Lecture 10

1. Lecture 10 Pressure and Wind

2. Also called “Barometric Pressure” We measure pressure with a barometer. The first barometers were mercury barometer. The aneroid barometer is commonly used in homes. Atmospheric Pressure

3. Atmospheric Pressure • An “altimeter” is a type of aneroid barometer which is calibrated to indicate altitude. • A “barograph” is a recording aneroid barometer.

4. Reading Pressure • When reading the pressure at a certain place we are reading the “station pressure”. • Pressure changes quickly in the vertical.

5. Sea Level Pressure

6. Wind • Thus far we have looked at vertical motion, in terms of cloud development. • We also have horizontal motions referred to as “wind”. • Let’s look at what causes the wind.

7. What happens when you open a can of coffee? • The noise you hear is due to air rushing into the can. • Air moves from high to low pressure. • Wind is nature’s attempt to balance inequalities in air pressure

8. What causes the wind? • The wind we experience is a result of a combination of forces: • Pressure Gradient Force • Coriolis Effect • Centripetal Force • Friction

9. To understand the wind we need to look at pressure…. isobars

10. High and Low Pressure Centers

11. Pressure Gradient Force

14. Let’s Play a Game!

15. Imagine that you are on the North Pole You want to launch an object towards Madison, where do you aim towards? Hint: Will Madison be in the same place by the time the object gets there?

17. Coriolis Effect • The coriolis effect modifies the wind. • All free moving objects or fluids in the northern hemisphere are deflected to the right. • In the southern hemisphere they are deflected to the left. • The coriolis effect is result of the earth spinning.

19. More movies!

20. The Earth’s Spin • The N. Hemisphere spins counter-clockwise  coriolis deflects to the right • The S. Hemisphere spins clockwise  coriolis deflects to the left • The wind is deflected by coriolis differently in each hemisphere.

21. Coriolis Effect

22. Coriolis at the Equator • Coriolis is zero at the equator! • However, as soon as an object moves just a little off of the equator, coriolis will act on it.

24. Coriolis Effect

25. Coriolis and Pressure Gradient Force Together

26. Curved Winds Gradient Wind

27. Winds Aloft • Winds that are not influenced by surface friction and therefore behave as geostrophic winds and gradient winds. • To look at winds at upper levels we look on constant pressure charts. • i.e. 850mb, 700mb, 500mb, 250mb charts • These charts are at a constant pressure level, and have contours of height.

28. A constant pressure surface may have different heights.

29. Air Aloft

30. Ridges and Troughs

31. SFC VS. Aloft • At the surface we must now consider friction! • Another example

32. Surface Flow

33. Southern Hemisphere • How would the winds flow around highs and lows in the Southern Hemisphere?

34. Convergence and Divergence

35. Convergence and Divergence

36. Wind as a Force • Near the surface the wind speed goes to zero as a result of friction. • This is why it is often more difficult on a windy day to drive a “tall” vehicle. • Or why it is windier on a bridge. • Wind exerts a force on objects, if the objects do not move the wind moves around the object.

37. Measuring Wind • Wind speed and direction are measured. • Winds are named for the direction they come from. • Wind vanes are used to measure wind direction. • When the wind consistently blows from one direction, that direction is called the prevailing wind.