Earth’s Atmosphere

The earth’s atmosphere is a thin gaseous envelope comprised mostly of nitrogen and oxygen with traces of other gasses such as water vapor, carbon dioxide.

~99% of the atmosphere lies within the 30km of the earth’s atmosphere.

Nitrogen occupies about 78% and oxygen about 21% of the total volume of the atmosphere. This percentages of N and O hold fairly constant up to an elevation of 80km.

In atmosphere there is a balance between output and input of these gasses. Nitrogen is removed from the atmosphere by biological processes involving soil bacteria. It returned to the atmosphere mainly through the decaying of plant and animal.

Oxygen on the other hand, is removed from the atmosphere when organic matter decays and when oxygen combined with other substances, producing oxides. It is also taken from the atmosphere during breathing. The addition of oxygen to the atmosphere occurs during photosynthesis.

The concentration of water vapor varies greatly from place to place. Close to surface in warm and tropical locations water vapor may account for up to 4%, whereas in colder arctic areas its concentration is mere fraction of percent.

Water vapor molecules become visible only when they transform into larger liquid or solid particles by condensation.

Water is the only substance that exist as a gas, a liquid, a solid at a temperature and pressure normally found near the earth’s surface.

During transformation from gaseous stage to solid/liquid, water vapor releases a large amount of heat- called latent heat.

Latent heat is an important source of atmospheric energy, especially for thunderstorms and hurricanes.

Moreover, water vapor is a potential greenhouse gas that absorbs the earth’s outgoing radiant energy.

Carbon dioxide is a natural component of the atmosphere occupying 0.037% of the volume of air. Carbon dioxide enters into the atmosphere mainly from decay of organic matter, volcanic eruption, exhalation of animals, burning of fossil fuels and deforestation.

The removal of CO2 takes place during photosynthesis of plants, by mixing with ocean water.

Ocean contains 50 times more CO2 than the atmosphere.

Presently CO2 levels increasing at a rate of 1.5ppm/Yr.

Like water vapor, CO2 is another greenhouse gas by absorbing portion of earth’s radiant energy. So, everything being equal, the increase in the atmospheric CO2 concentration will result in increase in average global average surface temperature.

Water vapor and CO2 are not the only greenhouse gasses. Other greenhouse gasses include methane (CH4), nitrous oxide (N2O) and chlorofluorocarbons (CFC).

Methane derives from the breakdown of the plant materials by certain bacteria in rice paddies, wet-oxygen-poor soil, biological activities of termite and biochemical reaction in the stomach of the cows.

Nitrous oxide forms in the soil through a chemical process involving bacteria and certain microbes.

Chlorofluorocarbons, mainly used as refrigerants, not only have potential for raising the global temperature but also they destroy the ozone in stratosphere.

Ozone forms when atomic oxygen combines with molecular oxygen. Most of the ozone is found in stratosphere.

Although at stratosphere the concentration of ozone is less than 0.002% by volume, but this small amount shields the plants, animals from UV rays.

Impurities from both natural and human sources are also present in the atmosphere collectively called aerosols. Ex- tiny salt particles, smoke, fine ash.

Some natural impurities let the water vapor to condense on them thereby forming the cloud.

Most-human made impurities create health-hazard called pollutants. Ex- carbon monoxide, nitrogen dioxide, hydrocarbons.

Burning of sulfur-containing fuels release colorless sulfur dioxide into the air. This sulfur dioxide transforms into sulfuric acids when combines with water vapor producing acid rain.

The Early Atmosphere

The earliest atmosphere of earth was composed mostly of hydrogen, helium, methane and ammonia. This early atmosphere escaped into the space from earth’s hot surface.

A second, denser atmosphere enveloped the earth by outgassing processes comprising of water vapor, carbon dioxide, nitrogen.

By the process of outgassing a huge amount of water vapor released into atmosphere forming the clouds.

By continuous rainfall rivers, lakes and oceans form in the earth. During this time large amount of CO2 were dissolved in the ocean.

With much of the water vapor already condensed and the concentration of the CO2 decreases, the atmosphere gradually becomes rich in nitrogen (which is chemically inactive).

Oxygen, the second most abundant gas in the atmosphere began with an extremely slow increase in the concentration as energetic solar ray splits water vapor into hydrogen and oxygen. Some oxygens are also believed to be generated from photosynthesis process.

Vertical Structure of the Atmosphere

Air molecules are held near the earth by gravity. This gravity force pulls down the air, so the air squeezes and number of molecules in a given volume increases.

As air density is the number of air molecules in a given volume, therefore air density is greatest at surface and decreases upward.

As air near the surface is compressed, so air density decreases rapidly at first, then more slowly as we move up from the surface.

The weight of all the air around the earth is 5600 trillion tons. The weight of the air molecules acts as a force upon the earth.

The amount of force exerted over an area of surface is called atmospheric pressure or air pressure. The air pressure at any point is measured in terms of the total mass of air above any point.

As the number of air molecules decreases with height, so atmospheric pressure decreases with increasing height.

Like density, air pressure decreases rapidly at first, more slowly at higher levels.

At sea level, the average or standard value for atmospheric pressure is 1013.25 mb = 1013.25 hPa = 29.92 in Hg

Notice that at an altitude of 50km, the air pressure is 1mb, which means 99.9% of all the air molecules are below 50 km.

Layers of the Atmosphere

We know both air pressure and density decrease with height- rapidly at first, then more slowly.

But air temperature has a more complicated vertical structure.

Air temperature normally decreases from the earth’s surface up to an altitude of 11 km.

This decrease in air temperature with increasing height is primarily due to the fact that sunlight warms the earth’s surface and surface in turn warms the air above it.

The rate at which the air temperature decreases with height is called lapse rate.

The average lapse rate of the lower atmosphere is 6.5°C/1000m.

If air becomes colder more quickly with height, then the lapse rate steepens and when the air cools slowly with increasing height the lapse rate is less.

Occasionally the air temperature increases with height, producing a condition called Temperature inversion.

When the temperature decreases with height, the lapse rate is positive (in Troposphere and Mesosphere).

When the temperature increases with height, the lapse rate is negative (in Stratosphere and Thermosphere).

The regions of the atmosphere from surface up to 11 km is well mixed by rising and descending air.

The regions of circulating air extending upward from the earth’s surface to where the air stops becoming colder is called the troposphere.

Above 11 km, the air temperature stops decreasing/ remains constant with height. Such isothermal zone is called tropopause.

Tropopause marks the top most limit of troposphere and beginning of the stratosphere.

Tropopause is found at higher elevation over equator and at lower elevation over pole.

Tropopause is higher in summer and lower in winter at all latitude.

In stratosphere the air temperature begins to increase with height, producing a temperature inversion condition.

This inversion restricts the air of the troposphere to spread out vertically.

Although the temperature increases with height, but the air at an altitude of 30 km is -46°C.

The reason behind this increase in temperature in stratosphere is the absorption of UV rays by ozone.

Above the stratosphere is the mesosphere. The air is extremely thin here and atmospheric pressure is quite low.

With a temperature of -90°C, top of the mesosphere is coldest place of our atmosphere.

The hot layer above mesosphere is the thermosphere. Here the oxygen molecules absorb the energetic solar rays, warming the air.

Although the temperature exceeds 500°C in thermosphere, but a person shielded from the sun would not necessarily feel hot because there are very few molecules are present in this region to bump against something.

Above thermosphere many of the lighter, faster-moving molecules travelling in the right direction escape the earth’s gravitational pull.

The region where atom and molecules shoot off into space is called Exosphere.

Below the thermosphere the composition of the atmosphere is uniform (78% nitrogen and 21% oxygen). This lower well mixed region is called homosphere.

In thermosphere due to very less number of air molecules stratification of air molecules (heavier atoms settle to the bottom and lighter float at the top). This stratified layer is called heterosphere.

Ionosphere

Ionosphere is an electrified region in the upper atmosphere, where large concentration of ions and free electrons exist.

The ionosphere starts about 60 km above the earth’s surface. So major portion of the ionosphere is in the thermosphere.

The ionosphere plays an important role in radio communication. The lower D region/layer absorbs the AM radio waves, thereby weakening the same.

During night the D regions disappears and AM radio waves are able to penetrate higher into the ionosphere (E & F regions). After reflecting from this AM waves are able to travel for many hundred kilometers at night.

Layers of the atmosphere based on temperature, composition and electrical properties are given.