The Atmosphere

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The Atmosphere
I. Introduction:

- The earth's atmosphere consists of a mixture of various gases surrounding the earth to a height of many km. Although almost all of the atmosphere (97%) lies within 29 km of the earth's surface, the upper limit of the atmosphere can be drawn approximately at a height of 10000 km. The science of meteorology deals with the physics of this atmosphere.

- Although other important properties of the atmosphere, such as temperature and pressure, can vary considerably in both time and place, its composition in terms of the relative proportions of the gases present in any unit volume, tends to remain remarkably constant, at least in the lower layers of the atmosphere. Besides gases, the atmosphere also contains water vapour and variable amount of solid material.
II. Composition of the Air:

A. Permanent Gases: The gases which are the major parts to composite the air. Most likely, they always keep in a fixed proportion in the Atmosphere. The main component gases of dry air are listed below. It will be noticed that nitrogen and oxygen make up about 99% of the volume and that the other 1% is chiefly argon.

Gases Percentage %

N2 78.084

O2 20.947

Ar 0.934

CO2 0.030

Ne 1.82 x 10-3

He 5.24 x 10-4

Kr 1.14 x 10-4

O3 6.00 x 10-5

H2 5.00 x 10-5

Xe 8.70 x 10-6

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B. Changing Gases: The gases which are changing in difference places and time. Generally speaking, they are Carbon Dioxide (CO2), Vapour (H2O) and Ozone (O3).
CO2: It is distributed in the lowest layer of the Atmosphere (Troposphere). Although constituting only about 0.03%, it is a gas of great importance in atmospheric processes because CO2 is one of the major gases which can absorb much terrestrial radiation (long wave radiation), thus the lower atmosphere can be warmed by heat radiation coming from the sun and from the earth's surface. The major source of CO2 is the combustion of fossil fuel such as Coal, Oil, Gas ...... etc..
H2O: The proportion of H2O in the Atmosphere is about 0-4% (It is various in difference places). The sources of H2O are evapotranspiration from the plant and evaporation from the water bodies on the earth. It is not found at great heights in the atmosphere, partly because mixing and turbulence is not sufficiently strong to carry it up very far, and partly because the upper atmosphere is too cold to absorb it. (about 50-60% H2O is distributed over the land 1.5km-2.5km). However, it presence in the atmosphere is fundamental to many essential meteorological processes and all the weather phenomena.
O3: O3 is distributed in the Ozone layer (over the land 20km-25km) which is in the Stratosphere. Ozone layer can absorb the short wave radiation from the universe and protects the living bodies of the earth.
Others: The remaining gases of the atmosphere are neon, helium, krypton, xenon, hydrogen, methane and nitrous oxide. They are present in extremely minute percentages.
C. Solid Impurities: Include dust (man-created pollution), chemical salt (introduced by evaporation over oceans), pollen, smoke ...... etc., they make the air look like dirty and reduce the visibility. They also can absorb much long wave radiation and cause the temperature increase. They provides the necessary nuclei on which water vapour can condense to form water droplets and eventually precipitation.
III. Structure of the Atmosphere:

The atmosphere can be divided conveniently into a number of rather well-marked horizontal layers, mainly on the basis of temperature.

A. Troposphere: The Troposphere extends from the earth's surface to a height of approximately 8 km in the polar regions and 16 - 19 km over the equatorial regions. It is the zone where weather phenomena and atmospheric turbulence are most marked and contains 75% of the total gaseous mass of the atmosphere and virtually all the water vapour and aerosols. In this layer, there is a general decrease of temperature with height at a mean rate of about 6.5oC per km (Environmental Lapse Rate), and the whole zone is capped in most places by a temperature inversion level. This inversion level or weather ceiling is called the tropopause (Ceiling of the troposphere). It marks the general upper limit of the transfer of atmospheric properties by large-scale vertical turbulence and mixing.

B. Stratosphere: The second major atmospheric layer is the stratosphere which extends upwards from the tropopause to about 50 km. The lower stratosphere has a remarkably stable, approximately isothermal temperature distribution. It is characterized by the persistence of its circulation patterns and high wind speeds. When circulation changes do occur, they take place very rapidly. Cirrus clouds occasionally form in the lower stratosphere, but visible weather phenomena usually are absent above the tropopause.

The isothermal character of the lower stratosphere terminates at a height of approximately 20 km. It is replaced by a very gradual temperature increase with height. This region is known as the upper stratosphere.

Upward through the stratosphere there sets in a slow rise in temperature until a value of about 0oC is reached at about 50 km. Here, at the stratopause, a reversal to falling temperature sets in.

C. Mesosphere: Above the stratopause average temperatures decrease to a minimum of about -90oC around 80 km. This layer is commonly termed the mesosphere. Above 80 km temperatures again begin rising with height and this inversion is referred to as the 'mesopause'. Pressure is very low in the mesosphere, decreasing from about 1 hPa at 50 km to 0.01 hPa at 90 km. The upper limit of the mesosphere is known as the mesopause. It is a transition zone between the mesosphere and the thermosphere.
D. Thermosphere: Extending from the mesopause and having no well-defined upper limit is the thermosphere. It is so called because of very high thermodynamic temperatures which may reach 1370oC. Auroral displays are a phenomenon of the lower thermosphere.

IV. Climatic System:

1. The Atmosphere as an energy system:

The Atmosphere is not a closed energy system. It is in contact with both the Earth and with space, and receives energy from both directions (earth and sun). However, the Earth itself directly contributes only a negligible amount of energy to the atmosphere, and its main role is to reflect energy from elsewhere. The ultimate sole source of atmospheric energy is in fact heat and light received through space from the sun. This energy is known as solar insolation.

All life processes as well as practically all exchanges of matter and energy at the interface between the earth's atmosphere and the surfaces of the oceans and lands are supported with radiant energy supplied by the sun. The planetary circulation systems of atmosphere and oceans are driven by solar energy. Exchanges of water vapour and liquid water from place to place over the globe depend upon this single energy source.

V. Atmospheric Energy - Energy Input : Solar Radiation

1. Importance:

The prime source of the energy injected into our atmosphere is the sun, which is continually shedding part of its mass by radiating waves of electro-magnetic energy and high-speed particles into space. This constant emission, called insolation, is important because it represents in the long run almost all the energy available to the earth.

All weather phenomena are affected by various meteorology parameters. They are interacted with each other. However, the main cause is the energy input - solar insolation.

Solar radiation is the source of energy required for photosynthesis. Animal lives, whether as herbivores or carnivores, are directly or indirectly dependent on plants for food and energy and so are indirectly dependent on solar energy.

Solar radiation is also important to human activities since the food crops required by man and the fodder crops carry out photosynthesis only in the presence of solar energy. Besides, solar energy sets the water cycle into motion and is the ultimate force for the circulation of moisture which supplies the water needed by human living.
2. Nature:

The surface of the sun, with an average surface temperature is about 6000oC. It emits an enormous amount of energy into space.

The energy emitted by the sun is a spectrum or combination of energy of different wavelengths (magnetic wave).
Most of the sun's energy is in the form of visible light rays which fall into the short-wave category. However, most of the energy emitted by the earth is in the form of in the infrared rays which fall into the category of longwaves (terrestrial radiation). As a whole, the sun's energy is more intense and shorter in wavelength than the energy emitted by the earth.

Generally speaking, the higher temperature of the object surface emits shorter radiation. The temperature of the sun's surface is 6000oC, but the earth's surface temperature in average is only 15oC.

a. Short-Wave Radiation (Wavelength < 0.4 micron )

These include ultra-violet rays, x-rays and gamma rays. They are invisible and harmful for the living organisms. Fortunately, most of them have been absorbed first by oxygen ions (Thermosphere) and ozone layer (Troposphere).

b. Medium-Wave Radiation (Wavelength 0.4 - 0.8 micron )

These are visible lights : violet, indigo, blue, green, yellow, orange and red. About 90% of the solar energy is concentrated in this visible group of radiation. It provides most of the heat energy to the atmosphere.
The visible light cannot be transformed into heat energy directly, only when it touches the ground and is reflected. So, the heat that produces air temperature is mainly derived from the ground. It is why air temperature decreases with height.
c. Long-Wave Radiation: (Wavelength > 0.8 micron )

These are infrared waves, micro waves and radio waves. Most of them would be absorbed by ozone, carbon dioxide and clouds when injecting into the atmosphere.

VI. Energy Transfer within the earth:

1. Receipt of Solar Radiation at the top of the earth's atmosphere

This is found to be dependent on four factors: solar output, distance between the sun and the earth, angle of solar incidence and length of daytime.
a. Solar Output:

The energy emitted by the sun is 105.3 x 106 cal/cm2 min. There are however 11-year cyclic variations of 1% in the output of solar energy because sunspots (dark areas visible on the earth surface) change in numbers and positions. More energy can be received at the top of the atmosphere when sunspot activity is less active.

b. Distance from the sun

The earth revolves around the sun in an elliptical orbit. It is closest to the sun on 3rd January (perihilion) at a distance of 147.3 million km and farthest away on 4th July (aphelion) at a distance of 152.1 million km. The is about 7% of the total energy difference received by the earth.

The amount of solar radiation received by any body varies inversely with the square of its distance from the sun. Assuming an average distance of 150 million km of the earth from the sun. The amount of solar radiation received at the top of the atmosphere is 1.9 cal/cm2min. Because this value is unvarying at all times, it is also called the Solar Constant.
c. Angle of Incidence:

Angle of solar incidence/ (angle of solar insolation)/ (altitude of the sun) is the angle between the sun's rays and the horizon.

Even at the top of the atmosphere, the greater the angle of solar incidence, the more concentrated is the radiation intensity per unit area and hence the higher is the temperature.
The angle of the sun's rays determines the intensity of insolation on the earth surface. The energy of vertical rays 'A' is concentrated in square 'a', but the same energy in slanting rays 'B' is spread over larger area 'b'.
Intensity of insolation is greatest where the sun's rays strike vertically. With diminishing angle, the same amount of solar energy spreads over a great area of ground surface. Hence, on the average, the polar regions receive the least heat per unit area. Therefore, average air temperatures are maximal at low latitudes and minimal near either pole. The inclination of the earth's axis helps redistribute the yearly total insolation toward higher latitudes, but deducts somewhat from the equatorial zone.
The angle of solar incidence is affected by latitude, the time of day and seasons.
i. Latitude: As the earth is a spherical body, the angle of solar incidence varies between different latitudes of the earth surface

ii. The Time of day: The angle of incidence in any one day is lowest during sunrise and sunset but is at a high angle during noon-time. In other words, the angle increases from sunrise to a maximum during noon-time and decreases in the afternoon to another minimum during sunset.
iii. Seasons: The earth completes one revolution around the sun on a horizontal plane in 365.25 days. As it revolved around the sun, the earth also rotates about its own axis of rotation (N-S axis) that is tilted at an angle of 66.5o to the horizontal. Because of the occurrence of rotation and revolution, the mean angle of solar incidence at any place is constantly changing.

d. Length of Daytime:

The longer the daytime (time between sunrise and sunset) the more solar radiation is received. The higher temperature of a summer day than that of a winter day is a direct result of the greater length of daytime in summer although it is also due to the attendant larger angle of solar incidence.

2. Receipt of solar radiation at the earth's surface:

The solar energy received at the earth surface is much less than that received at the top of the atmosphere. This reduction in energy receipt is mainly due to the effects of the atmosphere, cloud cover, differences between different surface covers, the effects of latitude, and elevation and aspect.

a. Basin terms:

i. Transmit: Solar radiation passes through an object (particle) and the object cannot gain any energy from this process.

ii. Absorb: Solar radiation passes through an object and some energy has been captured by the object.

iii. Reflect: Solar radiation cannot pass through an object, having been changed the moving direction.

iv. Diffuse reflection/ Scattering: Solar radiation cannot pass through an object, having been changed the moving direction in irregular paths.

b. Effects of the Atmosphere:

Incoming solar radiation possessing 100% of its original energy at 150 km above the earth surface loses its energy by way of absorption, reflection and scattering by atmospheric gases, water vapour, dusts and clouds.

i. Absorption at the Upper Atmosphere:

At the upper atmosphere, the radiation spectrum possesses almost 100% of its original energy. In penetration to an altitude of 88 km, most X-rays are absorbed and some of the ultra-violet radiation has been absorbed as well.

ii. Rayleigh Scattering and Diffuse Reflection:

In the lower atmospheric layers, gas molecules cause the visible light rays to be turned aside in all possible directions, a process known as Rayleigh Scattering. Where dust particles are encountered in the tropopause, further scattering occurs.

Blue light of shorter wavelength is scattered and reaches our eyes indirectly from all parts of the sky. Thus, the clear sky appears blue in colour. The red and infra-red rays are less subjected to scatter and largely continue to reach the earth. Therefore, the setting sun appears red.
As a result of all forms of short wave scattering, some solar energy is returned to space and forever lost, while at the same time some scattered short-wave energy is directed earthward.
iii. Absorption by Carbon Dioxide and Water Vapour:

Both carbon dioxide and water vapour can directly absorbing infra-red radiation. Absorption results in a rise of sensible temperature of the air.

According to the Hougton Model(Energy Budget Model), 19% of insolation is absorbed directly or indirectly by ozone and water vapour. Totally 34% of insolation is reflected back into space from the atmosphere, clouds and the earth's surface, only 66% to heat the earth and the atmosphere.

The surface absorbs 47% of the incoming energy available at the top of the atmosphere and re-radiated it outwards as long waves (infrared). Much of this re-radiated long-wave energy can be absorbed by the water vapour, carbon dioxide and ozone in the atmosphere, the rest escaping through atmospheric windows back into outer space.

c. Effects of Cloud Cover:

Cloud cover can form a significant barrier to the penetration of insolation. The amount of insolation reflected depends on the amount of cloud cover and thickness and cloud types. The proportion of incident radiation that is reflected is known as albedo.

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