The Structure of the Atmosphere

The atmosphere is composed of nitrogen, oxygen, argon, water vapor, and a number of trace gases
(Table 1). This composition has remained relatively constant throughout much of Earth's history.
Chemical reactions maintain the ratios of major constituents of the atmosphere to each other. For
example, oxygen is released into the atmosphere by photosynthesis and consumed by respiration.
The concentration of oxygen in the atmosphere is maintained by a balance between these two
processes:
Photosynthesis: CO2 + H2O + light ® CH2O" + O2
Respiration: CH2O + O2 ® CO2 + H2O + energy

"CH2O" denotes the average composition of organic matter.
Almost all weather occurs in the troposphere, the lowest layer of the atmosphere, which extends
from the surface up to 8 to 16 kilometers above Earth's surface (lowest toward the poles, highest
in the tropics). Earth's surface captures solar radiation and warms the troposphere from below,
creating rising air currents that generate vertical mixing patterns and weather systems, as detailed
further below. Temperatures decrease by about 6.5°C with each kilometer of altitude. At the topof the troposphere is the tropopause, a layer of cold air (about -60°C), which forms the top of the
troposphere and creates a "cold trap" that causes atmospheric water vapor to condense.
The next atmospheric layer, the stratosphere, extends upward from the tropopause to 50 kilometers.
In the stratosphere temperatures increase with altitude because of absorption of sunlight by
stratospheric ozone. (About 90 percent of the ozone in the atmosphere is found in the stratosphere.)
The stratosphere contains only a small amount of water vapor (only about one percent of total
atmospheric water vapor) due to the "cold trap" and the tropopause, and vertical air motion in this
layer is very slow. The stratopause, where temperatures peak at about -3°C, marks the top of the
stratosphere.
In the third atmospheric layer, the mesosphere, temperatures once again fall with increasing altitude,
to a low of about -93°C at an altitude of 85 kilometers. Above this level, in the thermosphere,
temperatures again warm with altitude, rising higher than 1700°C.
The atmosphere exerts pressure at the surface equal to the weight of the overlying air. Figure 1 also
shows that atmospheric pressure declines exponentially with altitude—a fact familiar to everyone who
has felt pressure changes in their ears while flying in an airplane or climbed a mountain and struggled
to breathe at high levels. At sea level, average atmospheric pressure is 1013 millibars, corresponding
to a mass of 10,000 kg (10 tons) per square meter or a weight of 100,000 Newtons per square meter
(14.7 pounds per square inch) for a column of air from the surface to the top of the atmosphere.
Pressure falls with increasing altitude because the weight of the overlying air decreases. It falls
exponentially because air is compressible, so most of the mass of the atmosphere is compressed
into its lowest layers. About half of the mass of the atmosphere lies in the lowest 5.5 kilometers (the
summit of Mt. Everest at 8850 m extends above about roughly two-thirds of the atmosphere), and 99
percent is within the lowest 30 kilometers.