Properties of the Earth’s Atmosphere

The earth’s atmosphere varies vertically and horizontally in: 

  • Pressure. 
  • Temperature. 
  • Density. 
  • Humidity. 

The earth’s atmosphere is fluid, supports life only at lower levels and is a poor conductor.


When air is compressed, at a constant temperature, more molecules occupy a given volume, thus the mass increases which results in an increase in density.

Conversely, when air expands, fewer molecules occupy the original volume, mass is less and therefore density decreases.

Pressure Increases – Air density Increases

Pressure Decreases – Air Density Decreases


When air is heated, while keeping the pressure constant, it expands and therefore, fewer molecules occupy a given volume and density decreases. The converse also applies.

Temperature increases – Air density decreases

Temperature Decreases – Air density Increases


As we have already discussed water vapor weighs less than dry air, therefore fewer molecules occupy a given volume.

Thus density decreases under the same conditions of temperature and pressure. In other words, water vapor molecules displace the molecules of air: less air per unit volume, decreased density. Therefore, the higher the humidity, the lower the density.


Air density varies with altitude, a decrease in air density ( increasing altitude ) results in a decrease in lift.

E.g. If we take two paper bags, one at Durban and one at Pretoria.

                    Durban              Pretoria

From the example, we can see that the bag at Durban contains more air molecules because:

  • Durban is situated at a much lower altitude than Pretoria thus air density is more
  • Durban is closer to the core of the earth meaning the gravity force between the air molecules is much more, thus pulling them closer together and increasing the air density.

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