The Atmosphere
Movement in the Atmosphere
Atmospheric Stability
Localised Winds
Visibility and Fog
Aircraft Icing
Weather Reports and Forecasts
Practice Exam

Air Pressure

High & Low Pressure

So we know the sun heats the ground, which in turn heats the air in contact with the ground. This warmer air then rises, which leaves an area of low pressure at the surface.

Conversely, where there is cooler air it will sink and create high pressure at the surface.

Pressure Gradient

This difference in pressure is called a Pressure Gradient, and it creates a force that causes air to flow from high pressure to low pressure. This is called the Pressure Gradient Force (PGF). The greater the difference in pressure, the stronger the force and the faster the air will flow (faster wind speed).

Coriolis Effect

However, this rising and sinking air doesn’t flow in straight lines. The rotation of the Earth causes the air to rotate as it rises and sinks.

In the northern hemisphere, sinking air (high pressure) rotates clockwise and rising air (low pressure) rotates counter-clockwise. This is known as the Coriolis effect.

Here’s an overhead view of a high pressure system and a low pressure system, with the white lines showing wind direction and the colours showing pressure:

Use the Right Hand Rule to remember which direction Coriolis will rotate the air

Right Hand Rule

  1. Point your right thumb in the vertical direction the air is traveling (up or down)
  2. Your fingers will curl in the direction of rotation

Remember: This only works with your right hand!

In the southern hemisphere, the direction of rotation is reversed