Earths Atmosphere

The vertical extend of the atmosphere is difficult to define, it surrounds the earth and becomes increasingly thinner until at some point space is reached. The lowest layer of the atmosphere is called the troposphere and contains almost all water vapour and approximately 75% of all molecular mass of the atmosphere, its height varies from 18 km at the equator to 8 km at the poles.

The sun warms the air or atmosphere indirectly, the earth receives the solar radiation (insolation) and heats the lower layers of the atmosphere by radiation and convection. It is in this layer where we humans live, breathe and fly.

Composition

The atmosphere consists of 78,08% Nitrogen (N₂), 20,95% Oxygen (O₂) and the rest includes 0-4% Water (H₂O), 0,93% Argon (A_R), 0,039% Carbon Doixide (CO₂), 0,0018% Neon (N_E), 0,0005% Helium (H_E), 0,00017% Methane (CH₄), 0,00005% Hydrogen (H₂), 0,00003% Nitrous Oxide (N₂O) and 0,000004% Ozone (O₃). Some traces of Krypton, Xenon and Radone are also found in the atmosphere.

The most effective greenhouse gases are (in order of importance): Water, Methane and Carbon Dioxide. Since 1750 the levels of these gases have increased by: 150% for Methane and 35% Carbon Dioxide (source: http://www.physicalgeography.net).

About half of these gases are below 5000 ft and 75% is below 11000 ft. Standard sea level pressure is 760mm/hg, at 18000 ft it is 380 mm/hg (50%) and at 34000 ft it is 190 mm/hg (25%). By volume this composition stays almost the same until a height of 75km is reached. Variations are also found in the amount of water vapor, ozone and carbon dioxide.

The amount of non gaseous matter can also vary widely, depending on the location. Volcanic ashes, dust and fine sand (Sahara) can travel hundred of miles and across continents.

Water (H₂O)

Found as vapor in the atmosphere below about 20000 ft, reaching levels from zero (poles) to 4% (tropical). Although small in quantity but its effect on weather can be dramatic. When water changes state from vapor to visible (water or ice) produces clouds, rain, fog and other phenomena it has a profound effect on temperature and humidity. Water vapor has its source from oceans, lakes, rivers, forest and plant respiration.

Ozone (O₃)

Formed in the upper levels of the stratosphere and the lower levels of the mesosphere by the sun and blocks dangerous UV radiation which kills life as we know it on earth. Consists of three oxygen molecules. The amount of ozone depends on the season, amount of solar radiation and certain human produced chemicals, which are banned these days.

Carbon dioxide (CO₂)

A colorless, heavy gas with no smell. Produced by oxidation processes in human (1 kg per day) and animal life (breathing), decomposing processes in the sea and the burning of fuels, wood and other materials containing carbon molecules. This molecule consists of 13 parts of carbon and 32 parts of oxygen by weight.

Large amounts of carbon dioxide are dissolved in the oceans, plant life is also a major remover of CO₂. Off all the CO₂ emitted in the atmosphere (0,039%) only 3% is of human origin (0,00117%). Worldwide aviation emissions causes only 2% of all man made carbon in the atmosphere (0,0000234%).

To learn more about carbon dioxide (CO₂) and its real effect on our atmosphere, follow this link to CO₂ Science. A higher CO₂ level encourages growth in plantlife and is eventually converted into oxygen needed for humans. It is an essential gas for life on this planet and not a pollutant. Read more about the carbon cycle on our planet.

Solid particles

As said above, the atmosphere also contains an amount of fine solid particles as soot (from fires and burning of fuel) dust, sand, salt and volcanic ashes. These do have an effect on the temperature as they can block incoming radiation and contribute to the condensation processes which forms clouds and fog in the lower levels. If fog is formed because of this then it is called smog.

Layered Atmosphere

The atmosphere of the earth consists of a number of layers like an onion. Research with rockets and satellites have confirmed this and these layers are defined by temperature differences and or well defined laps rates.
There are four named layers of atmosphere: troposphere, stratosphere, mesosphere and thermosphere.

Troposphere

Earths Atmosphere Effects (source: NASA)

This is the lowest layer where most of the weather takes place. Its altitude varies between 18 km at the equator to 8 km at the poles. This layer lowers in steps, sometimes overlapping, at which the jet streams are found and these have their influence on the weather systems and in the development of fronts.

The temperature decreases fairly constant with 1,98°C per 1000 ft (6°C/km). This lapse rate (usually positive) can be negative or even be constant, called an inversion and isothermal respectivily. The troposphere is capped by the tropopause and over the equator the temperature is about -80°C, over the poles (where it is lower) the temperature is around -48°C during summer.

Stratosphere

The stratosphere is some 35 km thick and initially the layer is isothermal (about 10 km) then the temperature slowly increases and the last 25% the temperature increases rather quickly with altitude. The cause being the ozone which absorbs large amounts of solar radiation at these levels. The density of the top part od the stratosphere is very low, in fact, if it was the same as at the surface the temperature would be around 15°C too.

The statosphere is mainly heated by absorption of solar radiation, where as the troposphere is heating from below by conduction and convection.

Mesosphere

Starts at the stratopause at some 50 km above the surface and about 90 km thick (90000ft). The top of this layer (mesopause) has a temperature of some -90°C. The pressure is around 1 hPA and almost nil near the mesopause.

Thermosphere

Here ends the homosphere (troposphere, stratosphere and mesosphere) at about 80 km from the surface, gas molecules are here bombarded by x-rays from the sun and form the ionosphere. A layer between 90 and 300 km. This process creates oxygen and nitrogen atoms with a positive charge capable of reflection short wave radio waves from radio stations on earth. Sometimes so intense that even VHF radio waves are being reflected around the earth.

Temperatures are high due to the very high ultraviolet radiation from the sun.