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INTRODUCTION

• The air is a mixture of various gases in different properties . The huge blanket or envelope of air which surrounds the earth is called atmosphere.
• There are other components such as water vapour and dust particles which combine with the gases to form the atmosphere.
• The atmosphere makes life possible on earth. The atmosphere is held to the earth by gravitational pull.
• Almost all of the atmosphere (97 percent) lies within 29 km of the earth’s surface.
• The upper limit of the atmosphere can be  drawn approximately at a height of 10,000 km.

STRUCTURE OF ATMOSPHERE    

• From the earth’s surface upward to an altitude of about 80 km,
• the chemical composition of atmosphere is uniform in terms of the proportions of its component gases. This layer is the homosphere.
• The homosphere can be divided into layer on the basis of temperatures and zones of temperature change.

(a)    Troposphere:

• It is the lowermost atmospheric layer   extending from about 8 km at the poles and 16 km at equator.
• All phenomena of weather and climate which physically affect man take place within this layer.
• Dust particles present in troposphere hold the water vapour and contribute to
• the occurrence of twilight and the red colours of sunrise and sunset and distribute insolation.

(b)    Stratosphere:

• The second layer of atmosphere is called the stratosphere
• . The level at which the troposphere gives way to stratosphere is called tropopause. (At this level, the fall in temperature stops.)
• The upper limit of this layer is called stratopause.
• Within the stratosphere, temperature increases from about –60°C at tropopause to about 0°C at stratopause. 
• The stratosphere provides ideal conditions for flying aeroplanes.

(c)    Mesosphere:

• Mesosphere is the atmosphere layer extending between the stratopause and mesopause - the upper limit of   mesosphere.
• Vertical air currents are not strongly inhibited in mesosphere and formation of ice
• crystal clouds called the noctilucent clouds takes place occasionally in the upper regions of the layer.

(d)    Thermosphere:

• Thermosphere is the uppermost layer of the atmosphere,
• extending from the  mesosphere at an altitude of about 85 km to 400 km of the atmosphere.

(e)    Exosphere:
        Exosphere is the boundary between the earth’s atmosphere and the interplanetary space

         It extends from about 400 km, above the earth’s surface.

ATMOSPHERIC PRESSURE

• The pressure exerted by atmosphere as a result of its weight above
• a unit area of the earth’s surface is called the atmospheric pressure.
• Atmospheric pressure is expressed in  millibars (mb) and measured with a   mercury barometer.

OZONE LAYER

• A zone within the atmosphere between 20 and 80 km,
• extending from upper stratosphere into the mesosphere, called the ozone layer,
• is of vital importance to all living forms on earth.
• The ozone layer serves as a shield  preventing most of the potentially damaging ultraviolet radiation of
• the sun from reaching the tropopause and earth’s surface.
• If the ultraviolet rays were to reach the earth in full intensity,
• all exposed  bacteria would be destroyed and animal tissue severely burnt.

AIR TEMPERATURE   

   Air temperature depend upon various geographical factors including elevation,

   aspect, proximity to sea, direction of  prevailing winds and patterns of insolation.

(a)    Land- water Differences:

• The absorption and radiation properties of land and water differ.
• Land gets heated up rapidly and intensely under the sun’s rays whereas
• the water surfaces get slowly and moderately heated. Land  gets cooled off faster than water when solar radiation is cut off
• . These differing qualities result in greater temperature contrasts over  land areas in the middle of the continents,
• while  over the water areas (near the coasts), these contrasts are moderate. The air  temperature over the ocean has two  features:

(i)    As water bodies heat and cool slower than land, maximum and minimum temperatures are reached a month later than on land.
(ii)    The yearly range of temperature in water is less than that over the land.

(b)    Annual Temperature Patterns:

• The air temperature varies at different latitudes.
• Isotherms (line connecting places having the same air temperature)
• which run more or less parallel to the lines of latitudes in the east- west zones reflect the general decrease of insolation from equator to poles.
• During the year, the isotherms follow  the declination of the sun and change position north or south.
• The pattern of temperature changes also varies at   different altitudes:

(i)    In equatorial areas:
    Annual temperature shows little seasonal variation as

   they receive constant amount of diurnal insolation throughout the year.
(ii) In mid and high-latitude areas:
     Seasonal variation of annual temperature is much more marked.
(iii) In areas between the tropics and  polar circle:
      In each hemisphere, air temperature and insolation amounts have a marked  seasonal pattern.

       This is due to the fact that the sun’s path in these areas shifts through a relatively

    large range of noon altitude and substantial differences exist in the length of the days.
(iv)    In polar regions:

• There are large seasonal contrasts in air temperature very low in winter or during polar nights and extremely high during the summer.

(c)    Daily Cycle of Temperature:

• The daily pattern of temperature changes illustrates energy changes on a small time- scale
• Air temperature falls if the ground is cooler than air owing to the fact that the atmosphere is largely heated form the earth’s surface

(d)    Vertical Changes in Temperature:

• Air temperature also varies according to the altitude .
• At higher altitudes as air  becomes less dense, it is unable to absorb heat, resulting in colder air temperature.
• In case where the temperature remains the same with increase in altitude, the layer of atmosphere is called isothermal.

PRESSURE AND WINDS

    Winds are dominantly horizontal air  motions on the earth’s surface.

(The dominantly vertical air motions are called up- drafts and down- drafts.)

The variable heating of different parts of the atmosphere results in variations in  pressure,

which in turn set the air in   motion. There is an intimate relationship between winds and pressure.
(a)    Pressure Belts:

• Atmospheric pressure decreases with height
• . The distribution of pressure is characterised by its zonal or belted nature.
• Each zone or belt constitutes elongated or circular cells of high or low pressure.
• There are seven pressure belts on the earth’s surface.
• The equatorial belt   separates
• the three pairs of belts in northern and southern hemisphere, namely, the polar high, the sub-polar low and the subtropical high.
• Location of belts is based on the annual average. 

(b)    Planetary Winds:

• The general, permanent circulation of  surface winds throughout the world is  denoted by the term ‘planetary winds’.

(i)    Doldrums:
    Doldrums is the name for the equatorial belt of low pressure lying between

5° South and 5° North latitude. This zone has no pressure gradients to induce a persistent flow of wind.
(ii)    Trade Winds:

• The word ‘trade’ comes from the Saxon word ‘tredon’ which means to tread and follow a regular path.
• Moving north and south of the equator, the main wind belts are trade winds, covering roughly the zone between 5° and 30° North and South.
• Trade winds blow from the subtropical high pressure area (Horse latitudes)  towards equatorial low pressure areas (doldrums).
• Under the influence of the Coriolis force, they blow from the north-east in
• the northern hemisphere (north-east trades) and from the south- east in the southern temisphere (south-east trades). They are also called tropical easterlies.

(iii)    Horse Latitudes:

• The subtropical belts of variable winds and columns that lie between
• the latitudes 25° and 35° South and North are called Horse Latitudes.

(iv)    Westerlies :

• These winds blow from subtropical high pressure areas (Horse Latitudes) to 
• subpolar low pressure areas and lie  between 35° and 60°N and S latitudes.
• Variable in direction and strength,           westerlies contain depressions.

(c)    Monsoon Winds:

• Derived from the Arabic word ‘mausim’, meaning season, ‘monsoon
• ’ is applied to winds whose direction is reversed   completely from one season to the next.

(i)    Summer Monsoon:

• During summer, a ‘thermal’ or ‘heat’ low is developed over southern
• Asia in the lower levels of the atmosphere. It is a cyclone with a considerable air flow.

(ii)    Winter Monsoon:

• Reverse air flow from that of summer takes place in winter in Asia.
• The land area is  dominated by a strong centre of high  pressure from which there is an outward flow of air.

(d)    Local Winds:

• These winds affects only a limited area and blow for short periods of time,
• and are generated by immediate influences of the surrounding area.
• Most local winds are developed by depression:
• these are of considerable environmental importance as
• they may  affect the plants and animals when dry and extremely hot or cold.

(i)    Land and Sea Breeze:

• When a region is hotter than the neighbouring region, air from the
• cooler region moves into the hot region to take the place of the hot air which has expanded and risen.
• During the day the land gets warmer than the sea producing low
• pressure over the land into which cooler air moves from the sea.
• Thus the local wind that blows from sea to land during the day is called sea breeze.
• Sea breeze reaches the maximum strength during the mid afternoon
• when  temperature difference between the land and the sea is greater,
• and dies out by the evening as the sun’s heat diminishes.
• During the night the land cools more quickly than the sea and a reverse  process sets in.
• Land breeze is a cool wind that blows from the land to the sea (or a large lake, etc.) during the night.
• Land breeze occurs when night time  radiation cools the land and the air in  contact with the ground surface. 
• Land breeze is most common in tropical areas but may
• also occur in high latitudes particularly when the weather is calm. Land breeze is not as strong as sea breeze.

(ii)    Mountain and Valley Winds:

• These are local winds responding to local pressure gradients set up by heating or cooling of the lower air.

(iii)    Katabatic Winds:

• A cold downslope wind caused by the gravitational movement of cold dense air near the earth
• ‘s surface is a katabatic or drainage wind. Such cold dense air may accumulate in winter over a high plateau or high interior valley.
• Favourable  conditions cause some of this cold air to spill over low divides and flow down as a strong cold wind.
• The strongest katabatic winds are those that blow from an ice cap off the Greenland or Antarctic ice caps. 

(iv)    Foehn and Chinook:
    These result when strong regional winds passing over a mountain range

    are forced to descend on the ice side with the result that the air is heated and dried.

GLOBAL CIRCULATION SYSTEM

The surface wind systems represent only a small part of the circulation pattern.

There is an upper air flow also- mainly westerlies flowing in a complete

circuit about the earth from latitude 25° almost to the poles, and equatorial easterlies between the high-pressure ridges.

(a)    Jet Stream:

• The term was introduced in 1947 by Swedish- born US meterologist  Carl-Gustaf Rossby,
• and it is used to describe a very strong steady westerly
• wind blowing at high altitudes (6,000 to about 1400  metres above the earth’s surface) just below the tropopause.
• Jet Streams are usually confined to a  narrow band and its speed reaches up
• to 350-450 kmph. The highest streams   occur during winter. There are two main jet streams:

(i)    Polar front jet stream:
    They are irregular in its location and     commonly discontinuous, 

(ii)    Subtropical jet stream:
    Are found between 20° and 30° latitudes, north and south, are fairly consistent for a given season.

• In the northern hemisphere, the strongest jets flow across Japan and the United States.
• The jets have an important role in the weather changes. High altitude flying has to take them into account.

DEPRESSIONS

• Depression is a mass of air whose isobars form an oval or circular shape,
• with low pressure at the centre. The air converges at the centre and rises to be disposed off.
• In a depression, the winds rotate anticlockwise in the northern hemisphere.
• While in the southern hemisphere, the  circular movement of winds is in a clockwise direction.

(a)    Depression or Temperate Cyclones:
          Also known as extra-tropical or wave cyclones.

•     Temperate cyclones are active over mid-latitudinal region between 35° and 65° latitudes in both hemispheres.

(b)    Tropical Cyclones:

• Tropical cyclones, the most destructive of nature’s phenomena,
• are known to form over all tropical oceans, except South Atlantic and the South Pacific, during certain seasons.

(i)    Tropical Cyclones in the Indian Ocean and Bay of Bengal Regions:

• The frequency, intensity and coastal  impact of cyclones vary region to region.
• Interestingly, the frequency of tropical cyclones is the least in the north Indian
• Ocean regions of the Bay of Bengal and the Arabian Sea; they are also of  moderate intensities.
• Tropical cyclones in the Bay of Bengal are more frequent than in the Arabian Sea. There could be three reasons for this-

 (i)    waters in the Bay of Bengal are comparatively shallow;
 (ii)   the coastline along the Bay of Bengal is more complicated; and 
 (iii)  more number of rivers drain into the Bay of Bengal than into the Arabian Sea.
 (c)    Anticyclones:

• This is the mass of air whose isobars also form an oval or circular shape
• but in which pressure is high at the centre, decreasing towards the outside.
• Winds in an anticyclone form a clockwise outspiral in the northern hemisphere,
• whereas they an anti clockwise outspiral in the southern hemisphere.

(d)    Tornadoes:

• Tornadoes are violently rotating storms, characterised by a funnel shaped cloud,
• in which winds whirl around a small area of extremely low pressure.
• It is more destructive than a cyclone as the speed of the winds is very high, exceeding 320 km per hour.
• Tornadoes occur mostly in the Mississippi Valley, and are sometimes known as twisters. 

ATMOSPHERIC MOISTURE 

   Water vapour is amongst the most important atmosphere gases.

The tmosphere gathers moisture by the process of evaporation, while it loses  moisture through condensation

and precipitation. Humidity, which refers to the condition of the air

with regard to water vapour, plays a decisive role in  evaporation, condensation and precipitation.

HUMIDITY

• It is the degree of water vapour present in the air. For any specified temperature,
• here is a definite limit to the maximum quantity of moisture that can be held by the air.
• This limit is known as the saturation point.
• Humidity can be measured by a  hygrometer.

(a)    Relative Humidity:

• Relative Humidity is the ratio between the amount of water
• vapour actually present in an air mass and the maximum amount that air mass can hold at that temperature.

(b)    Absolute Humidity:

• Absolute Humidity is the actual amount of moisture present in air.
• The amount of water vapour per unit volume of air is usually expressed in grams per cubic metre.
• Absolute humidity is a measure of the quantity of water that can be extracted from the atmosphere as precipitation.

(c)    Specific Humidity:

• Specific Humidity is the ratio of the weight of water vapour to the weight of moist air.

CLOUDS

• Clouds are masses of minute water  droplets and / or Ice crystals formed by
• the condensation of water vapour and held in the atmosphere.
• Condensation, which results from   cooling, usually takes place around nuclei such as dust,
• smoke particles and salt. The cooling may be caused by converction, uplift over mountains or scent in depressions.
• Clouds are classified on the basis of  appearance, form and altitude. On the basis of form, there are two major groups:

(a)    Stratiform or layered type clouds: 
    Stratiform Clouds are blanket-like, often covering vast areas but are fairly thin when  compared to their horizontal dimensions.

    They are sub-divided on the basis of  elevation.
(i)    High Clouds:     

• Are formed 6000 to 12000 metres above sea-level.
• Example :

         (i) Cirrus Cloud       (ii) Cirrocumulus        (iii) Cirrostratus
(ii)    Medium Clouds:

• Are formed 2100 to 6000 metres above sea level.
• Example :

         (i) Altocumulus Clouds     (ii) Altostratus 
(iii)    Low Clouds:

• Are formed below 2100 metres (6900 fett).
• Example : 

    (i) Stratus         (ii) Nimbostratus      (iii) Stratocumulus
(b)     Cumulus Clouds :

• These are clouds of great vertical extent from 1500 to 9000 metres
• They have a low base level (500-2000 m), the base being flat and cloud masses isolated,
•  either rounded or towering, and with a clear outline, resembling the head of a cauliflower.
• When these clouds are sunlit, they are brilliantly white and are called ‘wool clouds’.
• They occur mainly in summer and are caused by converction.
• Small cumulus clouds are associated with fair weather.

PRECIPITATION

• Formation of water particles or ice  within the cloud that fall towards earth’s surface is precipitation.
• It occurs when condensation takes place rapidly within the cloud.
• Main types are rain, drizzle, sleet, snow and hail. Any of these may evaporate before reaching the ground surface,      
• appearing as streamers below cloud base a phenomenon known as virgo.

(a)    Rain:

• Form of precipitation consisting of the drops of water.
• In strict terms, these drops have a diameter ranging from about 0.5 mm to 5.0 mm,
• although smaller drops can be called rain if they are widely  scattered .
• Rainfall is usually measured by an instrument called the rain guage.

(b)    Drizzle:
         Form of precipitation in which the water droplets are very fine (less than 0.5 mm) and are close together.
(c)    Sleet:
         Generally it implies a form of precipitation consisting of either partly-melted  snowflakes or rain and snow falling  together.
(d)    Snow:

• A form of precipitation consisting of  crystals of ice.
• Snow is produced when condensation takes place at a temperature below freezing point,
• so that the minute crystals (spicules) of ice form directly from the water vapour.

(e)    Hail:

• Precipitation in the form of ice- pellets (hail stones) that develop in and fall from cumulonimbus clouds,
• either at a cold front or where intense heating of surface causes rapidly-ascending convection currents.
• Hail consists or rounded lumps of ice, having an internal structure of concentric layers much like an onion.
• Conditions for Precipitation:

    There are three possible ways by which precipitation is produced.
(i)    Convectional Precipitation:

• Is caused by heating of moist air in the lower layers of atmosphere which rises, expands, and is cooled adiabatically to its dew point.
• It occurs in regions near the equator in the afternoon as a result of the constant high temperature and high humidity. 

(ii)    Orographic:

• Orographic rainfall means ‘related to  mountains’.
• The precipitation is caused by moisture-laden air being forced to rise over a relief barrier (mountain ranges).
• A belt of dry climate, often called a rainshadow. 

(iii)    Cyclonic Precipitation:

• Occurs when large masses of air of    different temperature meet
• The warm moist air of one air mass moves over the cold heavier air of another.
• Or, it is caused by air rising through horizontal convergence in an area of low pressure.