1.5.1 Structure & Composition of the Atmosphere
Troposphere: Extending from the surface to roughly 12 km, this layer contains 80% of the atmospheric mass and nearly all water vapor. It is characterized by a positive lapse rate, where temperature decreases with altitude at an average rate of .
Stratosphere: Located between 12 km and 50 km, temperature increases with height in this layer (a temperature inversion). This warming is caused by the absorption of ultraviolet (UV) radiation by the Ozone Layer.
Mesosphere: From 50 km to 85 km, temperatures decrease again, reaching the coldest point in the atmosphere (the mesopause). This layer lacks sufficient solar-absorbing gases to maintain heat.
Thermosphere: Beyond 85 km, temperatures rise sharply due to the absorption of high-energy X-rays and UV radiation by atomic oxygen and nitrogen. However, because the air is so thin, it contains very little total heat energy.
| Feature | Homosphere | Heterosphere |
|---|---|---|
| Altitude | Surface to ~80 km | Above 80 km |
| Composition | Uniform mixture of gases | Layered by molecular weight |
| Mixing Mechanism | Turbulent mixing (convection) | Molecular diffusion (gravity) |
| Primary Gases | (separated) |
Identify the Gradient: When asked to identify a layer, look at the temperature trend. If increases with height, it is either the Stratosphere or the Thermosphere. If decreases, it is the Troposphere or Mesosphere.
Ozone Logic: Always link the Stratosphere's warming to the Ozone layer. Without ozone, the stratosphere would not exist as a distinct thermal layer.
Mass Distribution: Remember that pressure and density always decrease exponentially with altitude, regardless of temperature changes. 99% of the atmosphere's mass is within the first 30 km.
Common Units: Be prepared to convert or identify altitudes in both kilometers (km) and miles, and temperatures in Celsius () and Kelvin ().
Heat vs. Temperature: Students often confuse high temperature in the thermosphere with high heat. Because the air is so sparse, an object in the thermosphere would not feel 'hot' because there are too few molecules to transfer kinetic energy.
The 'Empty' Space: Space does not begin abruptly. The atmosphere gradually thins out into the exosphere, with no definitive outer edge, though 100 km (the Karman line) is often used as a legal boundary.
Ozone Location: Misplacing the ozone layer in the troposphere is a common error. While 'bad' ozone exists at ground level as pollution, the 'good' protective ozone layer is strictly stratospheric.