The greenhouse effect appendix A: What is the atmosphere?

2022 August 08

The atmosphere is a mixture of gases that lies on the surface of the Earth. It tries to expand to fill the volume it is in like all gases, but it is drawn to the Earth by the Earth’s gravity. The balance, called hydrostatic equilibrium, between outward expansion and gravity causes it to be thickest at the surface of the Earth and thinner at higher altitudes. There is no sharp upper boundary to the atmosphere: it just continues to become thinner. (Similarly, the Sun and the gas giants do not have a sharp boundary between them and space either.)

Half of the atmosphere is within 5.5 km (3.5 miles) of the surface of the Earth. A commercial plane at a cruising altitude of 11 km (7 miles) is above 80% of the Earth’s atmosphere, with only 20% of the Earth’s atmosphere between it and space.

Gases in the atmosphere

The main components of the Earth’s atmosphere are as follows:

Gas	Chemical	Fraction of atmosphere
Nitrogen	N₂	78%
Oxygen	O₂	21%
Argon	Ar	1%
Water	H₂O	0.25%
Carbon dioxide	CO₂	0.04%
Neon	Ne	18 ppm
Helium	He	5 ppm
Methane	CH₄	1.9 ppm
Ozone	O₃	0.4 ppm

All of these chemicals except water and ozone are roughly uniformly distributed through the atmosphere; that is, at any location at any altitude you expect the air to be about 21% oxygen.

Nitrogen, argon, neon, and helium are all chemically unreactive and do not play a significant role in the climate.

Oxygen, of course, plays an important role in life. In fact, the Earth’s atmosphere did not have any oxygen for the first two billion years, until photosynthesis created the oxygen we have in our atmosphere today. While oxygen does not directly have a significant role in the climate, its initial appearance in the atmosphere caused enormous climactic upheavals¹This is called the Great Oxygenation Event, which may have involved a “snowball Earth” mostly or entirely covered in ice..

Carbon dioxide and methane are important greenhouse gases, and the effects of greenhouse gases are discussed in the other parts in detail. Carbon dioxide is mostly unreactive but is necessary for photosynthesis. Methane is only mildly reactive in atmospheric conditions, having a lifetime of 10 years in the troposphere or 120 years in the stratosphere, forming carbon dioxide and water. Methane is the main source of water in the upper stratosphere.

Water is the most exceptional of the components of the Earth’s atmosphere because it can readily change between gas, liquid, and solid at conditions typical in the atmosphere. Liquid and solid water will sometimes, depending on conditions, form large enough droplets or crystals to fall out of the atmosphere, whereas water vapor of course does not. Since the transition between phases strongly depends on the temperature, and temperature changes greatly with altitude, this means that the amount of water varies strongly with altitude. In particular, almost all of the water in the atmosphere is very near the surface; water that rises far above the surface of the Earth typically cools so much that it condenses to liquid or solid form and falls. In comparison, most of the other gases listed above are evenly mixed through the whole atmosphere.

Furthermore, since water readily enters and leaves the atmosphere through evaporation and precipitation, the amount of water in the atmosphere changes rapidly in just weeks. We see this in our day-to-day lives when we notice that one day is much more or less humid than normal. The other gases listed above typically remain in the atmosphere for very long periods of time.

Layers of the atmosphere

For the purpose of scientific study the atmosphere is typically organized into a series of layers. The layers differ from each other in their temperature and the types of phenomena that occur in each layer.

Anvil clouds as seen from the ISS. Clouds form when humid rising air condenses water vapor into water droplets; anvil clouds form at the tropopause, the boundary between the troposphere and stratosphere, as higher temperatures in the stratosphere prevent further rising. Overshooting tops, as seen in the middle of this anvil cloud, occur when particularly strong convection causes air to overshoot the tropopause and briefly enter the stratosphere before sinking back into the troposphere. — Anvil clouds as seen from the ISS. Clouds form when humid rising air condenses water vapor into water droplets; anvil clouds form at the **tropopause**, the boundary between the troposphere and stratosphere, as higher temperatures in the stratosphere prevent further rising. Overshooting tops, as seen in the middle of this anvil cloud, occur when particularly strong convection causes air to overshoot the tropopause and briefly enter the stratosphere before sinking back into the troposphere.

The bottom 10 to 15 km of the atmosphere is called the troposphere, and contains about 80% of the atmosphere. The troposphere is heated from below because the bottom is touching the Earth’s surface, which is warmed directly by sunlight. Because the troposphere is heated from below and hot air rises, it is a very active part of the atmosphere, and almost all of the weather that we are interested in occurs here. In particular, almost all of the water is in the troposphere, so clouds predominantly appear in this layer. The name “troposphere” refers to the constant “turning over” of the air there.

Above the troposphere and continuing to 50 km is the stratosphere. The important difference between the troposphere and the stratosphere is that the stratosphere is mostly heated from above, by the ozone layer. Because it is heated from above, and heat rises, the stratosphere is very stable and mixes slowly (that is, it is stratified, giving it its name). In particular, it does not mix readily with the troposphere. This lack of mixing can be best observed in strong thunderstorms that form anvil clouds at the boundary.

The mesosphere, until 85 to 100 km, and thermosphere, until 500 to 1000 km, are the next layers in the atmosphere. Around 100 km can be found the sodium layer which is formed by ablation of meteorites; it has a column density of about 1 milligram per square kilometer. This is also approximately the altitude at which gases begin to separate based on molecular weight, with lighter gases reaching higher altitudes. At 160 km the atmosphere is too thin to sustain sound waves within the range of human hearing²Higher pitches require denser air to be transmitted; if the time between collisions is longer than the frequency of the pressure wave, then the high and low pressures will simply be averaged out. The highest transmissible pitch drops by an octave roughly every 5 km of altitude.. The International Space Station orbits at 400 km and slowly falls over time due to drag from the air: it is boosted by rockets approximately once a month.

The upper boundary of the thermosphere depends strongly on daily variations in solar activity, referred to as space weather. Above the thermosphere is the exosphere, at which point the air is so thin that it no longer acts like a gas; instead, each of the molecules behaves independently of the others and their paths (or orbits!) are controlled by gravity. The exosphere could be considered part of space; different scientists use different definitions for when space “begins” according to what physical phenomena are relevant to their discipline.

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