture in the air condenses and rain falls.
- The Cyclonic (Frontal) Rainfall:
- occurs when warm air is forced to rise over cold air
- The moisture in the warm air condenses as it cools which causes clouds and rain.
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