Pressure Levels 3 forces that control wind direction? PGF or HGF, Coriolis, Friction What is geostrophic wind? PGF = Coriolis Wind blows parallel to isobars and height lines Is friction greatest at low or high elevations? Low, because of the earth’s surface! Friction slows the wind and reduces the effect of the coriolis force
Friction Air in motion near the earth’s surface is slowed by contact with the ground The magnitude of friction is dependent on the “roughness” of the surface Where is friction greater? Over land or over water?
Pressure Levels and Friction Surface Pressure What is the height of the “surface” pressure? Wind crosses isobars at a rather large angle
Pressure Levels and Friction 850mb-heights Wind crosses height lines at a small angle
Pressure Levels and Friction 500mb-heights Wind blows parallel to height lines
Buys-Ballot’s Law If you stand with the wind at your back on a constant pressure surface, lower heights (lower pressure) will be on your left in the Northern Hemisphere
Jet Stream! Vertical profile of winds measured at Caribou, ME Mandatory Pressure Levels – pressure levels which radiosondes always take measurements
Jet Stream The jet stream is caused by differences in temperature between the tropics and the polar regions The greater the temperature difference, the faster the jet stream Remember: Pressure decreases faster in cold air than in warm air!!!!!!!!