Pressure Gradient ForcePressure Gradient Force• How does the air move as a result of the

pressure gradient force?

996hPa

998hPa1000hPa

1002hPa

LOW PRESSURE

HIGH PRESSURE

(1) Right angle to isobars

(2) From high pressure to low pressure

Coriolis ForceCoriolis Force

• What you have learnt in F4?– Due to earth rotation– The air moves to right in NH– The air moves to left in SH– Ferrel’s law

New Things to learn in F6New Things to learn in F6

Factors Affecting Coriolis Force

- Cf = -2WVsin A

where W is the angular velocity of the rotation of the earth

(15/hour = 0.0000729rad/hour)

V is the velocity of the air mass

A is the latitude

Cf is the Coriolis force

Geostrophic Wind BalanceGeostrophic Wind Balance

996hPa

998hPa1000hPa

1002hPa

LOW PRESSURE

HIGH PRESSURE

Pressure gradient forceCoriolis force

Resulting wind

Necessary Conditions

. Geostrophic wind takes place above the influence of surface frictional drag, air movement is controlled by the horizontal pressure gradient force and the Coriolis force.

• For the purpose of explaining horizontal air movement, or wind, the forces of gravity and the vertical pressure gradient may be assumed to cancel each other out.

Ekman SpiralEkman Spiral

Cyclone in NH

Cyclone in SH

Why do we have Wind blowing clockwise inward in the cyclone in NH?

Why do we have Wind blowing anti-clockwise inward in the cyclone in SH?

Why do we have Wind blowing clockwise inward in the cyclone in NH?

Why do we have Wind blowing clockwise inward in the cyclone in NH?

Unresolved QuestionsUnresolved Questions