Chapter 6: Air Pressure and Winds

1. Chapter 6: Air Pressure and Winds • Atmospheric pressure • Measuring air pressure • Surface and upper-air charts • Why the wind blows • Surface winds • Measuring and determining winds

2. Atmospheric Pressure • air pressure at a given level is the weight of the air above • air pressure and temperature P = ρRT at constant P, cold parcel is denser; at constant T, higher P means denser air; at constant density, higher P means higher air T Q1: Because P = ρRT, higher T always leads to higher P a) true, b) false Q2: When we say “warmer air parcel is less dense and hence would rise”, the implicit assumption is a) Parcel pressure is the same as the environment; b) Parcel pressure is higher; c) parcel pressure is lower

3. Same density Stepped Art Fig. 6-2, p. 143

4. Q3: Which statement is correct? a) Warm air leads to high pressure in the atmosphere; b) Cold air lead to high pressure in the atmosphere Q4: Which statement is correct? a) It takes a shorter column of colder air to exert the same surface pressure b) It takes a taller column of colder air to exert the same surface pressure Q5: Air flows from high pressure to low pressure at the same altitude. a) true, b) false

5. Measuring air pressure • mercury barometer • digital barometer in weather observations Standard atmospheric pressure: 1013.25 mb = 1013.25 hPa = 29.92 in.Hg

6. Pressure Readings • station pressure: surface P at specific location if mercury barometer is used, corrections of temperature, gravity, and instrument error (surface tension of mercury) are needed • sea-level pressure: obtained from station P with corrections of altitude using 1 mb pressure increase for 10 m elevation decrease • Isobars constant pressure contour Q6: which statement is correct: a) 1 mb change for >10 m height change for warm air; b) 1 mb change for <10 m height change for warm air

8. Surface and Upper Air Charts • Surface map: isobars, high (H), low (L), cross-isobar flow • 500 mb map: height contour lines, ridges, troughs, flow parallel to height contours Q7: Why do height contours decrease in value from south to north? a) because temperature is higher in the south; b) because pressure is higher in the south

9. Figure 2, p. 150

10. Q8: Assuming pressure at point A is higher than that at B at the same height (e.g., around 5500 m), a) 500 mb height at A is greater than that at B; b) 500 mb height at A is less than that at B; c) 500 mb height at A is the same as that at B Q9: Assuming 500 mb height at A is greater than that at B, pressure at the same height (e.g., around 5500 m) would be a) higher at A; b) higher at B; c) equal at A and B Q10: Assuming pressure at point A is higher than that at B at the same height (e.g., around 5500 m), air temperature is a) higher at A; b) higher at B; c) equal at A and B

11. Why the Wind Blows • Newton’s first law of motion An object at rest (or in motion) will remain at rest (or in motion) as long as no force is exerted on the object • Newton’s second law of motion F = ma (force = mass times the acceleration) acceleration could be change of speed or direction Four forces include pressure gradient force, Coriolis force, centripetal force (or its opposite, centrifugal force), and friction Q11: if F = 0, does the object still move? a) yes, if it was moving; b) no, if it was at rest; c) both a) and b)

12. Forces that Influence the Wind • Wind is the result of a balance of several forces. • net force and fluid movement

13. Pressure Gradient Force • pressure gradient (pressure difference/distance) • pressure gradient force (PGF) (from high to low pressure) • strength and direction of the pressure gradient force • The horizontal (rather than the vertical) pressure gradient force is responsible for air movement. Q12: how to increase PGF? a) increasing pressure difference; b) decreasing distance between isobars; c) both a) and b)

14. Surface map Q13: What is the wind speed at point A? a) 40 knots; b) 40 miles/hour; c) 40 km/hour A

15. Coriolis Force • real and apparent forces • Coriolis force is an apparent force due to earth’s rotation • Its strength increases with the object’s speed, earth rotation, and latitude • Its direction: perpendicular to wind, to the right-hand side over Northern Hemisphere (NH), and to the left over SH

16. Q14: The claim that “water swirls down a bathtub drain in opposite directions in the northern and southern hemispheres” a) is true; b) is false Q15: The Coriolis effect is stronger if a) wind speed is faster; b) latitude is higher; c) both a) and b)

17. Straight-line Flow Aloft • balance of the pressure gradient and Coriolis forces • geostrophic wind: parallel to isobars with low pressure to its left (or right) in NH (or SH) • good approximation for flow aloft • Geostrophic winds can be observed by watching the movement of clouds.

18. Curved Winds Around Lows and Highs Aloft • cyclonic flow (with low P center) and anticyclonic flow (with high P center): direction opposite in NH versus SH • clockwise and anticlockwise: same direction in NH and SH • centripetal force (opposite to centrifugal force) • gradient wind: balance of PGF, Coriolis and centrifugal forces PGF = Co + Cen Co = PGF + Cen

19. Q16: what is the direction of PGF? a) from high P to low P; b) from low P to high P; c) depending on NH or SH Q17: what is the direction of Coriolis force? a) to the right of movement in NH; b) to the left of movement in NH; c) to the right of movement in SH Q18: what is the direction of centrifugal force? a) always outward; b) always inward; c) depending on NH or SH Q19: what is the balance of PGF, Co, and Cen for SH cyclonic flow? a) PGF = Co + Cen; b) Co = PGF + Cen

20. Winds on Upper-level Charts • meridional and zonal winds • wind is nearly parallel to the height contour • higher air T yields greater height contour value • Height contours on upper-level charts are interpreted in the same way as isobars on surface charts.

21. West wind over midlatitudes in NH and SH Figure 4, p. 157

22. Surface Winds • planetary boundary layer: bottom 1 km above surface • Friction: opposite to wind in direction; increases with wind • frictional effects on the wind: slow down wind • Wind rotates clockwise from near surface to free atmosphere in the NH

23. Wind always moves cross isobars toward the low pressure center in • both NH and SH; it moves outward for the high pressure center. • Wind rotates anticlockwise from near surface to free • atmosphere in the SH Fig. 6-21, p. 160

24. Q20: draw the three force (PGF, Co, Cen) balance and wind direction for a NH low pressure center. Q21: draw the three force (PGF, Co, Cen) balance and wind direction for a SH low pressure center. Q22: if surface wind is southwest in Tucson, the wind at 3000 m would be a) southerly; b) westerly; c) southwesterly; d) northeasterly

25. Winds and Vertical Motions • divergence and convergence • hydrostatic equilibrium (vertical PGF = gravity) Q23: Vertical PGF is much larger than horizontal PGF. a) true; b) false

26. Measuring and Determining Winds • wind direction: the direction where wind comes from • prevailing wind: wind direction that occurs most frequently • wind rose Q24: If the wind is southwesterly, the wind direction is a) 45o; b) 135o; c) 225o; d) 315o

27. Wind Instruments • wind vane • cup anemometer • aerovane • rawinsonde • wind profiler • By observing flags and smoke plumes, our eyes are also effective wind instruments.

28. Q25: at 14:00 local time, the surface wind is a) westerly; b) southerly; c) southwesterly; d) northeasterly Wind Power Fig. 6-29, p. 163