The earth's atmosphere (without considering water vapor) is mainly composed of two gasses:

Nitrogen (N₂) 78%

• This really doesn't do much in the atmosphere. It is needed in the ground by some plants (where it is taken in to the ground as nitrates any time lightning strikes the ground).  This constitutes another "cycle" know as the nitrogen cycle. You learn more about this in a biology class.

• You need this to breathe!
• It is/was produced as a byproduct of photosynthesis.  Earth's very early atmosphere did not have any (measurable) oxygen.  The first life forms on earth did not use photosynthesis (anaerobic bacteria)... in fact, oxygen was poisonous to them.  Photosynthetic bacteria changed all that by finding a way to use sunlight as an energy source. The byproduct was oxygen.
• Without oxygen, there is no ozone, O₃ , in the atmosphere.  Without ozone, life would be limited to the sea because ozone filters out much of the harmful UV radiation from the sun.

The rest is:

Argon (Ar) .9% - This is an inert gas which doesn't really do much.

Carbon Dioxide (CO₂) .03%
You already know the importance of this gas and the role it plays in the carbon cycle.  Specifically, it is important because the amount you find in the atmosphere directly effects:

• Chemical weathering of rocks
• Global warming - the greenhouse effect
• In the last century, global average temperatures have risen 1 degree Fahrenheit.
• The global average temperature now is 59 degrees F ... with no greenhouse gasses in the atmosphere (including other gasses such as methane and water vapor) ... the global average temperature would only be 0 degrees F.
• The atmosphere of Venus is almost all CO2 and the surface temperature there is hotter than your oven (in the self clean mode).
• The debate continues ... how much of the global warming comes from CO2 emissions from the burning of fossil fuels?

• Mostly concentrated in the part of the atmosphere known as the stratosphere.  This gas filters out much of the suns harmful UV (ultra violet) radiation.  The little amount of UV that does get through can cause sunburn when you stay on the beach too long.  Always use UV sunglasses and use plenty of UV blocker.
   
• Ozone is actually poisonous to us and is "manufactured" near the surface of the earth during lightning strikes (ever smell a storm?) and as part of a photochemical reaction with fossil fuel emissions (ozone alerts).  It can make breathing difficult and cause health problems.

Solar Radiation and the Atmosphere

Ultraviolet     = short wavelength ... less than .0004 mm
Visible        = wavelength between .0004 mm (violet) and .0007 mm (red)
Infrared    = long wavelengths ... longer than .0007 mm

How much of each type does the sun produce?  This is known as a radiation curve.  Click here, slide the temperature bar to around 6000 and click "new".  This will draw a radiation curve for the sun.  It should look something like this:

• 44% of its radiation in the visible.
• 7% of its radiation in the UV.
• 37% of its radiation in the near IR.
• 11% of its radiation in the far IR.
• 1% other types (x rays, microwaves, radio).

About:

• 5% is scattered (back to space) by the air.
• 27% is reflected (back to space) by clouds.
• 20% is absorbed by clouds and the air.
• 3% is reflected off the surface.

• Some objects absorb radiation better than other objects.  For example, black objects absorb better than white objects.  Also, a red object appears red because it is absorbing all the colors of the spectrum except red (which it reflects).
• Some objects absorb radiation and use it for food (which is why a patch of green grass doesn't get so hot).
• Different objects heat up to different temperatures when they absorb radiation.  For example, water will heat up very little when it absorbs radiation but land heats up quite a bit more (about 5 times more) when it absorbs the same amount of radiation.  A physicists would say that water has a higher specific heat than land.  Also, water will distribute much of the heat it absorbs to deeper layers because water is transparent (so it doesn't just absorb at the surface) and can transport heat easily from the surface to deeper layers by mixing.  You may be very familiar with the phrase "cooler near the lake" on a hot sunny summer day.  Now you know why.   Because of these properties, large bodies of water can moderate the climate (produce less extremes) than over land.  Land tends to get very hot in summer and very cold in winter (when compared to water).  This idea is important in understanding many ways water and land can effect the climate.

So what does an object do when it absorbs radiation?  It not only heats up, but it then re-radiates energy back to the sky ... but at a much longer wavelength than the type it absorbed in the first place.  In the case of the earth, the radiation coming off the surface is in the far infrared.  But gasses in the atmosphere like CO2, water vapor, and methane easily absorb this energy and heat up the atmosphere instead.  This is the greenhouse effect!  This means that the lower atmosphere is actually heated from the ground up by radiation emitted by the earth (and by conduction/convection originating from the surface).

Because the lower atmosphere is heated from the ground up, the higher up you go in the atmosphere, the cooler it gets (to a point).  On the average, for every 1000 feet you move up in the atmosphere, the temperature drops 3½ degrees Fahrenheit.  It is easy to see why glaciers can exist on the tops of mountains.

The Troposphere

The jet stream (covered later) can be found at the top of the troposphere.  It also marks the boundary for the next layer ...

The Stratosphere

This is good for us because UV radiation is harmful.  But why is ozone concentrated here?

To make ozone (O₃) you need diatomic oxygen (O₂) and monatomic oxygen (O) and lots and lots of tricky collisions.  Monatomic oxygen is found at the very top of the atmosphere but at that level, the air is so thin that there are not very many collisions .... (an important ingredient) to make ozone.  At the surface the air is more dense, so many collisions are possible ... but there is not enough monatomic oxygen to make ozone.  Only in the stratosphere is there the right combination of ingredients and necessary collisions to successfully manufacture ozone.

Perhaps you heard about the problems dealing with the "hole" in the ozone layer mostly over the south pole (in spring) ... but is seems to be thinning in vast regions as well.  It seems to be getting worse!  Is this caused by air pollution from "chloro- fluorocarbons".  The use of these chemicals in spray cans has been outlawed and the type of chemicals being used in refrigerators and air conditioners is being replaced with a new kind that is less harmful to ozone.

The top of the stratosphere marks the beginning of:
 

The Mesosphere

The Thermosphere

©Jim Mihal 2004 - all rights reserved