1. Chp. 07
Atmospheric Pressure & Wind

2. Pressure:
Few direct impacts
on our daily life-----
Rheumatism may flare up.
A bad tooth may ache.

3. H L The wind is a direct response to pressure
But it does not blow directly from H – L.

4. Atmospheric pressure. The “weight” of all. the molecules above
Atmospheric pressure The “weight” of all the molecules above a horizontal area
What is the pressure on the
surface of the moon?

5. James Burke discusses silver mining problems which led to ....

6. Continued

7. Continued

8. Mercury is often used to measure pressure:
Air pressure
Blood pressure

10. Aneroids are often used in home weather forecast stations

11. Barograph (aneroid barometer)

12. Chat with your neighbor and discuss
why this fellow is exiting this way.

14. To see what this means we go to the next slide

16. The station pressures are barometric
readings along the hills and valleys
in the top diagram.

17. Weather maps are used to describe pressure patterns
Weather maps are used to describe pressure patterns. The lines connect points of equal pressure. We interpret the patterns to help explain the weather (including winds).

18. A slice of the U.S. across the Rockies.
Oakland
Winnemucca
Salt Lake City
Denver
Dodge City ?
Nashville
Athens
A slice of the U.S. across the Rockies.
Since pressure drops about 10mb
for every 100m of height, mountains
cause big low pressure features.

19. This is a mess! What should we do?
This is a map of the staion pressures
for this date. Guess where the mtns are.
This is a mess! What should we do?

20. The trick is to “correct” each station to sea level by adding about 10mb for every 100m above sea level.
Thus the mtn
lows are “built
up”.

21. Now the map looks normal

22. It is insightful
to evaluate atmospheric
behavior at selected
height surfaces.

23. The governing equations are simpler if constant pressure surfaces are used. This is the reason the only constant height surface used is the surface map.

24. Changes in elevation of a constant pressure sfc (500 mb)
570
564
558
552
Changes in elevation of a constant pressure sfc (500 mb)
show up as contour lines on a constant pressure (500 mb)
map. Where the sfc dips fastest the lines are closest.

26. October mb Flow

27. Because warm air takes more space than cold air,
the distance between constant pressure surfaces is
greater when the air between them is warmer.

28. The wavelike patterns of a constant pressure sfc reflect the
changes of air temperature. The ridge (high heights) and the
trough (low heights) correspond with warm and cold temps.

29. Newton’s 2nd law: F=ma Or F/m=a
Force per unit mass equals (or causes) accelerations
Accelerations are changes in speed and/or direction

30. These forces are important for air flow
GRADIENT
These forces are important for air flow

31. p This causes the wind to blow L Pressure Gradient Force = ,
the change in air pressure for
some specified distance
Closely spaced lines represent large values
of pressure gradient, thus high winds!!

32. Closely spaced contours indicate
a strong pressure gradiant force
and high winds.

33. High pressure systems have widely
spaced contours and weak winds.

34. What do you predict the winds will be like in these two regions?
Sketch direction and speed

35. See how much you know?

36. Forces which influence horizontal winds
Pressure Gradient
Centripetal
Wind
Friction
Coriolis Force

38. Coriolis Example

40. Only an apparent force The Coriolis Force is proportional to :
wind speed
the spin of the system (earth)
the distance from the equator (latitude)
If it was a real force it could change the speed
of the wind. It only can change the direction!

41. Pressure Gradient
Direction of initial motion
Low Pressure
Isobars at the surface or
height contours at other levels
Coriolis Force only exists
when the speed is nonzero
High Pressure

42. Upper Air Wind Forces

43. Pressure Gradient
Low Pressure
Isobars at the surface or
height contours at other levels
Wind (“final”
motion!)
Coriolis Force
High Pressure

44. Geostrophic Flow For the upper air surfaces (no friction)
when the contours are more or less
straight and parallel the flow is called
Geostrophic Flow

45. L L H
This region has ~ geostrophic flow

46. Surface flow
Must consider the effects
of friction

47. The wind speeds up as it gets away from friction.

48. The surface winds cross the yellow isobars

49. Again compare the surface and upper levels.

50. Can the wind direction tell you where the low is?
Look at this diagram and try to invent a rule.

51. What causes the wind to follow curved paths?
When the PGF is greater than the Coriolis Force
(e.g., close contours in a low) inward flow occurs.

52. L L H

53. Centripetal acceleration pulls the riders
inward to make the path circular.

54. The imbalance toward the center of
motion is called the centripetal force.

55. Important consequences

57. Later we’ll put this together