Atmospheric Gases

Earth's atmosphere is an ocean of air hundreds of kilometers thick.  The air is densest at the Earth's surface in fact, about 90% of the atmosphere by weight is within 16 km of Earth's surface.  The greater the height above sea level, the thinner the air is.  The atmosphere eventually fade into the emptiness of outer space.

Air contains many gases, but most of its volume is made up of only two of them. Nitrogen and oxygen make up 99% of dry air.  These two gases are necessary for life processes.  Another gas necessary for live is carbon dioxide, which is used by plants in photosynthesis.  Carbon dioxide make up only about 0.0035% of dry air.

The atmosphere also contains small amounts of several other gases.  Water vapor ranges from almost none to about 4% of the air.  Water vapor in the atmosphere is an important part of the water cycle.

Another gas that exists in tiny quantities is ozone.  A molecule of ozone is made up of three oxygen atoms.  A molecule of oxygen is made up of two.  Most of the ozone is in the layer of the atmosphere that is between 10 and 50km high.  There it absorbs ultraviolet light from the sun, shielding Earth from rays that can be harmful to life.

Throughout the atmosphere, there are also varying amounts of dust particles.  These particles are not just dirt or soot.  Some, such as pollen grains, are from living things.  Others are remains of meteors that entered the Earth's atmosphere.  Dust particles affect the weather by providing a surface for water vapor to condense on so that it can become precipitation.

The atmosphere changes as the height about sea level increases.  That's why climber who challenge Mount Everest face more than sheer rock walls.  As they climb higher, the air grows colder and thinner.  So they carry supplies of oxygen.  Often mountain climbers must  use the supplies of oxygen because the are is too thing for them to breathe easily.  The top of Mount Everest is near the upper limits of the troposphere, the bottom layer of the Earth's atmosphere.  The troposphere stretches from sea level to about 10km above sea level.