# 5.5.2 Atmospheric Pressure

### The Atmosphere

• The Earth’s atmosphere is a thin layer (relative to the size of the Earth) of air around it
• It exerts a pressure of about 100 kPa at sea level

The Earth’s atmosphere

• The atmosphere extends more than 100 km into space and becomes less dense with increasing altitude (height above sea level)
• This means that the pressure becomes less too
• Atmospheric pressure various slightly from day to day, depending on the weather, and fine clear weather is usually associated with high pressure

### Atmospheric Pressure

• The graph below shows how the pressure varies with altitude:

Graph of atmospheric pressure against altitude

• Atmospheric pressure varies with height above a surface, for example, at sea level
• This is due to air molecules colliding with a surface creating atmospheric pressure
• These molecules create a force per area of the surface which creates the pressure
• The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases
• This is also known as the density of the air
• Therefore, as height increases, there is always less air above a surface than there is at a lower height and the atmospheric pressure decreases with an increase in height

Atmospheric pressure decreases as the density of the molecules decreases

#### Exam Tip

Explaining why atmospheric pressure decreases with increasing altitude is a common exam question – make sure you can summarise these points!

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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