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

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

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

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

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

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

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