CHAPTER 7 ATMOSPHERIC MOTIONS

1. CHAPTER 7 ATMOSPHERIC MOTIONS

2. Back to air pressure • Atmospheric Pressure is the force per unit area of a column of air above you • In other words, pressure is the weight of the column of air above you - a measure of how hard this column of air is pushing down • More fundamentally - atmospheric pressure arises from gravity acting on a column of air

3. Two columns - same temperaturesame distribution of mass 500 mb level 1000 mb 1000 mb

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

5. The level of the 500 mb surface changes; the surface pressure remains unchanged The 500 mb surface is displaced upward in the warmer column The level corresponding to 500 mb 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

6. A pressure difference in the horizontal direction develops above the surface The 500 mb surface is displaced upward in the warmer column The 500 mb surface is displaced downward in the cooler column new 500 mb level in warm air High Low 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

7. Air moves from high to low pressure in middle of column, causing surface pressure to change. Cold air aloft = low pressure Warm air aloft = high pressure Low High original 500 mb level 1003 mb 997 mb

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

9. Low High original 500 mb level Air diverges around the surface high Air converges around the surface low Low High 1003 mb 997 mb

10. Vertical motions above H & L • Rising air above the surface low leads to clouds and storms • Low pressure centers aka “cyclones” • Sinking air above the surface high leads to fair weather • High pressure centers aka “anticyclones”

11. Motions aloft Cold air aloft means low pressure (heights), warm air aloft means high pressure (heights).

13. Surface pressure maps (actually, pressure corrected to sea level)

14. Constant-pressure maps • Above the ground, we typically look at maps showing the height of a given pressure level • If there are no horizontal variations in pressure, the pressure at a constant height level, or the height at a constant pressure level, are the same thing

15. Constant-pressure maps • When there are horizontal variations, we see high heights in warm air aloft; low heights in cold air aloft

16. Steeper slope means the contour lines are closer together!

18. Comparison of surface and upper-air map

19. Table 7.1, p. 181

20. Forces

21. Pressure gradient force • The cause of the wind! • Horizontal pressure gradients lead to winds • PGF always directed from high to low pressure • The stronger the pressure gradient, the stronger the wind • Or, in other words, the closer the isobars are together, the stronger the wind will be

23. The length of the red arrows indicate the strength of the PGF

24. Except… • We don’t actually see the wind blow straight across from high pressure to low pressure • There must be other force(s) at work…

25. Coriolis force • “Apparent” force due to rotation • An outside (nonrotating) observer doesn’t experience it • An observer on the rotating body (like the earth, or the turntable) does experience it

26. Coriolis force • Since we (and the atmosphere) are rotating with the earth, we are affected by this force • Coriolis force turns moving objects/air parcels to the right in the northern hemisphere, to the left in the southern hemisphere • The faster the motion, the stronger the Coriolis force • Coriolis force is zero at the equator, relatively strong at the poles

28. So far: • Pressure gradient force (PGF) • Always from high pressure to low pressure • Coriolis force • Always toward the right (in the northern hemisphere) • When these two are in balance, it is called the geostrophic wind • Geostrophic = “earth turning” • If you’re traveling with the geostrophic wind, low pressure is always on your left! • “when the wind is at your back, lower pressure is to your left (NH)”

29. The geostrophic wind blows parallel to straight isobars

30. But what if the isobars (or isoheights) aren’t straight? • (They’re usually curved – troughs and ridges) • When there is curvature, an observer (or an air parcel) in the rotating frame of reference experiences a force directed outward – the centrifugal force – think of being in a car going around a curve • Magnitude of centrifugal force is related to the velocity and the radius of curvature • Faster speeds = greater centrifugal force • Tight curves = greater centrifugal force

31. Gradient wind • Involves the PGF, Coriolis, and Centrifugal forces – flow is parallel to curved isobars • This is a good estimate of the winds, except right near the ground

32. Stepped Art Fig. 8-29, p. 214

35. Winds near the ground • There’s one more force that’s important for winds near the ground

36. Friction • Near the surface, the wind is slowed by drag from the ground, trees, buildings, etc. • What happens to force balance of geostrophic wind when the wind slows down?

37. Friction • When wind speed slows down, Coriolis force also is reduced • Therefore, PGF is stronger than Coriolis, and wind blows across isobars toward lower pressure • Wind blows in toward a surface low, and away from a surface high

38. Summary - airflow Aloft – flow parallel to isobars or isoheights Near surface – flow in toward low, away from high Cyclonic flow (counterclockwise in NH) Anticyclonic flow (clockwise in NH)

39. The closer the isobars or contours, the faster the wind Fig. 7.17, p. 189

40. More about the wind at your back…

41. What about really small, really strong pressure gradients? • Like a tornado, or your water in your bathtub • In these situations, the balance is between the PGF and centrifugal forces (Coriolis is unimportant) • This is called a cyclostrophic wind • The water flowing out of your bathtub doesn’t change directions in different hemispheres!

42. Summary – forces affecting air

43. Summary – force balances

44. Fig. 7.13, p. 185

45. Barometers • Mercurial (Fortin) • Aneroid • Recording: Barograph • Electronic (Pressure Transducer)

46. Wind Measurements • Wind vane • Cup anemometer • Aerovane (Wind Monitor by R.M. Young) • Sonic • Rawinsonde (lifted by Weather Balloon) • Wind soundings • Wind Profiler

47. THE INFLUENCE OF EXTREME WINDS • A small increase in wind speed can greatly increase the wind force on an object • F ~ V2 • Turbulent whirls (eddies) pound against the car’s side as the air moves past obstructions, such as guard railings and posts • Similar effect occurs where the wind moves over low hills paralleling a highway • Weird Stuff: Wind erosion, desert pavements, sand ripples, snow ripples, snow dunes, snow rollers, snow fences, windbreak, shelter belt

48. Table 7.2, p. 196

49. Fig. 7.24, p. 196

50. Fig. 7.25, p. 197