Lecture Unit-19
Air Pressure and winds
Textbook Unit-8
Pressure: Definition:
Amount of force acting on a unit area (per amount of force per unit area)
Unit of Pressure: Pascal (not lb./sq. inch)
Barometer: The instrument that measures atmospheric pressure.
2992’’=101325Pa=1013.25 mil liber
US National weather service.
Relationship between temp charge and pressure change:
Inverse relationship:
If Tem. Increase, pressure decrease. If T decrease, P increase.
Isobar: A line of equal pressure.

a. At 25c Latitude in each hemisphere: Incoming radiation= outgoing radiation.
b. 0-25 latitude in both hemispheres: energy surplus.
c. 25-pole in each hemisphere: energy deficit
Surplus energy from the lower latitudes (0-25) in transferred to the high latitude (25-pole) deficit areas.
Agents of energy transfer:
1. Wind (70-80)
2. Ocean currents (20-30)
Forces involved in wind:
1. Pressure gradient force
2. Coriolis force, and
3. Frictional Force
1. Pressure gradient force: force due to pressure difference, which at 90c to the isobars.
Factors of PGF:
Isobaric spacing: The closer the isobars are, the stronger in the PGF.
(落下一些)
Two factors of CF:
1. Latitude: CF is zero at the equator, and increases toward both poles.
2. Velocity: CF increases with increasing velocity.
Frictional Force:
Retarding force due to surface friction.
Factor: Elevation above the surface.
Max. Frictional at the surface and it decreases rapidly with height.
Result: Wind