Week 7 (March 10)

1. Week 7 (March 10) • Tonight • Air Pressure & Winds (Chp 6) • Atmospheric Circulation (Chp 7) • Classwork/Homework #7 • Next Week (Mar 17) • Air Masses and Fronts (Chp 8) • El Niño/La Niña • March 24 • No Class – Spring Break • March 31 • No Class = Cesar Chavez Day

2. Wind

3. Weight Pressure = Force / Area Force = Weight of overlying column of air = mass x gravity

4. Pressure • The steady exertions of atoms and molecules, exchanging momentum with the walls of a container are “Pressure”.

5. Atmospheric Pressure • More air near the surface then number of molecules decreases with height • Air pressure, Air Density and Air temperature are all interrelated. • If one changes then the other 2 will change

6. Pressure Changes • Horizontal: Changes ~ 1 mb over 6000 meters • Vertical: 1 mb over 10 meters (600 X greater) • Vertical atmospheric motions are most important • Vertical pressure and temperature changes are much more dramatic

7. Two columns of air– same temperaturesame distribution of mass 500 mb level 1000 mb 1000 mb

8. Cool the left column; warm the right column The heated column expands The cooled column contracts 500 mb original 500 mb level 500 mb 1000 mb 1000 mb

9. The level of the 500 mb surface changes; the surface pressure remains unchanged 500 mb surface is displaced upward in the warmer column 500 mb level is displaced downward in the cooler column new 500 mb level in warm air original 500 mb level new 500 mb level in cold air The surface pressure remains the same since both columns still contain the same mass of air. 1000 mb 1000 mb

10. Air moves from high to low pressure in the middle of column, causing surface pressure to change. Low High original 500 mb level Note the new surface pressures 1003 mb 997 mb

11. Air now also moves from high to low pressure at the surface… Low High original 500 mb level Where would we have rising motion? Low High 1003 mb 997 mb

12. Air now also moves from high to low pressure at the surface… Low High original 500 mb level Low High 1003 mb 997 mb

13. What have we just observed? • Differential heating to uniform atmosphere • Different rates of expansion in the air • Results in hortizontal pressure differences • Pressure differences caused flow of air • Example of Atmosphere converting heating into motion

14. Measuring Air Pressure Mercury Barometer Aneroid Barometer

15. Station Pressure v. Sea Level Pressure

16. Pressure Maps • a) Surface map has altitude-adjusted station pressures to construct sea level pressure contours • b) Upper air map has constant pressure level delineated by height above sea level

17. Primary Levels 1000 mb = Surface 850 mb = 5,000’ 700 mb = 10,000’ 500 mb = 18,000’ (middle of the atmosphere) 300 mb = 30,000’

18. But contour lines are usually not straight. Ridges (elongated highs) occur where air is warm Troughs (elongated lows occur where air is cold Temperature gradients generally produce pressure gradients Isobars usually decrease in value from south to north (cooler temperatures) Troughs and Ridges

19. Near the surface in the N Hemisphere winds blow counterclockwise around and in toward the center of low pressure areas clockwise around and outward from the center of high pressure areas Surface pressure and winds Why doesn’t the wind blow directly from high to low pressure?

20. Upper Level Pressure Patterns • At upper levels, winds blow parallel to the pressure/height contours

21. Forces and winds • Differences in pressure produce fluid movement

22. Forces Controlling the Wind • Four forces act simultaneously to cause the wind • Pressure Gradient Force • Coriolis Force • Centrifugal Force • Friction Force

23. Pressure Gradient Force • Magnitude • Inversely proportional to distance • Closer together = stronger force • Direction • Always directed toward lower pressure and perpendicular to isobars

24. Coriolis Force Apparent force due to rotation • Magnitude • Dependent on latitude and speed of air parcel • Higher latitude = larger Coriolis force • zero at the equator, maximum at the poles • The faster the speed, the larger the Coriolis force • Direction • To the right he Northern Hemisphere • To the left in S Hemi • Does NOT influence speed

25. Coriolis Force • Acts to right in northern hemisphere • Stronger (i.e. more deviation) for faster wind

26. 994 mb 996 mb 998 mb Higher Pressure Geostrophic Wind • Geostrophic wind is flow in a straight line in which the pressure gradient force balances the Coriolis force. PGF=CF Lower Pressure

27. Geostrophic Wind • Wind speed constant if isobars are straight • Speed is proportional to Pressure Gradient • Bernoulli Effect • Same as nozzle on water hose

28. Geostrophic flow • With the inclusion of the Coriolis Force, air flows parallel to isobars of constant pressure.

29. Centripetal Force • Object on a curved path has an apparent inward force: centripetal force • Magnitude • depends upon the radius of curvature of the curved path taken by the air parcel • depends upon the speed of the air parcel • Direction • at right angles to the direction of movement

30. Friction near Earth’s surface • Friction of the ground slows wind down • Magnitude depends on • Speed of the air parcel • Roughness of the terrain • How uniform the wind field is • Direction • Always opposite to air movement • Importance of friction layer (aka PBL = Planetary Boundary Layer) • Approx. lowest 3,000 ft of the atmosphere

31. Frictional Effects • AGAIN Friction only slows wind speed, does not change wind direction • Therefore, in the Northern Hemisphere • Wind speed decreased by friction • Coriolis force thus decreased and thus will not quite balance the pressure gradient force • Force imbalance (PGF > CF) pushes wind in toward low pressure • Angle at which wind crosses isobars depends on surface roughness • Average ~ 30 degrees

32. Frictional Effects • Retards wind speed near the surface • Lowers the Coriolis Force • Therefore, wind direction is altered from parallel to crossing isobars.

33. Cyclonic & Anticyclonic Winds

34. Isobar Surface Map

35. Winds and vertical air motion • Surface winds blow • Toward low pressure (convergence) • Outward from high pressure (divergence) • Vertical movement to compensate • Surface convergence leads to divergence aloft • Surface divergence leads to convergence aloft VERY IMPORTANT CONCEPT

36. Naming Winds • Named for direction of origin • North wind comes from the north • Seabreeze comes from the sea • Exceptions: offshore/onshore • upslope/downslope

37. Measuring Winds • Instruments • Wind vanes • Anemometers • Combo • Aerovane • Wind sock • Profilers • Radar

38. Wind Measurements • Speeds • Sustained: 2 minute average in past 10 minutes • Gusts: greatest 5-second speed in past 10 minutes • Peak: greatest 5-second speed since last observation • Direction • 2 minute average direction • +/- 10 degrees

39. Wind Direction • Directional names • (16-point compass)

40. Beaufort Scale

41. Wind Rose

42. Wind Rose Application

44. Atmospheric Circulations

45. Scales of Motion • Microscale: meters • Turbulent eddies • Mechanical disturbance or convection • Minutes • Mesoscale: km’s to 100’s of km’s • Local winds and circulations • Land/sea breezes, mountain/valleywinds, thunderstorms, tornadoes • Minutes to hours • Synoptic scale: 100’s to 1000’s of km’s • High and low pressure circulations • Days to weeks • Global scale: systems ranging over entire globe

46. Surface Friction and Winds • Planetary Boundary Layer (PBL) • Wind speeds typically increase with height but rate depends on PBL B) smooth terrain = stable A) rough terrain = unstable

47. Eddies

48. Eddies • Produced by flow past a mountain range in a stable atmosphere • Can form lenticular and rotor clouds • Large gradients in wind speed over short distances cause strong wind shear • Clear air turbulence (CAT) can result, producing dangerous conditions for aircraft

49. Eddies

50. Eddies Von Karmann Eddies