PRESSURE, WIND & FORCES Dr. Sam Miller Weather & Climate – MTDI 1200OL Plymouth State University

1. PRESSURE, WIND & FORCES Dr. Sam Miller Weather & Climate – MTDI 1200OL Plymouth State University 1

2. Ahrens, Ch. 6

3. Atmospheric Pressure

4. Atmospheric Pressure • It is the pressure exerted because of the weight of the air above • Pressure = Force/Area • Varies much more in the vertical than the horizontal • Horizontal variations • Tens of millibars in thousands of kilometers • Vertical variations • Hundreds of millibars if ten kilometers • Always decreases with height

5. Less mass over your head here… …than here.

6. Atmospheric Pressure • Average sea-level pressure (SLP) • 1013.25 mb (a.k.a. hecto-Pascal; hPa) • 29.92 in. Hg • 14.7 lb/in2 • “Normal” range of sea level pressure • 960 to 1045 mb (hPa) • 960 – center of strong hurricane • 1045 – strong high in the winter • Average height of 500 hPa is 5.5 KM ASL (5500 meters)

7. Measuring Pressure • Barometers are used to measure pressure • Mercurial barometer • Classic barometer • Aneroid barometer • More recent • Digital barometer • Commonly used at modern airports • An altimeter is a type of barometer calibrated to show altitude, instead of pressure

8. Mercury Barometer

9. Aneroid Barometer

10. Station vs. Sea Level Pressure • Pressure always falls with height • A station above sea level will always have lower pressure than a station at sea level • Vertical changes in pressure overwhelm horizontal changes • As meteorologists, we’re interested in identifying weather systems, not making topographic maps

11. Station vs. Sea Level Pressure • It’s the horizontal differences in pressure that are meteorologically significant • Solution: Convert all station pressures to sealevel pressures • This is the pressure a given station would have if it were at sea level • Makes it possible to “cancel out” vertical changes in pressure, and visualize only the horizontal changes.

12. Correcting to Sea Level Observed station pressures at elevation Correction applied Resulting sea-level pressure Sea-level pressures with isobars

13. Correcting to Sea Level Observed station pressures at elevation LOOKS AS IF LOWEST PRESSURE IS HERE Correction applied Resulting sea-level pressure Sea-level pressures with isobars

14. Correcting to Sea Level Observed station pressures at elevation Correction applied Resulting sea-level pressure ADD ABOUT 1 MB FOR EACH TEN METERS OF ALTITUDE REDUCTION Sea-level pressures with isobars

15. Correcting to Sea Level Observed station pressures at elevation Correction applied Resulting sea-level pressure Sea-level pressures with isobars

16. Correcting to Sea Level Observed station pressures at elevation Correction applied Resulting sea-level pressure Sea-level pressures with isobars

17. Correcting to Sea Level Observed station pressures at elevation Correction applied LOWEST PRESSURE IS REALLY HERE Resulting sea-level pressure Sea-level pressures with isobars

18. Typical sea-level pressure chart

19. Typical sea-level pressure chart

20. Wind

21. Uneven heating of the Earth’s surface causes horizontal temperature differences • Horizontal temperature differences result in horizontal pressure differences

22. Cold Dense Heavy Warm Thinner Lighter

23. Uneven heating of the Earth’s surface causes horizontal temperature differences • Horizontal temperature differences result in horizontal pressure differences • Horizontal pressure differences push mass from high to low pressure

24. Wind • Movement of mass (air) in response to pressure differences • If no other factors affected it, wind would always blow directly from H to L pressure

25. How Fast Does the Wind Blow? • If pressure difference is large • wind blows faster • If pressure difference is small • wind blows slower

26. Pressure Gradient • Gradient = Change of some quantity over a distance • Pressure Gradient (PG): • Change in sea-level pressure per distance

27. Small PG Large PG

28. Light Wind Strong Wind

29. Measuring Wind • Wind vane • Measures wind direction • Anemometer • Measures wind speed

31. Wind Direction • Direction that it is COMING FROM

32. The wind speed and direction depend on the sum of the forces acting on the atmosphere • Pressure gradient is not the only force at work

33. Forces

34. Which forces affect the wind? • Pressure gradient force (PGF) • Due to horizontal differences in pressure • Coriolis force (CoF) • Due to the Earth’s rotation • Centrifugal force (CeF) • Due to curved motions • Friction force (FrF) • Due to interaction with the surface

35. Which forces affect the wind? • Pressure gradient force (PGF) • Due to horizontal differences in pressure • Coriolis force (CoF) • Due to the Earth’s rotation • Centrifugal force (CeF) • Due to curved motions • Friction force (FrF) • Due to interaction with the surface

36. Pressure Gradient Force (PGF) • Force that “pushes” the air from regions of higher pressure to regions of lower pressure • The “prime mover” • Gravity in disguise • Always directed away from H and toward L • Perpendicular to isobars • Strongest where pressure gradient is strongest • Important in entire atmosphere

38. Which forces affect the wind? • Pressure gradient force (PGF) • Due to horizontal differences in pressure • Coriolis force (CoF) • Due to the Earth’s rotation • Centrifugal force (CeF) • Due to curved motions • Friction force (FrF) • Due to interaction with the surface

39. Coriolis Force (CoF) • “Apparent” force due to the rotation and curvature of the Earth • Deflects objects from a straight path • Acts perpendicular to the wind direction • To the right in the Northern Hemisphere • To the left in the Southern Hemisphere • Affects all free-moving objects • Projectiles • Aircraft • Ocean currents • Air molecules • Important in entire atmosphere

40. Coriolis Force (CoF) • “Apparent” force due to the rotation and curvature of the Earth • Deflects objects from a straight path • Acts perpendicular to the wind direction • To the right in the Northern Hemisphere • To the left in the Southern Hemisphere • Affects all free-moving objects • Projectiles • Aircraft • Ocean currents • Air molecules • Important in entire atmosphere Gaspard-Gustave de Coriolis, early 19th Century French mathematician, scientist, and mechanical engineer

41. Strength of CoF depends on: • Rotation of the Earth • Speed and direction • Constant for our purposes • Latitude • CF zero at equator • CF stronger at higher latitudes • CF maximum at poles • Speed of the object • Faster objects deflected more • Slower objects deflected less

42. Which forces affect the wind? • Pressure gradient force (PGF) • Due to horizontal differences in pressure • Coriolis force (CoF) • Due to the Earth’s rotation • Centrifugal force (CeF) • Due to curved motions • Friction force (FrF) • Due to interaction with the surface

43. Centrifugal Force (CeF) • Another “apparent” force directed radially outward from the center of a system’s rotation. • Regardless of direction of rotation • Important throughout the entire atmosphere • Does not require “closed” circulation – only curvature • Points outward from focal point (center) of rotation

44. Which forces affect the wind? • Pressure gradient force (PGF) • Due to horizontal differences in pressure • Coriolis force (CoF) • Due to the Earth’s rotation • Centrifugal force (CeF) • Due to curved motions • Friction force (FrF) • Due to interaction with the surface

45. Frictional Force (FrF) • Force that “slows down” the wind, due to interactions with the surface. • Always directed opposite to the wind • Only Important from the surface up to about 1 km • Boundary layer – layer of the atmosphere where friction is important • Depth of the boundary layer varies, depending on time of day, wind speed, and surface roughness

46. If forces are balanced, air flows at constant speed and direction • If they are not balanced air will keep accelerating (changing direction and or speed) • The balance of forces is different close to the surface than it is in the upper atmosphere • Wind behaves differently at the surface than aloft • No friction aloft (above about 1 km)

47. Combining Forces

48. Wind Aloft • Applies to “free troposphere” above the boundary layer • Above ~ 1 km • Friction not important • Straight motion: PGF and CoF • Curved motion: PGF, CoF and CeF

49. Case 1: Geostrophic Wind • PGF balances CoF • No CeF or FrF • Wind direction parallel to straight isobars • Wind speed is constant • Remember Buys Ballot’s Law L isobars PGF wind CoF H

50. Case 1: Geostrophic Wind Buys Ballot’s Law “If you stand with your back to the wind in the Northern Hemisphere, lower pressure is on your left and higher pressure is on your right”