Structure Of The Atmosphere
The Structure of the atmosphere as interesting as it is intriguing due to its fascinating features which are mysterious also. With Lithosphere and Hydrosphere, the Atmosphere is also an integral part of mother earth, held to earth’s surface by the force of gravity. The atmosphere has evolved as a significant component of biospheric ecosystem through the process of de-gasing or vulcanism.
Nitrogen (78 %) and oxygen ( 21 %) together constitute 99 % of the total gaseous composition of the atmosphere. Nitrogen is not very active chemically and its main function is to regulate combustion by diluting oxygen. In contrast, oxygen is highly active chemically and readily combines with other elements (oxidation). Combustion represents a rapid form of oxidation while chemical weathering represents a very slow from of weathering . 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 reradiates it back to the earth , thus adding to the warming of the lower atmosphere.
- Ozone, less than 1.0001% by volume is another important gas. It protects the life layer from the harmful effect of ultraviolet rays by absorbing it. It is concentrated between the altitude of 20 and 30 km.
- About 90% of the total water vapour present in the atmosphere is found up to the height 5 km. Its content ranges between 0 and 5 % in the atmosphere. Vapour depends on temperature, so its amount decreases from the equator to poleward. This water vapour is responsible for various types of weather phenomena e.g. clouds, fog, rainfall, frost, dew etc., water vapour also absorbs not only long wave terrestrial radiation but also a part of incoming solar radiation , thus regulating the energy transfer through the atmosphere.
- Except the gases and the water vapour all the particles present in the air come under the particulate matter. These particles act as hygroscopic nuclei thereby helping various types of condensation. The blue colour of sky, the dawn and twilight are result of scattering by the rest particles.
Structure of the atmosphere:
- About 50% of the atmosphere lies below the altitude of 5.6 km and 97% of the atmosphere is confined to the height of 29 km. On the basis of temperature and pressure , following layers of atmosphere are recognized:
- This lowermost layer extends to an average altitude of 10 km which varies between 18 km above the equator and 8 km above the poles.
- contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapor and aerosols.
- Most weather phenomenon occur in this layer.
- The troposphere extends upward to about 10 km (6.2 miles or about 33,000 feet) above sea level.
- The height is more above the equator due to convective movement of the air.
- In the troposphere the temperature decreases with increasing at the rate of 6.50 centigrade per km which is called normal lapse rate. Most of the weather phenomena take place in the troposphere .
- The boundary line which separates troposphere from the next layer is called “Tropospause”.
- Therefore this layer plays a pivotal role in the structure of the atmosphere.
- Another key ingredient of the structure of the atmosphere is Stratosphere as it contains the Ozone layer without which life would not have been possible on earth.
- Lying above to a height of about 50 km.
- The temperature ceases to decrease in the lower part of the stratosphere after which is starts increasing. Strong and persistant winds blow in stratosphere from west to east.
- Stratosphere normally holds very little water vapour or dust, so weather disturbances are absent here. Now a days , jet air flights take place mainly in the stratosphere .
- One important feature of the stratosphere is the existence of ozone layer between 15 to 35 km which absorbs harmful solar ultraviolet rays. In fact the warming of the stratosphere with altitude is caused largely by the absorption of solar energy by ozone molecules.
This third layer extends from Stratopause at 50 km ( the dividing zone between stratosphere & Mesosphere ) to Mesopause at 80 km. In this layer, temperature decreases from 00C at the Stratopause to -800 at the Mesopause.
- Bulk of the meteors are destroyed in this region. Some material from meteors lingers in the mesosphere, causing this layer to have a relatively high concentration of iron and other metal atoms.
- The mesosphere is difficult to study, so less is known about this layer of the atmosphere than other layers. Weather balloons and other aircraft cannot fly high enough to reach the mesosphere. Satellites orbit above the mesosphere and cannot directly measure the traits of this layer.
- Scientists use instruments on sounding rockets to sample the mesosphere directly, but such flights are brief and infrequent. Since it is difficult to take measurements of the mesosphere directly using instruments, much about the mesosphere is still mysterious.
In this sphere temperature increases rapidly. Thermosphere is divided into two layers :
- Ionosphere: Extending between 80 km and 400 km , this is an electrically charged layer.Temperature remains constant. This layer reflects radio waves transmitted from the earth back to the earth.
- Exosphere: This uppermost layer extending beyond ionosphere imperceptibly merges with the outer space. Temperature becomes extremely high but the highly rarefied air holds little heat, so the high temperature is not felt. Hydrogen & Helium predominate in this region. This layer does not play a very important role in the structure of the atmosphere as it merges with the outer space.
Solar radiation that is intercepted by the earth Is known as insolation i.e., incoming solar radiation. 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 centimetre per minute. Insolation is measured with the help of pyronometers. The amount of insolation depends on following factors.
- The area and nature of the surface.
- The inclination of the rays of the sun.
- Lengths of the day.
- Distance between the earth and the sun.
- The transparency of the atmosphere.
- Vertical rays spread over minimum area of the earth’s surface bring more insolation than oblique rays. Hence as the angle of the sun’s rays decreases pole ward, the amount of insolation received also decreases in that direction.
- The amount of solar radiation received by earth is ½ billionth part of the total energy radiated from the outer surface of the sun.
- On January 3 the earth comes closer to the sun. Hence the amount of incoming solar radiation is about 7 per cent more in January.
Heat Budget of the earth
On an average , there is supposed to exist heat balance between the amount of solar radiation received by the earth’s surface and its atmosphere and the amount of heat lost by the outgoing terrestrial longwave radiation from the earth’s surface and loss of heat from the atmosphere out of the total incoming solar radiation entering the earth’s atmosphere , 35 % is sent back to space which is called albedo of the earth. Thus, this another aspect of the Structure Of The Atmosphere.
- Incoming shortwave solar radiation = 100%
- Amount lost to space
- Reflected by the clouds = 27 %
- Reflected by the grounds = 2 %
- Scattered energy lost to space = 5%
Total = 35% ( Albedo)
Remaining solar energy = 100 – 35 = 65
- Heat budget of the earth
- Received through direct radiation = 34%
- Received as diffuse day light = 17%
Total = 51%
- Heat budget of the atmosphere
- Absorption of incoming solar radiation = 14%
- Received from outgoing terrestrial radiation = 34%
Total = 48%
51% & 14% = 65%
- Energy sent back to space = 35% + 17 % ( through radiation from the earth) + 48% ( through radiation from the atmosphere ) = 100% . In this way whatever energy is received by the earth and its atmosphere is returned back to the space.
Heating and cooling of The Atmosphere
- By direct insolation: Atmosphere absorbs 14% of incoming shortwave solar radiation but it is too low to heat the atmosphere significantly.
- By conduction: it occurs through contact but air is a poor conductor of heat, so it is least important in the heat transfer.
- Convection & advection: These involve movement of gases for the heat transfer. Convection denotes vertical motions in the atmosphere , while advection comprises horizontal transport of heat.
- Terrestrial Radiation: The process of transfer of heat without the aid of a material medium is called radiation. Atmosphere is more or less transparent for incoming shortwave solar radiation but it absorbs more than 90% of long wave terrestrial radiation through carbon dioxide, water vapour, dust particles and ozone. Thus terrestrial radiation is the primary source of the heating of the atmosphere. That is why there occurs fall in temperature with altitude in the troposphere.
Latitudinal Heat Balance
There is large energy surplus area up to 400 latitude north and south due to more insolation received ( due to low angle of incidence of sun’s rays ) than is lost to space similarly there exists energy deficit areas in the high latitudes where outgoing terrestrial radiation exceeds insolation. However the fact is that the energy surplus area are not getting hotter or conversely the energy deficit area are not getting cooler! The oceans and atmosphere are giant thermal engines which transfer heat from the low latitudes to high latitudes through the movements of air masses, winds & ocean current.
World Temperature Patterns
- Temperature decrease from the equator to the poles.
- Lowest temperature are experienced in January over the northern continents of Asia & North America in the Artic and sub-arctic zone.
- The highest temperature for both January and July are found over the continents.
- All the isotherms to over northward between January and July.
- Seasonal changes are less prominent over the southern continents than over the northern continents.
- In the northern hemisphere, isotherms bend poleward over the ocean and equatorward over the continents in the month of January since continents cool faster than ocean.
- Generally the annual range of temperature increases from the equator to the poles.
- Around 600 latitudes of north America and Asia the greatest range of temperature occurs but not at the poles.
- Coastal regions have smaller range of temperature than continental interiors.
- In the same latitude , the eastern sides of north America and Asia have greater range of temperature.
- Highlands are always colder than surrounding lowlands.
Inversion of Temperature
Normally air temperature decreases with increasing height at the rate of 6.50C per 1000 meters (called normal environmental lapse rate). But under certain weather conditions and over a limited height range air temperature increases with elevation, called inversion of temperature, in which warmer air overlies a colder layer. The temperature inversion is commonly experienced in valley and hollow. The favorable conditions for inversion to take place are :
- Calm clear winter nights
- When radiation has caused considerable cooling and the cold air has sunk down into them.
- When sky is cloud free and anticyclonic conditions prevail.
- Dry air near the ground surface.
- When the earth surface in covered with ice, snow & frost.