Chapter 8: Air Pressure and Winds

1. Chapter 8: Air Pressure and Winds

2. Wind What is wind? What determines the direction of the wind? What determines the wind speed? How can we use weather charts to discern both wind speed and direction?

3. Wind The answers have a lot to do with pressure variations! Recall that the pressure decreases with altitude (a reasonable approximation is 1 mb for every 10 meters) But how does the pressure vary in the horizontal?

4. Wind In the horizontal, the pressure varies only slightly, i.e., only a few millibars over several hundred miles/kilometers.

5. Wind In the horizontal, the pressure varies only slightly, i.e., only a few millibars over several hundred miles/kilometers. It is these horizontal variations in pressure that drive winds.

6. Wind In the horizontal, the pressure varies only slightly, i.e., only a few millibars over several hundred miles/kilometers. It is these horizontal variations in pressure that drive winds. Because these changes are small, we have to remove the effects of altitude to better represent the horizontal variability.

7. Wind Thus, to create the figure at left, observed station pressures are adjusted to sea level, i.e., the pressure that would be observed in the station were located at sea level.

8. Wind Correcting station pressures allows a surface pressure chart (which is a constant height chart) to be constructed.

9. Wind Example surface pressure chart Joaquin

10. Wind Horizontal pressure differences can be generated by changes in temperature

11. Wind The pressure difference aloft, pushes air toward the left, which decreases the pressure in the second column, forming a region of low pressure at the surface

12. Wind Recall the ideal gas law: p ~ r x T For a constant pressure, warming means that the density decreases!

13. Wind At the surface, we use a surface pressure chart, which is a constant height chart, to depict pressure. What about pressure variations aloft?

14. Wind We use constant pressure charts. We showed how pressure varies with temperature; thus, constant pressure surfaces (e.g., the 500-mb surface) slopes down from warm to cold air.

15. Wind The altitude at which the given pressure is found, is recorded on a constant pressure chart.

16. Wind

17. Wind

18. Wind The isoheight lines display a wave-like pattern with ridges and troughs. Ridges are associated with air that is warm. Troughs are associated with air that is cool.

19. Wind: Quick Recap Constant Height Chart(e.g., sea level chart): Locations with the same pressure linked by isobars. Constant Pressure Chart:Locations with the same height linked by lines of constant height (isoheights). Thus the constant pressure chart (isobaric chart) shows height variations along an equal pressure surface. At each location, the chart gives the height at which the specified chart pressure is found.Note that high heights are associated with warm air, and vice versa. To recap, constant height charts depict pressure variations; constant pressure charts show height variations.

20. Wind Our consideration of atmospheric pressure at the surface and aloft acts as a necessary preamble to a discussion of the wind. What causes the wind to blow?

21. Wind Our consideration of atmospheric pressure at the surface and aloft acts as a necessary preamble to a discussion of the wind. What causes the wind to blow? Horizontal variations in pressure that create a pressure gradient, which is indicated by a change in pressure over a certain distance.

22. Wind Our consideration of atmospheric pressure at the surface and aloft acts as a necessary preamble to a discussion of the wind. What causes the wind to blow? Horizontal variations in pressure that create a pressure gradient, which is indicated by a change in pressure over a certain distance. The resulting force is called the pressure gradient force.

23. Wind We can relate forces to moving objects by applying Newton’s second law, namely Force = mass x acceleration (F = ma). Thus, assuming an object’s mass is constant, an exerted force is directly related to acceleration, which the the change in velocity (speeding up, slowing down, or changing direction).

24. The magnitude of the pressure gradient can be assessed by noting the spacing of the isobars. A wide isobar spacing implies a small pressure gradient, and vice versa. Wind Horizontal pressure gradients result from unequal heating of the Earth’s surface.

25. Wind The pressure gradient force is directed inward, towards the center of a surface low (L). The pressure gradient force is directed outward from the center of a surface high (H).

26. Wind The magnitude of the pressure gradient force is proportional to the pressure gradient itself

27. Wind Because the pressure gradient force acts at right angles to the isobars (or isoheights), we might expect that winds would also blow at right angles to the isobars (or isoheights). A glance at a 500 mb map shows that contrary to expectation; the winds blow approximately parallel to the isoheight lines!

28. Wind The winds aloft do not blow parallel to the isobars and isoheights because of the action of the Coriolis Force. Properties of the Coriolis Force (CF) 1) The CF only occurs on Earth because it is rotating. 2) The CF always acts at right angles to the direction of motion. (Right in NH) 3) The strength of the CF depends on the speed of the object on which it is acting. 4) The strength of the CF also depends on the latitude. It is equal to zero at the equator and is largest at the poles

29. Wind 3) The strength of the CF depends on the speed of the object on which it is acting. 4) The strength of the CF also depends on the latitude. It is equal to zero at the equator and is largest at the poles

30. Wind Consider an air parcel initially at rest experiencing a constant PGF above the surface in the NH

31. Wind When the parcel starts to move as a result of the PGF, the CF increases in magnitude from zero and deflects the parcel off to the right.

32. Wind As the parcel accelerates, the CF increases in magnitude, pushing the parcel further off to the right. Notice that the PGF remains constant.

33. Wind Eventually the parcel is deflected by the CF so much that it is traveling parallel to the isobars, with equalpressure gradient and Coriolis forces acting in opposite directions. The resulting geostrophic windblows parallel to the isobars.

34. Wind Fun fact: Driving down a highway, your car would deviate to the right by about 1500 feet for every 100 miles if it were not for the friction between your tires and the road!

35. Wind It is the Coriolis force that causes air to move counterclockwise (cyclonic) around low-pressure systems and clockwise (anticyclonic) around high-pressure systems.

36. Winds on a non-rotating planet would blow directly from centers of high pressure to centers of low pressure Winds on a rotating planet like Earth, blow around centers of high and low pressure. Wind

37. Wind Oh if only it were that simple . . . On top of the PGF and CF, we have another force that we must account for

38. NH Wind Friction with the surface slows the wind speed. Slower wind speeds reduce the magnitude of the CF; thus the deflection of air parcels to their right is reduced. Consequently, the wind no longer blows parallel to the isobars, but angles across them toward lower pressure.

39. Wind

40. Wind Example of flow patterns around high- and low-pressure systems

41. Wind It is this ageostrophyc component that gives rise to convergence (divergence) at the centers of low- (high-) pressure systems