1. Pressure, and the forces that explain the wind
Chapter 6
Pressure, and the forces that explain the wind

2. Dz area Hydrostatic balance: The upward pressure gradient force is
equal and opposite to the gravity

4. aneroid
barometer
mercury
barometer
aneroid barograph

5. What is the typical SLP? How much does it vary ?

6. 780
Average air pressure in Laramie

8. We need to reduce station pressure to a standard height, for instance sea level
Why?
Because winds are driven by horizontal pressure differences

9. Isobars and pressure patterns

10. Where are you more likely to find a pressure value of 994 mb? At A or B ?

11. Becoming acquainted with contouring and frontal analysis
Defining patterns on a surface weather chart
lows and highs
trofs and ridges
saddle

13. ridge
trof

14. examine the current weather analysis

15. What drives the wind?

16. Pressure gradient force (PGF) and wind

17. here, Dx=100 km and Dp=4 mb

18. The PGF is directed from high to low pressure, and is stronger when the isobars are more tightly packed

19. in reality, winds do not blow from high to low, at least not along the shortest path
… so there must be other force(s)

20. Coriolis force
Geostrophic wind balance: a balance between the PGF and the Coriolis force
link

21. L

22. Buys-Ballot law When you face downwind, the low will be on your left
Vice versa in the southern hemisphere
you (seen from above)

23. The geostrophic wind blows along the isobars (height contours), counterclockwise around lows (in the NH), and at a speed inversely proportional to the spacing between the isobars (height contours)

25. in the southern hemisphere, the low is on your right when you look downwind

26. There is a third force, important only near the ground
Friction slows the wind

27. Interplay between 3 forces
1008
1004
1000
Interplay between 3 forces
Pressure gradient force
Coriolis force
Friction (near the ground)
Check out how they affect the wind!
Guldberg-Mohn balance

28. > 30° near mountainous terrain
Trajectories
spiral out of a high,
and into a low
~ 10° over oceans
~ 30° over land
> 30° near mountainous terrain

30. finally, a fourth force: centrifugal force
CFF
PGF
PGF
Coriolis
Coriolis
CFF
slower-than-geostrophic wind
(subgeostrophic)
faster-than-geostrophic wind
(supergeostrophic)

31. The jet stream wind is subgeostrophic in trofs, and supergeostrophic in ridges
slow
fast
fast
slow

32. Where does the air, spiraling into a low, end up?
height

33. rising motion leads to cloudiness and precipitation
subsidence leads to clear skies

34. Fig

35. 300 mb height, 9 Nov 1975, 7 pm
Find the trofs
Fig

36. upper-level divergence, low-level convergence slow
fast
upper-level divergence,
low-level convergence
slow
surface low
300 mb height, 9 Nov 1975, 7 pm
Fig

37. Today’s upper-air maps
Today’s surface weather analysis
Today’s upper-air maps

38. Upper-level winds, and upper-level charts

39. Upper level charts are NOT plotted at constant height, eg 18,000 ft
Upper level charts are NOT plotted at constant height, eg 18,000 ft. Rather, they display the topography of a pressure surface, eg 500 mb

40. Approximate conversion of pressure level to altitude
Approximate Height
Approximate Temperature*
1013 mb
0 m (sea level)
0 ft
15 °C
59 °F
1000 mb
100 m
300 ft
850 mb
1500 m
5000 ft
5 C
41 F
700 mb
3000 m
10000 ft
-5 C
23 F
500 mb
5000 m
18000 ft
-20 C
-4 F
300 mb
9000 m
30000 ft
-45 C
-49 F
200 mb
12000 m
40000 ft
-55 C
-67 F
100 mb
16000 m
53000 ft
-56 C
-69 F
Approximate conversion of pressure level to altitude

41. 1000 mb – near sea level

42. 850 mb - ~5,000 ft

43. 700 mb - ~10,000 ft

44. 500 mb - ~18,000 ft

45. 300 mb - ~30,000 ft

46. 200 mb ~ 40,000 ft

47. pressure at a fixed height (sea level)

48. elevation of the 1000 mb surface

49. contours: sea-level pressure color fill: 1000 mb height

50. height Why do isobar and height contour charts look (almost) the same?
high
low
1500 m
pressure surface
sea level
New York
Boston
Pressure decreases with height at about 10 mb every 100 m

52. Locate the trofs

54. Thickness and temperature

56. thickness
between 2 material surfaces
( mb)
temperature

57. L

58. Pop quiz: why is their a ‘pit’ in the 500 mb surface over Antarctica?
- because it is much colder there than over Australia and other surrounding places
- because of the ozone hole
- because there is less sunshine
- I give up

59. L calm calm calm calm Jet stream is due to the cold pool below
(circumpolar vortex)
calm
calm

60. Jet stream
why does it exist?
why does it vary in strength?
The jet stream is the result of a horizontal temperature gradient
… and thus a thickness gradient
thickness = 20.3 * Tmean
thickness is in meters between 1000 and 500 mb
Tmean is the layer-mean temperature in Kelvin

61. pretty flat pretty steep 1000 mb height (m) 500 mb height (m) 5000 100
5200
5400
5600
150
5800
1000 mb height (m)
500 mb height (m)
near the ground: weak PGF, weak wind
near 18,000 ft: strong PGF, strong wind
Where is the mb thickness lower? Where is it higher?
Where is the colder airmass – where is the warmer one?

62. B B A A 1000 mb height (m) 500 mb height (m) 5000 5100 100 5200 5400
5600
150
5800 A A
1000 mb height (m)
500 mb height (m)
Calculate thickness at A and B
at A: Z500-Z1000 = = 5700 m
at B: Z500-Z1000 = = 5000 m
… answer: the lower atmosphere
is less thick at B up north

63. B A 700 mb mean temperature (C)
indeed, it is colder where the air is less thick B A
700 mb mean temperature (C)

64. Relation between wind and temperature ...
Key : colder air is less thick, therefore upper level winds will blow cyclonically around cold pools

65. For instance, look at the pole-to-pole variation of temperature with height (in Jan)

66. Around N, temperature drops northward, therefore westerly winds increase in strength with height

67. The N-S temperature gradient is large between 30-50N and 1000-300mb
Therefore the westerly wind increases rapidly from 1000 mb up to 300 mb J J
cold
warm
cold

68. ‘thermal wind’
The increase of wind with height parallel to the isotherms, cyclonically around cold pools

69. Illustration : compare the 300 mb height over the northern hemisphere ...

70. … to the temperature
700 mb

71. Question:
Why, if it is colder at higher latitude, doesn’t the wind continue to get stronger with altitude ?

72. There is definitively a jet ...
stratosphere
troposphere

73. Pop quiz: Why is there a jet maximum in the upper troposphere?
because the air is too thin in the stratosphere
because it is warmer over the poles than over the equator, in the stratosphere
because of the ozone hole
because there is too much friction with outer space in upper layers of the atmosphere.

74. Answer: above 250 mb, it is no longer colder at higher latitudes...
60 kft
stratosphere
troposphere
18 kft
equator
pole

75. Now explain why a jet stream is found above a frontal zone
wind speed (kts)

76. The jet stream is there because of low-level temperature differences
polar
front jet
(PFJ)
The jet stream is a current of fast moving air found in the
upper levels of the atmosphere. This rapid current is typically
thousands of kilometers long, a few hundred kilometers wide,
and only a few kilometers thick. Jet streams are usually found
somewhere between km (6-9 miles) above the earth's
surface. The position of this upper-level jet stream denotes the
location of the strongest surface temperature contrast

77. Pop quiz: why is the jet stream stronger in winter?
because the north-south temperature gradient is larger
because cold air is lighter and can be blown around easier
because there is less sunshine
because there are fewer thunderstorms that act as obstacles to the upper-level flow.

78. Pop quiz: why is the jet stream stronger in winter?
because the north-south temperature gradient is larger
because cold air is lighter and can be blown around easier
because there is less sunshine
because there are fewer thunderstorms that act as obstacles to the upper-level flow.
Change the equator-to-pole temperature gradient, and see what happens to the jet stream!

79. Pop quiz: according to climate change models and observations, the arctic is warming up faster than low latitude regions. What does this imply about the strength of the jet stream and the intensity of storms spawned by the jet stream?
they weaken
they strengthen
it can go either way
I give up

80. Summary There are four key forces driving the wind:
pressure gradient force (to start the motion)
Coriolis force
friction (only near the ground)
centrifugal force
As a result the wind blows counterclockwise around lows (in the NH)
friction makes the low-level wind spiral into lows
the centrifugal force slows the wind in trofs, and speeds it up in ridges
Weather changes (as we know it) is the result of passing jet streams, with
rising motion & clouds ahead of a trof, with a low at the surface
sinking motion & clear skies upstream of a trof, with a high at the surface
the deep vertical motion is due to changes in wind speed in the jet, as the wind in trofs (ridges) is slower (faster) than expected from geostrophic balance
The jet stream tends to occur above regions with a large temperature difference
The jet blows counterclockwise around cold pools (in the NH)

81. Let’s cover chapter 7 (global winds) and skip chapter 8 (air-sea interaction) then we ‘ll do chapter 9 (air masses and fronts) and chapter 10 (mid-latitude weather)