Chapter 4. Air

Composition of Atmosphere
• earth is surrounded by a huge blanket of air known as atmosphere.
• Nitrogen and oxygen are two gases that make up bulk of atmosphere. Carbon dioxide, helium, ozone, argon & hydrogen are found in lesser quantities.

• This is second most plentiful gas available in air.
• Humans and animals inhale it as they breathe.
• Plants produce oxygen by process of photosynthesis [which takes place in presence of sunlight]. It is way oxygen content in air remains unchanged.
• But due to cutting of large number of trees its balance has been disturbed.

• This is most plentiful gas in air.
• Plants need it for their survival but they can not absorb it directly from air.
• Plants take nitrogen from bacteria [present in soil and roots of plants] which directly absorb nitrogen from air and change its form.

Carbon dioxide:
• Plants use carbon dioxide to make their food and release oxygen as a by product. Humans and Animals release carbon dioxide. It creates a perfect balance.
• Carbon dioxide released in atmosphere trap head radiation and keeps earth’s atmosphere warm which create a green house effect. But when level of carbon dioxide increased the, retained heat start increasing and it cause an increase in atmospheric temperature, this phenomenon of over heating of planet is known as global warming.
• However, Humans add billions of tons of carbon dioxide in atmosphere by burning of fossils that affects earth’s weather and climate.
• effects of Global warming are:
(1) Melting of ice
(2) Rise in sea level
(3) Increased floods in coastal areas
(4) Extinction of some animals and plants

Structure of Atmosphere:
• Our atmosphere is divided into five layers starting from earth’s surface.
• These are Troposphere, Stratosphere, Mesosphere, Thermosphere and Exosphere.

• This layer is most important layer of atmosphere. Its average height is 13 km.
• air we breathe exists here.
• Almost all weather phenomena like rainfall, fog & hailstorm occurs in this layer.

• Stratosphere lies above troposphere. It extends up to a height of 50 km.
• This layer is almost free from clouds and associated weather phenomena, making conditions most ideal for flying airplanes. It contains a layer of ozone gas that protects us from harmful effect of Sun rays.

• It lies above stratosphere and extends up to height of 80 km.
• Meteorites burn up in this layer on entering from space.

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

• upper most layer of atmosphere is called exosphere.
• This layer has a very thin air.
• Light gases like helium and hydrogen float into space from here.

Weather and Climate
• Weather is this hour-to-hour, day to day condition of atmosphere.
• average weather condition of a place for a longer period of time represents climate of a place.

• degree of hotness and coldness of air is called temperature. It changes day to day and season to season.
• Insolation is incoming solar energy intercepted by Earth which is a prime factor that influences temperature.
• As amount of Insolation decreases from equator to poles, temperature decreases from equator to poles.
• cities are built up of concrete and metals that heats up during day therefore temperature of Cities is higher than villages areas.

Air Pressure:
• pressure exerted by weight of air on Earth’s surface is known as air pressure.
• Air pressure decreases with height. highest pressure is on Sea level.
• When earth’s temperature increases, air gets heated and rises which creates low pressure. Heavy air sinks and creates high pressure.
• air always moves from high pressure areas to low pressure areas.

• Wind is movement of air from high pressure areas to low pressure areas.

Belts and Wind System
• Winds can be broadly divided into three types:
(1) Permanent winds: trade winds, westerlies and easterlies are permanent winds.
(2) Seasonal winds: These winds change their direction in different seasons, for example monsoons in India.
(3) Local winds: These winds blow only during a particular period of day or year in a small area for example land and sea breeze.
• Cyclone devastated large portions of Odisha state in 1999. damages caused were mainly due to three factors: wind velocity, rain & tidal surge.

Moisture and Humidity:
• Moisture in air at any time is called humidity.
• Warm air has more capacity to hold water vapour than cold air.
• Clouds are just masses of water droplets which are formed by condensation.
• These water droplets come down as precipitation as they become too heavy for air to hold.
• moisture from Jet plane’s engines condenses and forms a white trail behind them.

• On basis of mechanism, there are three types of rainfall:
(1) convectional rainfall
(2) orographic rainfall
(3) cyclonic rainfall

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