What is air made of?  (Composition of air)

    Nitrogen 78% (essential for life)
    Oxygen 21% (essential for life)
    Carbon dioxide 0.03% (essential for plant life)
    Argon 0.9%
    Water vapor (from 0% to 4%
    Trace gases (neon, helium)
        (see a graph of gases in atmosphere, today and early earth, here
The Oxygen - Carbon Dioxide Cycle - a cycle which keeps the  total amount of oxygen and carbon dioxide the same.

Plants
*convert carbon dioxide to oxygen and carbon through photosynthesis
*  dead plants decompose, returning carbon to the earth
* if plants burn, carbon dioxide is released.

Animals 
* animals breathe in oxygen and release carbon dioxide
* dead animals decay, returning carbon to the earth

Fossil fuels
* dead plants that did not decompose (this takes millions of years) and become fossil fuels- coal, oil, natural gas.  When we burn these, they add carbon dioxide to the atmosphere.

The Nitrogen Cycle - a cycle that keeps the amount of nitrogen on earth the same.

The nitrogen in the air can't be used directly by most organisms.

    * Lightning converts nitrogen to compounds that are brought
        to earth by rain

    * Bacteria living in plant roots remove nitrogen from air and
        release it to the soil

    * Bacteria in soil help decay dead organisms and when they do  
        this, they produce nitrogen.

    * When coal or gas is burned, nitric oxides are released to
        the air.

Air density - Air is matter.       Air has density.
Density is affected by gravity.  Gravity decreases as you go higher.  Density of the air decreases too.

Temperature also affects density.  Cold air is more dense than
    warm air.

Air pressure - * warm air is less dense and exerts less pressure.
                       * cooler air is denser and exerts more pressure.
                       * water vapor in the air affects pressure.  Water
                            molecules are lighter than gas molecules.  If
                            more water molecules are in the air, then the
                            air pressure decreases.
                            http://www.usatoday.com/weather/wbarocx.htm   (learn more about air

Conduction - the direct transfer of heat from particle to particle. (This happens in the solid state of matter)
 
 

Convection - The transfer of heat in a gas or a liquid. Heated air rises, then cools. It sinks forming a convection cell. Uneven heating of the earth between the poles and the equator sets up large convection cells in the atmosphere.
 
 

Radiation - Energy that travels through empty space. The earth receives radiant energy from the sun. Some of the radiant energy is absorbed by the atmosphere. Some is reflected back into space.
 

It is important to protect your skin from absorbing too much of the sun's radiant energy. Use sunscreen, wear protective clothing, stay out of the sun between 10:00 A.M. and 3:00 P.M.