Atmospheric Pressure (AQA GCSE Physics)

• 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

• 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