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, 100 km is usually used as the border to space.
The lowest layer of the atmosphere is called the troposphere and contains almost all water vapor 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.
The composition of the atmosphere is important for sustainable life on this planet. In the lower part we find most of the weather creating circumstances so that life as we know it can prosper on this planet.
This is a natural effect and its largely self regulating combined with the daily/night exposure to the sun due to the rotation of our planet.
These pages can only scratch on the surface of a very interesting subject, as not everything is fully understood by science. Being the chaotic system our atmosphere is, I wonder if it ever will be understood.
The atmosphere consists of 78,08 % Nitrogen (N2), 20,95 % Oxygen (O2) and the rest includes 0-4 % Water (H2O), 0,93 % Argon (AR), 0,04 % Carbon Dioxide (CO2), 0,0018 % Neon (NE), 0,0005 % Helium (HE), 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 our atmosphere.
The most effective gases are (in order of importance): water, carbon dioxide and methane. Since 1750 the levels of these gases have increased by: 150 % for methane and 35 % carbon dioxide (source: http://www.physicalgeography.net). After 1998 these increases have slowed down considerably.
But to assess the effect of these gases we need to take a look at the absorption of each of them at the molecule level. See image to the right. As it turns out that, for example methane (CH4) is totally masked by water vapor!
Source (image too): WUWT article, Methane: The Irrelevant Greenhouse Gas .
About half of these gases are below 5000 ft and 75 % is below 11000 ft. Standard sea level pressure is 760 mm/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 an altitude of 75 km 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.
Found as vapor in the atmosphere below about 30000 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, animal and human respiration.
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. It consists of three oxygen molecules. The amount of ozone depends on the season, amount of solar radiation and some say certain human produced chemicals, which are banned these days.
A colorless, heavy gas with no smell. It is produced by oxidation processes and expelled by breathing in humans (average 1 kg per day) and animal life, decomposing processes in the sea and the burning of fuels, wood and other materials containing carbon molecules. Which is normal as this planet and everything on it is carbon based. 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 this gas. They are made of carbohydrates (carbon, hydrogen, oxygen) living of the CO2 in the air. It is also known that the best food crop yields occur when plants are grown in atmospheres that are triple or quadruple current levels compared to our planet today (Source: http://wattsupwiththat.com/2011/01/06/co2-is-plant-food-clean-coal-say-watt/).
A higher level encourages growth in plant life and is eventually converted into oxygen and food needed for humans. It is an essential gas for life on this planet and NOT a pollutant! One could even come to the conclusion that the current levels are just enough to sustain plant life. Basically: without it there would be no life possible as we know it today. Trying to reduce it could be seen as a crime to humanity. Click the next link for an impression of important CO2 levels or here for CO2 levels in the past, click the back button to return. Increasing CO2 levels results in even less effect in the atmosphere.
Off all the CO2 emitted in the atmosphere (0,04 % / 400 ppm in 2017) only about 3% is of human origin (0,0012 % / 12 ppm) (source WUWT: EPA document supports ~3% of atmospheric carbon dioxide is attributable to human sources ). Worldwide aviation emissions results only 2% of all man made carbon dioxide in the atmosphere (0,000024 % / 0,24 ppm).
On top of that: NASA and Science Daily concluded that CO2 even cools and greens the planet. (source: NASA Science News March 22, 2013 and Science Daily, July 2013: Deserts are greening from extra CO2 and CO2 fertilization .
Any attempt to remove the human origin part of this gas from the atmosphere will have absolutely no measurable effect at all on the climate. Financially, our economy WILL go bankrupt trying to accomplish this!
Like carbon dioxide, methane is a trace gas and with an amount of about 1,7 ppm (0,00017 %) it is not very abundant in our atmosphere. Its specific gravity is 0,55 (lighter than air) and specific heat is 1,305 so it heats and cools slower than air.
The amount of methane has increased from 1,7 ppm (1990) to 1,75 ppm in 1998, after which the amount added to the atmosphere has decreased reaching levels of around 1,8 ppm in 2010. (source WUWT: Methane Madness )
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 might 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.