1. PRESSURE & WIND, GENERAL CIRCULATION, JET STREAMS

4. Current weather map
Current wind map

5. Animation

6. Deflection...
Increases with latitude . Increases with wind speed. Increases with mass of object.

7. Minimal friction aloft > 3000 ft in troposphere “friction layer” : 0 – 3000 ft
Winds aloft blow parallel to isobars: geostrophic wind

8. geostrophic balance
Show air parcel moving in response to pressure gradient, then deflected, as in Fig 8.9 , until pressure gradient force and Cor effect are in opposite directions; eventually this balance is reached : “Geostrophic balance”; the resulting wind is geostrophic wind and it is parallel to isobars;
(Note: Coriolis effect is to the right of MOTION not to the right of gradient; define geostrophic balance as” balance between pressure gradient and Coriolis forces acting on a parcel so that the forces are equal in magnitude but in opposite directions; wind produced is geostrophic.)
“balance between pressure gradient and Coriolis forces acting on a parcel
so that the forces are equal in magnitude but in opposite directions”

9. GEOSTROPHIC WIND
Northern Hemisphere L H
Around and clockwise
Around and counterclockwise
Southern Hemisphere L H
Around and clockwise
Around and counterclockwise

12. Current upper air map

13. Friction and Surface Winds
Drag produced by surface. Frictional force is applied opposite to direction of air motion; causes wind to blow across the isobars.

14. At surface, friction reduces wind speed, which reduces Coriolis effect.
Coriolis can not balance PGF so wind crosses isobars.

19. Current weather
Current satellite IR

21. General Circulation
REMINDER: Go Over wind speed

22. Global Wind Systems driven by Highs and Lows at surface
Where are Highs and
Lows?

23. Imagine the earth with no rotation
HIGH
There would be a single
cell of convection in each
hemisphere
LOW

24. But the earth rotates Coriolis deflection causes air to be deflected
from those simple
convective pathways
Creating 3 cells in each
hemisphere and a surface
High Pressure in subtropics

25. Let’s look at SURFACE
Components of each cell

26. Hadley Cells Strong and persistent Warm air rising at
Intertropical Convergence
Zone (ITCZ)
At top of troposphere,
spreads poleward, sinks at
Subtropical Highs
Blows towards ITCZ at
Surface, creating…

27. Trade Winds Between subtropical Highs and ITCZ NE in N. Hem
SE in S. Hem

28. Ferrel cells Not as strong, persistent, well- defined

29. Westerlies (surface component of Ferrel cells)
35o - 60o N & S
not steady or persistent

30. Polar Front Zone 60o - 65o N & S
zone of conflict between differing air masses

31. Polar Easterlies 65o - 80o N & S
more prevalent in Southern, variability in Northern

32. Distribution of land masses disturbs this idealized
system of Highs, Lows, winds
Why?
Uneven heating of land and water creates temperature differences
and therefore pressure differences over land vs water with seasonal changes

33. Canadian High
Siberian High
Icelandic Low
Aleutian Low
Azores Bermuda High
Pacific High

34. Pacific High
Azores Bermuda High
Monsoonal Low

35. Upper Air Movement

36. 500
625
Isobaric
surfaces
750
875
1000
City A
City B
COOL
HEAT

37. DECREASED DENSITY
INCREASED DENSITY
It takes a shorter column of cold air to exert the same surface pressure as a tall column of warm air.
500
625
500
625
750
750
875
875
1000
1000
HEAT
COOL

38. Hot Cold 2300 meters 500 Constant Pressure Map (isobaric maps) 625 750
850
750
850
1000
1000
Hot
Cold

39. Constant pressure map shows elevation of a certain pressure.
Low heights and troughs
represent cold air.
High heights and ridges
represent warm air.

40. 5400
5520
5580
5640
5700

43. Currently: Current surface temperature map
Current map of heights of 500 mb layer

44. Constant Altitude Map
Shows pressure at a given altitude

45. 500
625
550 mb
750
810mb
500
1000m
1000m
1000m
625
850
750
850
1000
1000
Hot
Cold

46. On a constant altitude map:
low pressures indicate Cold Air
high pressures indicate Warm Air

47. High heights on a constant pressure surface map are equivalent to high pressures on a constant altitude map
Low 500 mb heights are associated with low pressure at any given altitude;
High 500 mb heights are associated with high pressure at any given altitude.

48. Therefore, high and low heights tell you where high and low pressures are (for a given altitude)

49. Upper Level Winds Westerly in mid- and high latitudes Easterly
(20°-90° N & S)
Easterly
in Tropics (15°N - 15°S)

50. Upper Level Westerlies have ridges and troughs:
“Rossby Waves” (Longwaves)
Wavelength = 1000s km
3 - 6 loops around earth
above 500 mb layer
influence surface weather

51. c

53. Converging height lines make wind speeds increase

56. On warm side, pressure drops less rapidly with altitude than on cold side; Note isobaric surfaces slope and slope increases with altitude

57. Therefore, wind speed increases with altitude
JET STREAMS : zones of high wind speed (Narrow bands, speed increases toward center (up to 150 mph))
Embedded in upper level Westerlies
below tropopause
Jet streams are located above strong temperature contrasts at surface

58. Polar Jet Stream
Subtropical Jet Stream

59. Polar Front Jet Stream
Between midlatitude tropopause and polar tropopause

60. Polar Jet above Polar Front Zone :
Where cold dense polar air meets warmer air from mid-latitudes

63. Can see polar jet on 300 mb maps
Current 300 mb map

64. Subtropical Jet Stream
Between midlatitude tropopause and tropical tropopause

66. Subtropical Jet due to Conservation of Angular Momentum:
greatest wind speed at North edge of Hadley cell
due to Conservation of Angular Momentum:
(smaller radius of rotation, faster the spin)