The Atmosphere: Composition and Structure

The atmosphere can be described as a gaseous envelope held to the earth by gravitational forces. The composition of the atmosphere is fairly homogenous from the earth’s surface upward to an altitude of about 80km. it is fairly homogenous because there are internal variations in the composition even within the range.

This fairly homogenous part of the atmosphere is referred to as the homosphere. From the altitude of 80km upward, the composition of the atmosphere is no longer uniform and this layer is referred to as the heterosphere.

Factors Influencing the Composition of the Atmosphere

1. Altitude

The dense gases are found in the lower layer of the atmosphere, especially at a distance of up to 15km from the earth’s surface. The lighter gases such as hydrogen and helium are found in the upper layer of the atmosphere.

It should be noted that turbulence can bring a distortion in the atmospheric composition since the atmosphere is not static but rather dynamic.

2. Latitude and season

Gases that vary with latitude and season include ozone, water vapor, and carbon dioxide. Ozone concentration is lower around the equator and densely concentrated over latitudes 500N and S of the equator.

3. Time

Gases such as carbon dioxide present in the atmosphere vary with time. The period of time when changes take place in a society that is urbanized and industrialized to the time it achieves a high level of urbanization and industrialization is referred to here as “Time”.

It has been observed that the carbon dioxide content of the world’s atmosphere increased by 9% between 1900 and 1935 arising from urbanization and industrialization all over the world.

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The Structure of the Atmosphere

The structure of the atmosphere refers to the stratification or division of the atmosphere using the factors of temperature. In other words, it refers to the division of the atmosphere into temperature zones.

The various layers or divisions in the atmosphere are the troposphere, stratosphere, mesosphere, and thermosphere.

1. The Troposphere

This is the lowest layer of the atmosphere. It extends to a height of 16 -18km over the tropical region, and 8-10 km over the polar region, although an average of 15km is often adopted as the vertical extent of this layer.

The major characteristics of this layer include:

i. Uniform decrease in temperate with an increase in height. This rate of temperature decrease with increasing height is called Environmental Lapse Rate. This decrease in temperature takes place at a rate of 6.50C per kilometer.

ii. Increase wind speed with an increase in height.

iii. Higher quantity of moisture at the lower part of this layer.

iv. Considerable movement of air vertically.

There is considerable atmospheric instability or what we call air turbulence arising from alternating heating (during the day and cooling at night) of the earth’s surface. The upper boundary of this layer is called “The tropopause”.

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2. The Stratosphere

This is the second layer of the atmosphere. It extends from an average of 15km to 50km from the earth’s surface. It has the following characteristics:

i. Horizontal air movement.

ii. High wind speeds.

iii. Low concentration of water vapor and consideration of nuclei.

iv. Absence of clouds (except for cirrus clouds) at the lower parts.

v. Temperature is constant at the lower part about 2-3km but begins to rise with a height from the height of 20 km.

vi. This is the layer that has ozone.

vii. Maximum temperature is about 70C at the upper limit of the layer.

The combination of stratosphere and troposphere constitutes what is referred to as the lower atmosphere.

3. The Mesosphere

This is the third layer of the atmosphere and this extends from about 50km to 80 km.

General characteristics of the mesosphere:

i. Temperature generally decreases with increased height or elevation to a minimum of about 900C at its upper limit

ii. It is also characterized by very high atmospheric pressure iii. Some clouds-like particles believed to be a layer of ash produced by the oxidation of meteors are found in this layer.

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4. The Thermosphere

This is the fourth layer. Temperature increases with elevation or height up to a maximum of about 12320C at its upper limit of 480km.

Functions of the Atmosphere

i. It serves as a medium for the exchange of water and heat between the earth and the atmosphere

ii. It provides the oxygen for breathing by man and other living organisms

iii. It provides the gas (ozone) that shields the earth from the ultraviolet radiation of the sun.