1. The Wind: PGF Pressure gradient force is what sets air in motion
Directs air from higher toward lower pressure
Thus direction of PGF is perpendicular to isobars toward lower pressure
Wind speed is proportional to the PGF (all else being equal)
Vertical PGF >> Horizontal PGF, so why doesn’t the atmosphere blow straight up and away?
Compensating force, gravity
No such compensating force in the horizontal

2. The Wind: Coriolis Effect
An apparent force that, from our frame of reference, deflects winds to the RIGHT in the northern hemisphere
Always directed 90 degrees to the right of the wind direction (northern hemisphere)
Does not cause motion
Geostrophic Balance
PGF and Coriolis
Wind blows parallel to isobars, low pressure to the left
90 degrees to right of PGF
Generally applies to the atmosphere well above the ground where there is no FRICTION

3. The Wind: Friction I Decreases Wind Speed
Friction acts directly opposite the wind direction
Wind speed generally decreases as height above ground decreases
As friction increases, wind speed decreases
Amount of friction depends upon the roughness of the surface; think about air movement above…
Forest vs field
Mountains vs plains
Land vs Water

4. The Wind: Friction II Alters Wind Direction
Unlike geostrophic flow, wind direction will not turn a full 90 degrees to the right of the PGF
More friction = less turning = greater crossing angle (air will flow across isobars toward lower pressure)
Counterclockwise and inward toward low pressure center
Clockwise and outward from high pressure center

5. The Wind: Friction II Alters Wind Direction
Low pressure = converging air at surface = rising air = cooling air = increased relative humidity = clouds and precipitation if T cools to dewpoint
High pressure = diverging air at surface = sinking air = warming air = decreased relative humidity = few clouds

6. The Wind: Mid-Latitude Jet Stream
A relatively narrow channel of fast-moving air in the upper troposphere embedded in the westerlies
A result of horizontal temperature gradients
Stronger in winter than in summer
Greater north-south temperature gradient

7. Key Figures
6.17, 6.20, 6.27, 6.29, 6.30, 6.35, 6.36, 7.20, 7.28, 7.30
Remember the following facts:
PGF always perpendicular to isobars, from higher toward lower pressure
Coriolis effect always 90 degrees to the right (in the northern hemisphere) of wind direction
Friction always 180 degrees (opposite) the wind direction