Composition of the Modern Atmosphere
Our atmosphere has gone through significant changes since it was formed, eventually producing the atmosphere we have today. The composition of our modern atmosphere is shown below:
|Neon||Ne||18.20 parts per million|
|Helium||He||5.20 parts per million|
|Krypton||Kr||1.10 parts per million|
|Sulfur dioxide||SO2||1.00 parts per million|
|Methane||CH4||2.00 parts per million|
|Hydrogen||H2||0.50 parts per million|
|Nitrous oxide||N2O||0.50 parts per million|
|Xenon||Xe||0.09 parts per million|
|Ozone||O3||0.07 parts per million|
|Nitrogen dioxide||NO2||0.02 parts per million|
|Iodine||I2||0.01 parts per million|
(Atmospheric data courtesy of NOAA.gov http://www.srh.noaa.gov/jetstream/atmos/atmos_intro.htm)
Water vapor, carbon dioxide, methane, nitrous oxide, ozone, and carbon monoxide are all greenhouse gases. A greenhouse gas is a substance in the air that can absorb light as it enters the atmosphere. Different greenhouse gases absorb different spectra of light.
* - Water vapor is an extremely important greenhouse gas and it plays a significant role in climate, but it is different than other greenhouse gases because it cannot accumulate in the atmosphere beyond local relative humidity thresholds. Once the relative humidity gets high enough water vapor condenses and falls as precipitation. BTW, the average length of time it takes for all of the water vapor in that atmosphere to be replaced is between 5-9 days. Other greenhouse gases don't do this, they accumulate in the air.
** - The amount of ozone in the troposphere varies greatly with proximity to industrialized areas. Plus, ozone is a reactive gas with other atmospheric compounds like CO2 and CH4, removing them from the atmosphere. Alas, ozone is itself harmful to us, so a strategy of producing ozone to remove other greenhouse gases is not advisable. Ozone in the stratosphere, however, is extremely beneficial to us since it absorbs UV radiation that can cause things like skin cancer and cataracts in our eyes. But human-produced ozone tends to stay in the lower troposphere.
Physical Structure of the Atmosphere
There are four successive layers of the atmosphere, starting from the ground and increasing in elevation they are the troposphere, the stratosphere, the mesosphere, and the thermosphere.
(Image courtesy of NOAA - http://www.srh.noaa.gov/jetstream/atmos/atmprofile.htm)
The troposphere extends from the surface to an altitude of ~12km in altitude (~8 miles); it's thinnest at the poles. About 80% of the mass of the atmosphere and 99% of all water vapor is found in this layer. The average global temperature of the troposphere is about 15oC (~60oF) at the Earth's surface, cools to about -50-60oC at the top of the troposphere. The troposphere is warmest near the surface because this is where greenhouse gas concentrations are highest. Virtually all weather takes place in the troposphere. The creation of surface winds and large-scale movement of air masses in the troposphere will be the topic of another posting.
The stratosphere is between ~12-50km (~8-32 miles) in altitude. Most of the remaining 20% of the mass of the atmosphere is found here, as is most of the rest of the water vapor. The ozone layer that protects us from dangerous UV radiation is found here. The lower part of the stratosphere is warmest (this is where most of the ozone is), and it cools to about 0oF at its highest altitude. Ozone is formed naturally here as O2 molecules are split apart by energy from the sun and individual oxygen atoms bond with molecules of O2 to form O3 (ozone). While ozone is extremely effective at absorbing UV radiation, other wavelengths of light are not captured here.
The mesosphere is between ~50-85km (~30-55 miles) in altitude. Though atmospheric concentration is dwindling greatly at these heights, there are still enough molecules here that this is where most meteorites burn up producing "shooting stars." Since there are fewer air molecules the higher you go in this part of the atmosphere less light is captured, and less heat is released, so the higher you go in this layer, the colder it is (see the figure above).
The thermosphere is between ~85-140km (~55-87 miles) in altitude. There is so little atmosphere in this layer that if you were there you would be subjected to the full intensity of the sun's energy. This is why the top of the thermosphere is the hottest layer in the atmosphere at ~60oC (~140oF). Though the thermosphere is the top layer of the atmosphere, the space shuttle flies at even higher altitudes (~300-530km (~190-330 miles).
OK, so what!? You need to know something about the atmosphere if you want to understand climate, because climate is determined by long term averages of weather events including precipitation, relative humidity, temperature, and seasonality. That's why this matters. Enjoy!