The Wind: PGFThe Wind: PGF Pressure gradient force is what sets air in motionPressure gradient force is what sets air in motion Directs air from higher toward lower pressureDirects air from higher toward lower pressure

– Thus direction of PGF is perpendicular to isobars toward Thus direction of PGF is perpendicular to isobars toward lower pressurelower pressure

Wind speed is proportional to the PGF (all else Wind speed is proportional to the PGF (all else being equal)being equal)

Vertical PGF >> Horizontal Vertical PGF >> Horizontal PGF, PGF, so why doesn’t the so why doesn’t the atmosphere atmosphere blow straight up blow straight up and away? and away?– Compensating force, gravityCompensating force, gravity– No such compensating force No such compensating force in in

the horizontalthe horizontal

The Wind: Coriolis EffectThe Wind: Coriolis Effect An An apparent apparent force that, from our frame of force that, from our frame of

reference, reference, deflectsdeflects winds to the RIGHT in the winds to the RIGHT in the northern hemispherenorthern hemisphere

AlwaysAlways directed 90 degrees to the right of the directed 90 degrees to the right of the wind direction wind direction (northern hemisphere)(northern hemisphere)

Does not cause motionDoes not cause motion Geostrophic BalanceGeostrophic Balance

– PGF and CoriolisPGF and Coriolis– Wind blows Wind blows parallelparallel to to

isobars, low pressure toisobars, low pressure to the the leftleft

– 90 degrees to right of PGF90 degrees to right of PGF– Generally applies to theGenerally applies to the atmosphere atmosphere

well above the well above the ground where ground where there is no there is no FRICTION FRICTION

The Wind: Friction I The Wind: Friction I Decreases Wind SpeedDecreases Wind Speed

Friction acts Friction acts directly oppositedirectly opposite the wind direction the wind direction Wind speed generally decreases as height above Wind speed generally decreases as height above

ground decreasesground decreases As friction increases, wind speed decreasesAs friction increases, wind speed decreases Amount of friction depends upon the roughness of the Amount of friction depends upon the roughness of the

surface; think about air movement above…surface; think about air movement above…– Forest vs fieldForest vs field– Mountains vs plainsMountains vs plains– Land vs WaterLand vs Water

The Wind: Friction II The Wind: Friction II Alters Wind DirectionAlters Wind Direction

Unlike geostrophic flow, Unlike geostrophic flow, wind direction will not wind direction will not turn a full 90 degrees turn a full 90 degrees to the right of the PGFto the right of the PGF

More friction = less More friction = less turning = greater turning = greater crossing angle (air will crossing angle (air will flow across isobars flow across isobars toward lower pressure)toward lower pressure)– Counterclockwise and Counterclockwise and

inward toward low inward toward low pressure centerpressure center

– Clockwise and outward Clockwise and outward from high pressure from high pressure centercenter

The Wind: Friction II The Wind: Friction II Alters Wind DirectionAlters Wind Direction

Low pressure = converging air at surface = rising air = Low pressure = converging air at surface = rising air = cooling air = increased relative humidity = clouds and cooling air = increased relative humidity = clouds and precipitation if T cools to dewpointprecipitation if T cools to dewpoint

High pressure = diverging air at surface = sinking air = High pressure = diverging air at surface = sinking air = warming air = decreased relative humidity = few warming air = decreased relative humidity = few cloudsclouds

The Wind: Mid-Latitude Jet StreamThe Wind: Mid-Latitude Jet Stream A relatively narrow channel of fast-moving air in the A relatively narrow channel of fast-moving air in the

upper troposphere embedded in the upper troposphere embedded in the westerlieswesterlies A result of horizontal temperature gradientsA result of horizontal temperature gradients Stronger in winter than in summerStronger in winter than in summer

– Greater north-south temperature gradientGreater north-south temperature gradient

Key FiguresKey Figures 6.17, 6.20, 6.27, 6.29, 6.30, 6.35, 6.36, 7.20, 7.28, 6.17, 6.20, 6.27, 6.29, 6.30, 6.35, 6.36, 7.20, 7.28,

7.307.30 Remember the following facts:Remember the following facts:

– PGF PGF alwaysalways perpendicular to isobars, from higher perpendicular to isobars, from higher toward lower pressuretoward lower pressure

– Coriolis effect Coriolis effect alwaysalways 90 degrees to the right (in the 90 degrees to the right (in the northern hemisphere) of wind directionnorthern hemisphere) of wind direction

– Friction Friction alwaysalways 180 degrees (opposite) the wind direction 180 degrees (opposite) the wind direction