UPSC Geography Atmosphere / वायुमंडल Short Notes - Atmosphere

Atmosphere

• Composition: The atmosphere is a mixture of many discrete gases as Nitrogen (78%), Oxygen (21%), Argon (0.93), Carbon dioxide (0.03)%, and others are Hydrogen, Helium, Krypton, yenon, Methane, Neon and ozone,
• Troposphere: Lower most part of the atmosphere. All weather phenomena takes place in this layer. Height of troposphere at poles is about 8 km while at equator is about 16 km due to greater heating at the equator.
• Stratosphere: Above troposphere is the stratosphere, which is primarily important because of the presence of ozone. This layer absorbs and scatters the solar ultraviolet radiation. It varies from 18-50 km.
• Mesosphere: It is a transitional layer. This is the coldest region of the atmosphere. Its height varies from 50- 85 km. Meteors burn up in this layer.
• Thermosphere: It starts just above the mesosphere and extends to 600 km high. Aurora and satellites occur in this layer.
• Ionosphere: It is a layer of electrons and ionized atoms and molecules from 48 km to 965 km which has electrically conducting layers that help in Radio Communication. It overlaps into the mesosphere and thermosphere.
• Exosphere: The outermost layer of the Earth's atmosphere is known as Exosphere. It extends from the top of the thermosphere upto 10,000 km.

Structure of the Atmosphere

• Energy transfer in the atmosphere takes place in 3 ways: radiation, conduction and convection.
• The Earth's surface receives solar radiation at the rate of 1.94 calories per square centimetre per minute. This is called Insolation (Incoming Solar Radiation).
• The vast amount of energy coming to and leaving the Earth's surface is through radiation.

Atmospheric Pressure           

• Atmospheric pressure is the force per unit area exerted on a surface by the weight of air above that surface.
• Mean Sea level pressure of the atmosphere is 2 mb.
• Air pressure at sea level is higher than it is at the top of a mountain.
• Air pressure increases when air descends,
• Air pressure always decreases with increase in altitude,
• Similarly, when air rises, its volume increases arid its pressure decreases.
• The temperature of air rises when its pressure rises and temperature of air falls when its pressure falls.
• The pressure of air falls when its temperature rises.
• The pressure of air rises when its temperature falls.
• High temperature along the equator causes the air to expand low pressure, it is called Doldrums, low pressure develops.
• Atmospheric pressure is measured with Barometer.

Pressure belts

• Atmospheric pressure is distributed across the latitudes in the form of pressure belts.
• These belts are seven in number: Polar High, Sub Polar Low, Sub Tropical High, Equatorial low, Sub Tropical High, Sub Polar Low and Polar High.

Wind                    

Because of horizontal differences in air pressure, air flows from the areas of high pressure to the areas of low pressure, the horizontal movement of the air is called wind.

• Planetary Winds are the surface air flow in global scale which affects the climate acoss pressure belts.

Some of the well-known planetary winds are:

• Doldrums: Between 5° N to 5° S Latitude.
• Equatorial Westerlies: They blow from 15° N to 35° N Latitude.
• Trade Winds: They blow from the sub-tropical High Pressure Belt to the Equatorial Low Pressure Belt in the tropics between 30° North and 30° South latitudes.
• Polar Winds: They blow from the polar High Pressure Belt to the Sub- polar Low Pressure Belt between 60° latitude and the Pole on both sides of the equator.
• Westerlies: The Westerlies, anti- trades, or Prevailing Westerlies, are prevailing winds from the west towards the east in the middle latitudes between 30 and 60° latitude. They originate from the high-pressure areas in the horse latitudes and tend towards the poles. These are :

(i) Roaring forties - blow between 40°S to 50°S, it is known as roaring forties latitude winds.

(ii) Furious fifties have its location around 50° to 60°s latitude.

(iii) Screaming (shrieking) Sixties wind blows 60° Onwords in southern hemisphere.

• Periodic Winds: They reversed their direction periodically with season. E.g. Monsoon, land and sea breeze, mountain and valley breeze.
• Local Winds: They develop as a result of local differences in temperature and pressure. Loo is an example of local wind.

Local winds

Cyclone

Cyclones: The system of wind rotating inward to an area of low pressure zone from its surrounding high pressure area.

• It is characterized by inward spiraling winds that rotate anti-clockwise in Northern hemisphere and clockwise in Southern hemisphere.

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On the basis of the location cyclone are of two types.

Tropical Cyclone: Tropical cyclones are intense storm which follows circular tract over tropical oceans.

Temperate Cyclone: Temperate cyclones are formed in the middle and high latitude where there is large temperature variation owing to front genesis.

Anti-Cyclone

• Rapid outward movement of air masses with a high pressure at centre. The relation of wind is clockwise in northern hemisphere and anti-clock wise in southern hemisphere.

Hurricane

• Also called tropical cyclone. It is a disturbance of about 600 km across, spinning around a central area of very low pressure with wind speed above 118km/hr.

Precipitation

This is the process when the atmosphere moisture which are previously in gaseous forms get condensed and comes down to earth's surface in the form of tiny droplets. Precipitation are of three types convectional, orographic and cyclonic.

Humidity

• The amount of water vapour in the air is called humidity.
• The humidity of air depends upon the temperature, e.g. if the temperature rises then air can hold more water vapour.

Clouds

• A cloud is a mass of small water droplets or thin ice crystals.
• Different types of clouds are as follow:
• Cirrus: Feather like.
• Cirrocumulus: Ripples like.
• Cirrostratus: Transparent sheet like causes the Sun and the Moon to have 'halos'.
• Altocumulus: Have bumpy-look
• Altostratus: Sheet like.
• Stratocumulus: Large globular masses.
• Nimbostratus: Dark grey and rainy looking, gives continuous rain.
• Stratus: Low clouds foggy in appearance.
• Cumulus: Round topped and flat based.
• Cumulonimbus: Special type of cumulous clouds spread out in form of an anvil. Often indicate convectional rain, lightning and thunder.