The Thermal Wind Relationship

1. LAB 9 April 12, 2006 The Thermal Wind Relationship

2. A Thought Experiment: Start with a column of air.

3. A Thought Experiment: The base of this column is at the surface, so lets say its pressure is about 1000mb. 1000mb

4. A Thought Experiment: The top of this column is quite high—let’s say that its pressure is 500mb. 500mb 1000mb

5. A Thought Experiment: This column has some thickness: it is some distance between 1000mb and 500mb. 500mb 1000mb

6. A Thought Experiment: If we heat the column of air, it will expand, warm air is less dense. The thickness of the column will increase. 500mb is now farther from the ground. 500mb 1000mb Warmer

7. A Thought Experiment: If we cool the column of air, it will shrink, cool air is more dense. The thickness of the column will decrease. 500mb is now closer to the ground. 500mb 1000mb Colder

8. A Thought Experiment: In fact, temperature is the ONLY factor in the atmosphere that determines the thickness of a layer!

9. A Thought Experiment: It wouldn’t have mattered which pressure we had chosen. They are all higher above the ground when it is warmer….

10. …which is what this figure is trying to show.

11. In the tropics, 700mb is quite high above the ground… 700mb

12. …whereas it is quite low to the ground near the poles. 700mb

13. These layers are much less “thick”. See how “thick” these layers are.

14. Let’s think about what this means near a polar front, where cold air and warm air are meeting.

15. This is a cross section of the atmosphere. North COLD South WARM

16. Cold air is coming from the north. This air comes from the polar high near the North Pole. North COLD South WARM

17. Warm air is coming from the south. This air comes from the subtropical high near 30°N. North COLD South WARM

18. These winds meet at the polar front. POLAR FRONT North COLD South WARM

19. Now, think about what we just learned about how temperature controls the THICKNESS of the atmosphere. POLAR FRONT North COLD South WARM

20. On the warm side of the front, pressure levels like 500mb and 400mb are going to be very high above the ground. 400mb 500mb POLAR FRONT North COLD South WARM

21. On the cold side of the front, pressure levels like 500mb and 400mb are going to be very low to the ground. 400mb 500mb 400mb 500mb POLAR FRONT North COLD South WARM

22. Above the front, the thickness of the atmosphere changes rapidly. 400mb 500mb 400mb 500mb POLAR FRONT North COLD South WARM

23. Now, let’s think about the pressure gradient force above the front. 400mb 500mb 400mb 500mb POLAR FRONT North COLD South WARM

24. Let’s draw a line from the cold side of the front to the warm side. 400mb A 500mb B 400mb 500mb POLAR FRONT North COLD South WARM

25. What is the pressure at point A? 400mb A 500mb B 400mb 500mb POLAR FRONT North COLD South WARM

26. The pressure at point A is less than 400mb, since it is higher than the 400mb isobar on this plot. Let’s estimate the pressure as 300mb. 400mb A 500mb 300mb B 400mb 500mb POLAR FRONT North COLD South WARM

27. What is the pressure at point B? 400mb A 500mb 300mb B 400mb 500mb POLAR FRONT North COLD South WARM

28. The pressure at point B is more than 500mb, since it is lower than the 500mb isobar on this plot. Let’s estimate the pressure as 600mb. 400mb A 500mb 300mb B 400mb 600mb 500mb POLAR FRONT North COLD South WARM

29. The pressure gradient force between point B and point A is huge! 400mb A 500mb 300mb B 400mb 600mb 500mb POLAR FRONT North COLD South WARM

30. Therefore, all along the polar front, there will be a strong pressure gradient force aloft, pushing northward. 400mb A 500mb 300mb B 400mb 600mb 500mb POLAR FRONT North COLD South WARM

31. Key Points: This strong pressure gradient force happens: Aloft (above the surface) Directly above the Polar Front Also, this force pushed toward the north (in the Northern Hemisphere).

32. Polar Front and The Jet • So, how does this all cause the midlatitude jet stream?

33. Polar Front and The Polar Jet • Suppose we have a polar front at the surface. This purple line is the polar front at the surface. As we’ll learn, this is NOT how fronts are correctly drawn, but it will work for now.

34. Polar Front and The Jet • All along the front, there is a strong pressure gradient force pushing northward.

35. Polar Front and The Jet • Winds aloft are in geostrophic balance…

36. Polar Front and The Jet • …so the true wind will be a WEST wind, directly above the polar front.

37. Another View: Here’s the same diagram, shown from a slightly different angle, which might make this all more clear.

38. In Perspective: Here is the polar front at the surface.

39. In Perspective: Remember, it’s a polar front because it is where warm air from the south meets cold air from the north.

40. In Perspective: The midlatitude jet stream is found directly above the polar front.

41. Conclusions: The Midlatitude Jet Stream is found directly above the polar front, with cold air to the LEFT of the flow. This is because of the changes in THICKNESS associated with the polar front. This process is known as the THERMAL WIND RELATIONSHIP.