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The Earth's atmosphere is its outermost layer. The atmosphere is a mixture of gases and extends about 700 km above the Earth's surface.

Scientists identify different layers of the atmosphere, according to the height above the surface and the properties of the gases there.

Altitude and pressure

The atmosphere exerts pressure on the Earth's surface and on objects placed in the atmosphere. This is called atmospheric pressure.

Atmospheric pressure at sea level is about 100,000 N/m², or 100 kPa. However, this decreases as the height above the surface of the planet (altitude) increases. For example, at the highest points on Earth's surface, atmospheric pressure is about 30 kPa.

As the height above the surface increases:

• the number of gas molecules in a given volume of air decreases
• the frequency of collisions with objects in the air decreases
• the pressure decreases

This explains why a helium weather balloon expands as it rises. These balloons are only partly filled at launch. This is so that they do not expand so much that they eventually burst.

Example

A box with side length 0.50 m is raised from sea level to the top of a mountain. The gas pressure on the box changes from 1.0 × 10⁵ Pa to 4.0 × 10⁴ Pa. Calculate the change in the force exerted on one side of the box.

change in pressure = (1.0 × 10⁵) - (4.0 × 10⁴)

= 6.0 × 10⁴ Pa

area of a side is (0.50 × 0.50) = 0.25 m²

Rearrange the equation:

force = pressure × area

= 6.0 × 10⁴ × 0.25

= 15,000 N