|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|
The atmosphere is a cloud of gas and suspended solids extending from the Earth's surface out many thousands of miles, becoming increasingly thinner with distance but always held by the Earth's gravitational pull.
The atmosphere is made up of layers. it surrounds the Earth and holds the air we breathe; it protects us from outer space; and holds moisture (clouds), gases, and tiny particles. In short, the atmosphere is the protective bubble we live in.
This protective bubble consists of several gases (listed in the table to the right) with the top four making up 99.998% of all gases. Of the dry composition of the atmosphere nitrogen, by far, is the most common. Nitrogen dilutes oxygen and prevents rapid burning at the Earth's surface. Living things need it to make proteins. Oxygen is used by all living things and is essential for respiration.
It is also necessary for combustion or burning.
Argon is used in light bulbs, in double-pane windows, and to preserve the original Declaration of Independence and the Constitution. Plants use carbon dioxide to make oxygen. Carbon dioxide also acts as a blanket that prevents the escape of heat into outer space.
These percentages of atmospheric gases are for a completely dry atmosphere. The atmosphere is rarely, if ever, dry. Water vapor (water in a 'gas' state) is nearly always present up to about 4% of the total volume. In the Earth's desert regions (30°N/S) when dry winds are blowing, the water vapor contribution to the composition of the atmosphere will be near zero.
Water vapor contribution climbs to near 3% on extremely hot/humid days. The upper limit, approaching 4%, is found in tropical climates. The table below shows the changes in atmospheric composition with the inclusion of different amounts of water vapor.