Geography: Movement of Air

Movement of Air

Relationship between air pressure, temperature and wind

• Hot air expands and rises – LP – convergence
• Cold air is dense and heavy and subsides to Earth’s surface – HP – divergence


Convergence is when air moves to areas of LP
Divergence is when air moves away from areas of HP



PGF
• What: Pressure Gradient Force causes air to move from an area of HP to one of LP
• Cause: Pressure gradient (the horizontal difference in pressure)
• Direction: Across the isobars from HP to LP
• Strength: The greater the difference in pressure, the steeper the pressure gradient,
the stronger the PGF and the stronger the wind



Coriolis Force
• What: Coriolis force is a force that causes the deflection of winds
• Cause: The surface of the earth rotating at different speeds
• Direction: Ferrel’s law states that winds are deflected to their left in the Southern
Hemisphere and to their right in the Northern Hemisphere
• Strength: As wind speed increases, CF becomes stronger



Geostrophic flow
• What: When air blows parallel to isobars due to equilibrium between PGF and CF
being reached
PGF causes air to move from HP to LP. As air speed increases, CF deflects wind. No
friction – CF is strong and deflection increases until PGF and CF are in equilibrium