Topic 4.4 Notes – Earth’s Atmosphere

What Earth’s Atmosphere Is

The atmosphere is the gravitationally bound envelope of gases surrounding Earth. It interacts with the geosphere (land), hydrosphere (water), and biosphere (living things), and it is the medium where weather and atmospheric circulation happen.

A few jobs of the atmosphere show up all over APES:

• It provides gases needed for photosynthesis and respiration.
• It helps control Earth’s temperature.
• It absorbs some harmful solar radiation, especially UV.

Two ways APES organizes the atmosphere here:

• by composition of gases
• by vertical layers based on temperature change with altitude

As altitude increases, the atmosphere gets less dense. Air pressure also decreases because there is less air above you pushing down. There is no sharp outer edge. The atmosphere gradually thins into space.

Atmospheric Composition

When APES gives atmospheric percentages, it usually means dry air, which leaves out water vapor because water vapor changes a lot.

Major gases in dry air

A common APES trap is forgetting that argon is more abundant than carbon dioxide.

Percent and ppm

You may see carbon dioxide written as a percent or in ppm.

$$1\% = 10{,}000\ \text{ppm}$$

So:

$$0.04\% = 400\ \text{ppm}$$

Water vapor and trace materials

Water vapor $\mathrm{H_2O}$ is variable, from nearly 0% up to about 4%.

• enters through evaporation and transpiration
• leaves through condensation and precipitation
• important for weather, clouds, and the greenhouse effect

Other greenhouse gases you should recognize:

• carbon dioxide $\mathrm{CO_2}$
• methane $\mathrm{CH_4}$
• nitrous oxide $\mathrm{N_2O}$
• water vapor $\mathrm{H_2O}$
• ozone $\mathrm{O_3}$

Ozone depends on location:

• stratospheric ozone protects from UV
• tropospheric ozone is an air pollutant

The atmosphere also contains aerosols, which are tiny particles or droplets such as sea salt, dust, smoke, pollen, and soot. They are part of the atmosphere, but not part of the gas-percentage numbers.

The Atmospheric Layers

The five main layers, from the surface upward, are:

1. troposphere
2. stratosphere
3. mesosphere
4. thermosphere
5. exosphere

These layers are defined by temperature gradients, not by gas composition. The pattern to memorize is down, up, down, up, which is exactly what the temperature line in the diagram shows:

Atmospheric layers and temperature profile

• troposphere: temperature decreases
• stratosphere: temperature increases
• mesosphere: temperature decreases
• thermosphere: temperature increases
• exosphere: very thin outer atmosphere fading into space

Boundaries between layers are transition zones:

• tropopause
• stratopause
• mesopause
• thermopause or exobase

Key Features of Each Layer

• Troposphere
  Lowest layer, about 10 to 12 km. Most atmospheric mass, almost all water vapor, clouds, and weather. Average lapse rate is about $6.5^\circ\text{C}$ per km.

• Stratosphere
  About 10 to 12 km up to 50 km. Contains the ozone layer, mostly around 15 to 35 km. Ozone absorbs UV, so temperature rises with altitude.

• Mesosphere
  About 50 km to 80 to 85 km. Temperature falls again. Coldest region is near the mesopause. Many meteoroids burn up here.

• Thermosphere
  Begins around 80 to 85 km and extends several hundred km. Sparse particles absorb high-energy solar radiation, so temperature rises sharply. Auroras occur mainly here. Much of the ionosphere overlaps this layer.

• Exosphere
  Outermost layer. Extremely low density. Hydrogen and helium become more important. It gradually merges with space.

How to Read an Atmosphere Diagram

On an exam diagram, identify the layers by the temperature trend first.

• Check the axes. Altitude goes up. Temperature may be shown in reverse.
• If temperature falls near the surface, that is the troposphere.
• If it rises above that, that is the stratosphere.
• If it falls again, that is the mesosphere.
• If it rises again, that is the thermosphere.

Helpful clues:

• weather and clouds = troposphere
• ozone layer = stratosphere
• meteors = mesosphere
• auroras and some satellites = thermosphere
• fading into space = exosphere