Exam 1: Earth and Its Atmosphere

Correct Answer:

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The tropopause is the boundary layer between the troposphere, which is the lowest layer of the Earth's atmosphere, and the stratosphere above it. The troposphere is where most of the Earth's weather phenomena occur, and it contains approximately 75-80% of the atmosphere's mass and water vapor.

There is very little water vapor above the tropopause in the stratosphere for several reasons:

1. Temperature Inversion: The tropopause acts as a barrier because of the temperature inversion that occurs at this boundary. In the troposphere, the temperature generally decreases with altitude, but at the tropopause, this trend reverses, and the temperature starts to increase with altitude in the stratosphere. This temperature inversion prevents the convective uplift of air and moisture from the troposphere into the stratosphere.

2. Condensation and Freezing: As air rises in the troposphere, it cools and expands. When it cools to the dew point, water vapor condenses into water droplets or ice crystals, forming clouds and precipitation. By the time air reaches the tropopause, most of its moisture has already condensed out and fallen back to the surface as precipitation.

3. The Brewer-Dobson Circulation: This is a large-scale atmospheric circulation that moves air from the tropical tropopause up into the stratosphere and towards the poles. While this circulation does transport some water vapor into the stratosphere, it is minimal because the rising air must pass through the cold trap of the tropical tropopause, where temperatures are extremely low and any remaining water vapor is likely to freeze out and precipitate.

4. Stratospheric Dryness: The stratosphere itself is inherently dry because it lacks the mechanisms for producing and retaining moisture that are present in the troposphere, such as evaporation from oceans, transpiration from plants, and the aforementioned convective processes.

5. Photodissociation: In the rare instances when water vapor does make it into the stratosphere, it can be broken down by the intense ultraviolet (UV) radiation from the sun in a process called photodissociation. This further reduces the amount of water vapor that can exist in the stratosphere.

Due to these factors, the stratosphere contains only a tiny fraction of the water vapor found in the troposphere, leading to very dry conditions above the tropopause.