The atmosphere 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 in which we live.
This protective bubble consists of several gases (listed in the table below), with the top four making up 99.998% of all gases. Of the dry composition of the atmosphere,nitrogenby faris 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 (burning).Argon is used in light bulbs, in double-pane windows, and to preserve museum objects such as the original Declaration of Independence and Constitution. Plants use carbon dioxide to make oxygen. Carbon dioxide also acts as a blanket that prevents the escape of heat into outer space.
| Gas | Symbol | Content | |
|---|---|---|---|
| Nitrogen | N2 | 78.084% | |
| Oxygen | O2 | 20.947% | |
| Argon | Ar | 0.934% | |
| Carbon dioxide | CO2 | 0.035% | |
| Neon | Ne | 18.182 parts per million | |
| Helium | He | 5.24 parts per million | |
| Methane | CH4 | 1.70 parts per million | |
| Krypton | Kr | 1.14 parts per million | |
| Hydrogen | H2 | 0.53 parts per million | |
| Nitrous oxide | N2O | 0.31 parts per million | |
| Carbon monoxide | CO | 0.10 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 | |
| Ammonia | NH3 | trace | |
These percentages of atmospheric gases are for a completely dry atmosphere. The atmosphere is rarely, if ever, dry. Water vapor (water in a gasstate) is nearly always present, up to about 4% of the total volume.
| Water Vapor | Nitrogen | Oxygen | Argon |
|---|---|---|---|
| 0% | 78.084% | 20.947% | 0.934% |
| 1% | 77.30% | 20.70% | 0.92% |
| 2% | 76.52% | 20.53% | 0.91% |
| 3% | 75.74% | 20.32% | 0.90% |
| 4% | 74.96% | 20.11% | 0.89% |
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.
