ATMOSPHERE

 ATMOSPHERE

The atmosphere is a thick gaseous envelope which surrounds the earth from all sides and is attached to the earth's surface by gravitational force. It is composed of (i) gases, (ii) vapour and (iii) particulate matter.

It provides all necessary gases for the sustainance of all life forms in the biosphere, filters the incoming solar radiation and also acts as a blanket by protecting the earth from becoming too hot or too cold.

The height of the atmosphere is estimated upto 29,000 kilometres from the sea level.

Gases

Nitrogen and Oxygen are major gases which constitute 99% of the total gaseous composition of the atmosphere. The remaining one per cent is represented by other gases.

Oxygen is the most important gas from the standpoint of living organisms because they inhale it for their survival. Nitrogen acts as dilutent and is generally chemically inactive.

Carbon dioxide is used by green plants for photosynthesis. It absorbs most of radiant energy from the earth and re-radiates it back to the earth. Thus, carbon dioxide, a greenhouse gas, increases the temperature of the lower atmosphere and the earth's surface.

Vapour

The atmospheric vapour is received through the evaporation of moisture and water from the water bodies, vegetation and soil covers.

The vapour content in the atmosphere ranges between zero and 5% by volume. The content of vapour decreases upward. More than 90% of the total atmospheric vapour is found upto the height of 5 km.

Particulate matter

Includes dust particles, salt particles, solid particles which are kept in suspension in the atmosphere. These particulates help in the scattering of solar radiation which adds varied charming colour of red and orange at sunrise and sunset. 

The sky appears blue in colour due to selective scattering of solar radiation by dust particles.

Salt particles become hygroscopic nuclei and thus help in the formation of water drops and clouds.

 LAYERS OF ATMOSPHERE

(A) On the basis of the characteristic of temperature and air pressure there are four layers from the earth's surface upward e. g. (1) Troposphere, (2) Stratosphere, (3) Mesosphere and (4) Thermosphere,

1. Troposphere:

The lowermost layer of the atmosphere and almost all of the weather phenomena (e.g., fog, cloud, dew, frost, rainfall, hailstorm, storms, cloud- thunder, lighting, etc.) occur in this layer.

Temperature decreases with increasing height at the rate of 6.5°C per 1000 m. This rate of decrease of temperature is called normal lapse rate.

The average height of the troposphere is about 16 km over the equator and 6 km over the poles. The upper limit of the troposphere is called tropopause which is about 1.5 km thick. The height of tropopause is 17 km over the equator and 9 to 10 km over the poles.

2. Stratosphere:

The layer just above the troposphere is called stratosphere. There is circulation of feeble winds and cirrus clouds in the lower stratosphere. On an average the upper limit of the stratosphere is taken to be 50 km. The upper limit of the stratosphere is known as stratopause.

There is concentration of ozone layer between the height of 15-30 km. It acts as a protective cover, it absorbs almost all of the ultraviolet rays of solar radiation and thus protects the earth's surface from becoming too hot.

3. Mesosphere:

Mesosphere extends between 50 km and 80 km. Temperature again decreases with increasing height.

At the uppermost limit of mesosphere (80 km) temperature becomes - 80°C. This limit is called mesopause.

4. Thermosphere:

Part of the atmosphere beyond mesopause is known as thermosphere wherein temperature increases rapidly with increasing height. The temperature at its upper limit becomes 1700°C.

Thermosphere is divided into two layers viz. (i) Ionosphere, and (ii) Exosphere.

(i) Ionosphere

. It contains electrically charged particles known as ions, and hence, it is known as ionosphere. Radio waves transmitted from the earth are reflected back to the earth by this layer.

Extends from 80 km to 400 km. There are a number of ionic layers (with increasing heights) in this sphere e.g. D layer, E layer, F layer and G layer.

D Layer (between the height of 60 km - 99 km) reflects the signals of low frequency radio waves. This layer disappears with the sunset because it is associated with solar radiation.

E Layer, also known as Kennelly - Heaviside layer, is confined in the height between 99 km - 130 km. This layer reflects the medium and height frequency radio waves back to the earth. It also disappears with the sunset.

Sporadic E Layer reflects very high frequency radio waves. E₂ layer is generally found at the height of 150 km. Layer also disappears during nights.

F Layer consists of two sub layers e.g. F₁ and F_{2} layers (150-380 km) and are collectively called Appleton Layer. These layers reflect medium and high frequency radio waves back to the earth.

G Layer (400 km above) most probably persists day and night.

(ii) Exosphere

 Represents the uppermost layer of the atmosphere.

The density becomes extremely low and the atmosphere resembles a nebula. The temperature becomes 5568°C at its outer limit.

(B) On the basis of chemical composition, the atmosphere is divided into two broad zones viz. (i) Homosphere Layer and (ii) Heterosphere Layer.

(i) Homosphere Layer

Represents the lower portion of the atmosphere and extends upto the height of 90 km from sea level. The main constituent gases are oxygen, nitrogen and other gases. This zone is called homosphere because of the homogeneity of the proportion of various gases.

On the basis of thermal conditions the homosphere has been divided into three layers viz. (i) Troposphere, (ii) Stratosphere and (iii) Mesosphere.

(ii) Heterosphere Layer

Extends from 90 km to 10,000 km. Different layers of this sphere vary in their chemical and physical properties. There are four distinct layers of gases in this sphere.

(i) Molecular nitrogen layer extends upward upto the height of 200 km (90 to 200 km).

(ii) Atomic oxygen layer extends from 200 to 1100 km.

(iii) Helium layer which extends upto the height of 3500 km.

(iv) Atomic hydrogen layer is the topmost layer of the atmosphere and extends upto the outermost limit of the atmosphere.

INSOLATION AND HEAT BUDGET

Insolation is the energy received on the earth's surface from the sun. It is the most important single source of atmospheric heat.

The earth's surface does not absorb all the energy that it receives. The proportion of the solar radiation reflected from the surface is called Albedo.

The atmosphere loses an amount of heat equal to the gain through insolation. This mechanism of maintaining the same temperature by the atmosphere is called the Heat Budget or Heat Balance.

Assuming 100 units of energy, 14 units are absorbed directly by the atmosphere and 35 units are lost to space through reflection. The remaining 51 units reach the earth's surface and are absorbed by the earth due to which the surface gets heated. The heated surface of the earth starts radiating energy in the form of long waves and this process is called Terrestrial Radiation. Out of the total 51 units given up by the surface in the form of terrestrial radiation, the atmosphere (mainly carbon dioxide and water vapour) absorbs about 34 units and the remaining 17 units escape to space.

In this manner, the atmosphere receives a total of 14 + 34 = 48 units and this amount is radiated back to space by the atmosphere. The total loss of energy to space thus amounts to 100 units: 35 units reflected by the atmosphere, 17 units lost as terrestrial radiation and 48 units from the atmosphere. In this manner, no net gain or loss of energy occurs in the earth's surface.

The he at/energy is transferred from the lower latitudes to the higher latitudes througi winds and ocean currents.

 ATMOSPHERIC AIR PRESSURE

 Air moving in a particular direction is called wind. The principal cause of winds is difference in pressure.

 Air always moves from areas of high pressure to those with low pressure. The slope of the pressure from high to low is known as Pressure Gradient.

A The direction of winds is affected by the Coriolis Force which is caused by the rotation of the earth. So, winds are deflected to their right in the Northern Hemisphere and to their left in the Southern Hemisphere. This is referred to as Farrel's Law.

Global Pressure Belts

Air is extremely compressible gas having its own weight. The pressure exerted by air due to its weight is called atmospheric pressure on earth's surface.

(i) Equatorial Low Pressure Belt (or Doldrums) 

From 5° North to 5º South latitudes. Zone of low pressure caused thermally near the equator. This belt is known as doldrums (Belt of Calm) because of calm winds prevailing here.

(ii) Tropical High Pressure Belt or Horse Latitudes

Standing between 25° and 35° latitudes in both hemispheres, this high pressure belt is dynamically induced by the rotation of the earth and sinking down of winds.

This zone is characterised by anticyclonic conditions.

(iii)Sub-Polar Low Pressure Belt

Situated between 60° and 65° latitudes in both the hemispheres.The area of contact between cold and warm air masses form a contact zone is known as Polar front, near 60° latitude.

(iv) Polar High Pressure Belt

Arctic and Antarctic Polar regions are the belts of high pressure characterised by permanent anticyclone. Polar easterly winds spiral outward form this belt.

WINDS TYPES

Three broad categories are 

(A) Regular Winds/Prevailing Winds/Planetary Winds: (e.g., Trade winds, Westerlies and Polar Easterlies)

Trade Winds

Trade in German means Track. To blow trade means to blow steadily in the same direction and in a constant course. They are regular winds.

These are steady currents of air blowing from the sub tropical high pressure belts towards the equatorial low pressure areas (doldrums). Under the influence of the Coriolis force they blow from the north-east in the northern hemisphere and from the south-east in the southern hemisphere.

Between the two trade winds systems is found the Inter Tropic Convergence Zone (ITCZ).

Westerlies

Blows from subtropical high pressure to sub-polar low pressure belt. In the northern hemisphere, land masses cause considerable disruption in the westerly wind belt.

Between 40° and 60° South lies the almost unbroken belt. Westerlies are strong and persistent here, giving rise to mariner's expressions- Roaring Forties, Furious Fifties and Shrieking Sixties.

Polar Easterlies

Move from high pressure poles to sub-polar low pressure areas.

These are deflected by the Earth's rotation to become east winds, or the polar easterlies.

(B) Periodical Winds (which blow seasonally): 

Monsoon (Precipitation)

It refers to falling of water, snow or hail from the clouds and results when condensation is occurring rapidly within a cloud.

The most common form of precipitation is rain and it is formed when many cloud droplets coalesce into drops too large to remain suspended in the air. Rainfall occurs when the dew point of air is above the freezing point.

Conditions for Precipitation

There are three possible ways by which precipitation is produced.

1. Convectional Precipitation: It 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 is accompanied by lightning and thunder. It occurs in regions near the equator in the afternoon as a result of the constant high temperature and high humidity.

2. Orographic Precipitation: In this, precipitation is caused by moisture-laden air being forced to rise over a relief barrier (mountain ranges). As the air rises in the windward side, it is cooled at the adiabatic rate, if sufficiently cooled then precipitation results.

When the air descends on the leeward side, it gets warmed and dry, having no source from which to draw up moisture. A belt of dry climate, often called a rain shadow, may exist on the leeward side.

3. Cyclonic or Frontal Precipitation: When the air is caused to rise upwards due to cyclonic circulation, the resulting precipitation is said to be of the cyclonic type.

(C) Variable Winds: (Cyclones and other local winds)

1. Cyclones

It is a system of very low pressure in the center surrounded by increasingly high pressure outwards. In this, the winds blow in a circular manner in

1. Anticlockwise direction in the Northern Hemisphere.

2. Clockwise direction in the Southern hemisphere 

Cyclone are mainly of two type

(1) Tropical Cyclone

(ii) Temperate Cyclone

2. Anticyclones

They are opposite to cyclones in all respects. They are the centers of high pressure with gentle outward flow of air. The air circulation is clockwise in the northern hemisphere and anticlockwise in the southern hemisphere.

Weather associated with an anticyclone is fair weather.

 HUMIDITY AND CLOUDS

Humidity refers to the amount of water vapour present in the air.

Humidity is measured by an instrument called hygrometer. Another instrument used for the same purpose is sling psychrometer.

Condensation, Dew Point and Related Aspects

The physical process of transformation from the vapour to the liquid state is condensation. This is the basis of all types of precipitation (fall of water from the atmosphere to the ground in any form).

 Dew point is the temperature at which the air is fully saturated and below which condensation normally occurs.

 Dew is the deposition of water droplets on the ground. It occurs when the temperature of the ground surface falls and the air in contact with it is cooled below its dew point. Dew is likely to occur on clear and calm nights.

 Frost is a weather condition that occurs when the air temperature is at or below freezing point. Then the moisture on the ground surface and objects freezes to form an icy deposit.

 Fog is made of the droplets of water suspended in the lower layers of the atmosphere, resulting from the condensation of water vapour around nuclei of floating dust or smoke particles. A visibility of less than 1 km is the internationally recognised definition of fog. Fog is not considered as a form of precipitation.

 Smog (Smoke + Fog) is a form of fog that occurs in areas where the air contains a large amount of smoke.

Mist is the term for reduction of visibility between condensation producing water droplets within the lower layers of atmosphere.

Haze is formed by water particles that have condensed in the atmosphere and the visibility in this case is more than 1 km but less than 2 km. Haze may also be produced by presence of dust and smoke, which reduce visibility.

Clouds

Clouds are masses of minute water droplets or ice crystals formed by the condensation of water vapour and held in suspension in the atmosphere. Condensation usually takes place around nuclei such as dust, smoke particles and salt. Such particles are called condensation nuclei.

Clouds are of different types and they can be classified on the basis of their form and altitude. On the basis of form, there are two major groups: (i) Stratiform or Layered types and (ii) Cumuliform or Massive types.

(i) Stratiform Clouds

These clouds, which are fairly thin and blanket like, are sub-divided into three main categories on the basis of altitude.

(a) High Clouds (height 6-20 km)

1. Cirrus: Indicates fair weather.

2. Cirrocumulus: Forms the mackerel sky.

3. Cirrostratus: Produces a halo around sun and moon.

(b) Middle Clouds (height 2.5-6 km)

1. Altocumulus: Indicates fine weather.

2 Alto-stratus: Associated with development of bad weather.

c) Low ( Clouds (height, ground surface 'upto' together 2.5 km)

1. Stratus: Brings dull weather, usually accompanied with a drizzle.

2. Nimbostratus: If rain or snow is falling from a stratus cloud, it is called nimbostratus.

3. Stratocumulus: Indicators of fair or clearing weather.

(ii) Cumulus Clouds

They are massive clouds having a vertical extent from 1,500 to 9,000m. They resemble 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 produced by convection.

Under different weather conditions, a cumulus cloud may develop into cumulonimbus the thunderstorm cloud mass of enormous size which brings heavy rainfall, thunder and lightning and dusty winds.

CLIMATE

The average weather conditions over a large area is called the climate of a place. Weather conditions over a specific length of time, usually a period of 31 years, are taken into consideration.

Climate can be classified on the basis of temperature, rainfall, evaporation, transpiration and water balance.

Isopleth

Lines drawn on map along which the value of a particular phenomenon is uniform.