Home General Knowledge GEOGRAPHY NOTES : OUR ATMOSPHERE
GEOGRAPHY NOTES : OUR ATMOSPHERE
Saturday, 18 January 2014 07:45

 

OUR ATMOSPHERE

  • The atmosphere has evolved as a significant component of biospheric ecosystem through the process of degassing or volcanism.

 

COMPOSITION

  • Nitrogen (78 per cent) and Oxygen (21 per cent) together constitute 99 per cent of the total gaseous composition of the atmosphere.
  • Oxygen is essential for the survival of living organism since they require it to convert food into energy.
  • Carbon dioxide acts as a ‘Green House Gas’ as it absorbs most of the radiant long wave energy from the earth and re-radiates it back to the earth, thus adding to the warming of the lower atmosphere.
  • Ozone, less than 0.0001 per cent by volume, is another important gas. It protects the life on earth from the harmful effects of ultraviolet rays by absorbing it. It is concentrated between the altitude of 20 and 30 km.
  • About 90 per cent of the total water vapour present in the atmosphere is found up to the height of 5 km. Its content ranges between 0 and 5 per cent in the atmosphere.
  • The blue colour of sky, the dawn and twilight are the result of scattering by the rest particles.

COMPOSITION

GAS

VOLUME (IN PER CENT)

Nitrogen (N2)

78.084

Oxygen (O2)

20.947

Argon (Ar)

00.934

Carbon Dioxide

00.314

Neon

00.0018

Helium

00.0005

Methane

00.0002

Krypton

00.0001

Hydrogen

00.00005

STRUCTURE OF THE ATMOSPHERE

  • About 50 per cent of the atmosphere lies below the altitude of 5.6 km and 97 per cent of the atmosphere is confined to the height of 29 km.
  • Based on temperature and pressure, following layers of atmosphere are recognised.

TROPOSPHERE

  • This is the lowermost layer.
  • Extends to an average altitude of 10 km which varies between 18 km above the equator and 8 km above the poles.
  • The height is more above the equator due to convective movement of the air.
  • In the troposphere the temperature decreases with increasing height at the rate of 6.50 centrigrade per km which is called Normal Lapse Rate.
  • Most weather phenomenon takes place in the troposphere.
  • The boundary line separating troposphere from the next layer is called “Tropopause”.

STRATOSPHERE

  • This layer lies above troposphere.
  • Extends upwards to a height of about 50 km.
  • The temperature ceases to decrease in the lower part of the stratosphere after which it starts increasing.
  • Strong and persistant winds blow in stratosphere from west to east.
  • Normally holds very little water vapour or dust, so weather distrubances are absent here.
  • Jet air flights take place mainly in the stratosphere.
  • The existence of ozone layer between 15 to 35 km which absorbs harmful solar ultraviolet rays is an important feature of the stratosphere.

MESOSPHERE

  • Extends from stratopause at 50 km (the dividing zone between Stratosphere and Mesosphere) to Mesopause at 80 km.
  • In this layer, temperature decreases from 0oC at the stratopause to -80oC at the mesopause.
  • It is a zone of chemical reactions.
  • Bulk of the meteors are destroyed in this region.

THERMOSPHERE

In this sphere temperature increases rapidly. Thermosphere is divided into two layers:

(i) Ionosphere: Extending between 80 km and 400 km, this is an electrically charged layer. Absorption of heat by certain ions       present in this layer increases the temperature. This layer reflects radio waves transmitted from the earth back to the earth.

(ii) Exosphere: This uppermost layer extending beyond ionosphere imperceptibly merges with the outer space. Temperature becomes extremely high but the highly rarified air holds little heat, so the high temperature is not felt. Hydrogen & Helium predominate in this region.

 

 

INSOLATION

  • Solar radiation that is intercepted by the earth is known as Insolation.
  • The amount of insolation reaching the outer limit of the atmosphere is called solar constant which is of the value of 2 gram calories per square centimeter per minute.
  • Insolation is measured with the help of pyronometres.
  • The amount of isolation depends on following factors:

(i) The area and nature of the surface.

(ii) The inclination of the rays of the sun.

(iii) Length of the day.

(iv) Distance between the earth and the sun.

(v) The transparency of the atmosphere.

  • As the angle of the sun’s rays decreases poleward, the amount of insolation received also decreases in that direction.
  • On January 3 the earth comes closest to the sun. Hence, the amount of incoming solar radiation is about 7 per cent more in January.

RECORDS OF EXTREME TEMPERATURES

RECORD

OC

LOCATION

Highest official air temperature

58o

Al Azizia, Libya

Highest mean annual temperature

34.4o

Dalol, Ethipoia

Coldest place (Lowest average Annual temperature)

-56.7o

Plateau, Station Antarctica

Lowest world surface air temperature

-88.3o

Vostoc, Soviet Station

Lowest official temperature in Northern Hemisphere

-68o

Verkoyansk, Siberia

WORLD TEMEPRATURE PATTERNS

1. Temperature decreases from the equator to the poles.

2. Lowest temperatures are experienced in January over the northern continent of Asia & North America in the Arctic and Sub-Arctic zone.

3. The highest temperature for both January and July are found over the continents.

4. All the isotherms seen to move northward between January and July.

5. Seasonal changes are less prominent over the Southern Continents than over the Northern Continents.

6. In the Northern Hemisphere, isotherms bend poleward over the oceans and equatorward over the continents in   the month of January since continents cool faster than oceans.

7. Generally the annual range of temperature increases from the Equator to the poles.

8. Coastal regions have smaller range of temperatures than continental interiors.

9. In the same latitude, the eastern sides of North America and Asia have greater range of temperature.

10. Highlands are always colder than surrounding lowlands.


 

PRESSURE AND WINDS

  • Pressure is exerted on the earth’s surface by the overlying weight of the air.
  • The standard air pressure at sea level is 1013.25 millibars.
  • The global air pressure varies from region to region due to combined effect of rotation of the earth and conditions of temperature as follows:

(a) Effect of Temperature

(i) Low Temperature at the poles causes contraction of air and so high pressure is formed.

(ii) High Temperature at the equator causes expension of air, so low pressure called Doldrum Low Pressure Belt is formed

(b) Effect of Rotation

(i) As the air moves away from the poles it crosses latitudes that are longer, it has to spread to occupy more space and so pressure falls as at 600 N and S latitudes.

(ii) At the equator the spinning earth causes the rising air to spread towards the poles so it has to cross latitudes which are shorter. So the air has to occupy less space, thus, it contracts and sinks at latitudes 300 N and S, forming Horse Latitude High Pressures.

  • Coriolis Forces (Ferrel’s Law): Winds deflect to their right in the Northern Hemisphere and to their left in the Southern Hemisphere due to Coriolis Force caused by rotation of the earth.
  • Some important facts related to atmospheric circulation:

(i) The winds are strong where the isobars are closely spaced and weak where they are wide apart.

(ii) Along and near the earth’s surface wind does not move freely in a horizontal plain. The irregularities of the earth surface (e.g. mountains, valleys, etc.) influence the direction of winds.

(iii) The maximum speed of wind usually occurs in the early afternoon and the minimum in the early morning.

(iv) Winds are named after the direction from which they come. A wind blowing from east to west is a westerly wind.

PATTERN OF PRESSURE BELTS AND PERMANENT WIND SYSTEM

(i) Equatorial Lowa Pressure Belt:

  • Zone of low pressure caused thermally near the equator.
  • Extending between 50 N and 60 S latitudes, this belt is known as ‘Doldrum’ because of calm winds prevailing here.

(ii) Sub-Tropical High Pressure Belts (Horse Latitudes): Extending between 25o and 35o in both hemispheres,           this high pressure belt is dynamically induced by the rotation of the earth and sinking down of winds.

  • This zone is characterized by anticyclonic conditions which cause atmosphere stability and aridity.

(iii) Sub-Polar Low Pressure Belts: Situated between 600 and 650 latitudes in both the hemispheres.

  • This low pressure belt is dynamically produced by the rotation of the earth.
  • The area of contact between cold and warm air masses forms a contact zone, known as Polar front, near 60o latitude.

(iv) Polar High Pressure Belts: The Arctic and Antarctic Polar regions are the belts of high pressure characterized by permanent anticyclone.

  • Polar easterly winds spiral outward from this belt.

TRADE WINDS

  • These are regular winds in direction and force.
  • They blow from the sub-tropical High Pressure Belt to Doldrums.
  • In the Northern Hemisphere, they are deflected to their right to form North-East Trades and in the Southern Hemisphere from South-East Trades.
  • Between the two trade wind systems is found the Inter-Tropical Convergence Zone (ITCZ).
  • A zone of westerly winds intervenes between the two trades in the doldrums, called the Equatorial Westerlies.

WESTERLLIES

  • They blow from the Horse Latitudes to Sub-Polar Low Pressure Belt.
  • Due to Coriolis Force, their direction is from south-west in the Northern Hemisphere and from north-west in the Southern Hemisphere.
  • The westerlies become more vigorous in Southern Hemisphere between 40 and 60 degrees, and are called the Roaring Forties, Furious Fifties and Shrieking Sixties in the respective latitudes.
  • Where these warm westerlies meet with the cold polar winds, a cyclonic front, called the Polar Front, is formed and temperature cyclones are originated.

POLAR WINDS

  • They blow from Polar High Pressure to Sub-Polar Low Pressure.
  • They are easterly winds due to the effect of Coriolis Force.
  • They are generally sporadic and of low velocity.

JET STREAM

  • Jet Stream is a very high speed wind blowing from west to east in a meandering couse at high altitude, usually near the level of tropopause (12 km).
  • The speed of the jet stream varies from a mean of 110 km /hour in summer to 190 km/hr in winter.
  • Has an important influence in the formation of global climatic phenomena, e.g. cyclones, variability of the monsoons etc.

CYCLONES AND ANTICYCLONES

  • Cyclone is a system of low pressure in which the barometric gradient is steep.
  • In a cyclone, winds circulate, blowing inwards in an anti-clockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere.

TEMPERATE CYCLONES

  • There are cyclones of temperate latitudes caused by contrasting cold air from polar regions and warm moist air from tropical regions.
  • Their general direction of movement is from west to east along with the westerlies.
  • The rainfall is light to moderate which occurs in the form of light showers.

TROPICAL CYCLONES

  • It is a system of a low pressure occurring in tropical latitudes.
  • They move westward through the trade wind belt.

ANTICYCLONES

  • These are characterized by the system of high pressure in the centre.
  • In the Northern Hemisphere, the wind circulation is clockwise and anticlockwise in the Southern Hemisphere.

 

 

HUMIDITY, CLOUDS & PRECIPTATION

HUMIDITY

The water wapour content of air is known as humidty. Warmer air has a greater capacity for holding water vapour whereas cooler air has a lesser capacity.

RELATIVE HUMIDITY

Relative Humidity is the ratio of water vapour actually present in the air to the maximum water vapour holding capacity of the air at that temperature, expressed as a percentage, as follows:

 

DEW POINT

The temperature at which a given mass of air becomes saturated is termed as the Dew Point.

ABSOLUTE HUMIDITY

The amount of water vapour present in a unit volume of air is called Absolute Humidity. It is expressed as gram/m3.

SPECIFIC HUMIDITY

The mass of water vapour contained in a unit mass of air is termed as Specific Humidity. It is expressed as grams of water varpour per kilogram of air.

 

CLOUDS

  • A cloud is an aggregation of grouping of moisture droplets and ice crystals that are suspended in the air and are great enough in volume and density to be visible to the naked eye.

 

TYPES OF CLOUDS

A. LOW CLOUDS (FROM THE SURFACE TO 2000 METRES)

(i) Stsrato cumulous clouds – arranged in globular masses.

(ii) Stratus clouds – resembles fog, comprises uniform layer, appear dull.

(iii) Nimbostratus – known as rain clouds, dark and thick enough to beat the sun (The term Nimbus is associated with any cloud that gives rain).

 

 

B. MIDDLE CLOUDS (FROM 2000 – 6000 METRES)

(i) Altostratus – in form of continuous sheet or veil.

(ii) Altocomulus – a grayish sheet cloud.

C. HIGH CLOUDS (ABOVE 6000 METRES)

(i) Cirrus clouds – long-drawnout wisps, known as mare’s tails are produced, delicate fibrous or wispy clouds consisting of tiny spicules of ice, an indication of an approaching cyclone.

 

D. CLOUDS WITH CONSIDERABLE VERTICAL DEVELOPMENT

(i) Cumulus Clouds – a convention cloud, have ‘cauliflower heads’, appear puffy like ‘cotton balls’.

(ii) Cumulonimbus clouds – cloud with a considerable vertical extent, its top acquiring an ‘anvil shape’, usually associated with heavy precipitation and thunder storms.

  • The amount of cloud cover in the sky is expressed in eights or oktas such as 2/8 (quarter covered), 4/8 (half cast), etc.

 

 

PERCIPITATION

FORMS OF PRECIPITATION

(i) Rain – In it the liquid water particles falls with the drops of more than 0.5 mm diameter.

(ii) Drizzle – Drops of water are fine, less than 0.5 mm of dia.

(iii) Snowfall – Fall of white and opaque grains of ice.

(iv) Sleet When water droplets and snow all in combination, it is called sleet.

(v) Hail It refers to falling of small pieces of ice with a diameter of 5 to 50 mm.

TYPES OF RAINFALL

(i) Convectional Rainfall: This type of rainfall generally occurs in the equatorial climate where constant high temperature and high humidity produces convectional rainfall – occurs in early summer.

(ii) Orographic Rainfall: Moisture-laden winds are forced to rise with the slope of the mountain and the air gets cooled           adiabatically, causing rainfall on the windward side of the mountains. The leeward side of the mountain becomes a rain-shadow area. Orographic rainfall is larger than any other type of rainfall in the world.

(iii) Cyclonic or Frontal Rainfall: These rain are associated with the passage of a cyclone or depression and are caused by a warm moist air mass moving upwards over colder heavier air. It usually occurs in the middle and high latitudes.

LOCAL WINDS

WINDS

NATURE

REGION

1.

Fohn

Warm

Alps/Europe

2.

Chinook (Snow eater)

Warm

Rockies (USA & Canada)

3.

Sirocco

Hot

North Africa

4.

Khamsin

Hot

Egypt

5.

Harmattan (The Doctor)

Hot

Sahara to Guinea Coast (Ghana, Nigeria etc.)

6.

Zonda

Warm

Argentina, Chile / Andes

7.

Brick Flelder

Hot

Australia

8.

Samun

Hot

Iran

9.

Leveche

Hot

Algeria, Morocco

10.

Santa Ana

Warm

USA (California)

11.

Berg

Warm

South Africa

12.

Yoma

Warm

Japan

13.

Karaburan

Hot

Tarim Basin

14.

Black Roller

Hot/Dusty

North America

15.

Kalbaisakhi

Hot

North India

16.

Buran

Cold

Siberia

17.

Mistral

Cold

France to Mediterranean (Rhine Valley)

18.

Pampero

Cold

Argentina

19.

Bora

Cold

Yugoslavia

20.

Southerly Burster

Cold

Australia

21.

Blizzard

Cold

Siberia, Canada, USA

22.

Purga

Cold

Russian, Tundra

23.

Bire

Cold

France

24.

Cape Doctor

Cold

South Africa’s Coast

FOG

  • Fog is regarded as a layer of cloud on the ground, with visibility reduced to less than 1 km.
  • The cause of fog is the consideration of water vapour in the lower layers of the air due to cooling of the air below its dew point mainly through radiation from the earth’s surface.
  • Favourable condition for the formation of fog are (i) very light wind, and (ii) a clear sky at night.

HAZE

  • It is a state of obscurity in the lower layers of the atmosphere due to the presence of large number of condensation nuclei such as dust, smoke, or salt spray.

SMOG

  • It is a special case of fog that is heavily laden with smoke or similar atmospheric pollutants. It frequently occurs in the industrial cities and densely populated urban areas. Intense smog can be a major health hazard.

DEW

  • When terrestrial radiation from the earth cools the lower layers of air below Dew Point, the water vapour condenses into drops on the ground surface, grasses, leaves, etc., in the form of dew.

FROST

  • Moisture on the ground surface condenses directly into ice, i.e. when condensation occurs below freezing point.





Last Updated on Saturday, 18 January 2014 07:57