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 101 kPa at sea level
  • Scientists identify different layers of the atmosphere according to: 
  • The height above the Earth's surface
  • The characteristics of the gases there
• The Atmosphere as a Fluid

  • A fluid is either a liquid or a gas
  • When an object is immersed in a liquid, the liquid exerts a pressure upon the object
  • This liquid pressure is in addition to the pressure already exerted by the atmosphere
  • For example, an object at sea level (on the surface of the sea) experiences a pressure of 101 kPa due to the atmosphere
  • If this object is now immersed to a depth of 10 metres underwater, it experiences an extra pressure of 100 kPa due to the water
  • This means that the object will experience a total pressure of

  101 kPa + 100 kPa = 201 kPa
• • Therefore, overall pressure arises due to both:
  • The water (liquid) pressure
  • Atmospheric (gas) pressure
  • These are both sources of fluid pressure
• Atmospheric Pressure

  • 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 varies with height above a surface, for example, at sea level

  • This is due to air molecules colliding with a surface creating atmospheric 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