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Chapter 6: Air Pressure and Winds

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1 Chapter 6: Air Pressure and Winds
Atmospheric pressure Surface and upper-air charts Why the wind blows Surface winds Winds and vertical air motions Determining wind direction and speed

2 Atmospheric Pressure air pressure at a given level is the weight of the air above air pressure and temperature P = ρRT (where R is a constant) at constant P, cold parcel is denser; at constant T, higher P means denser air; at constant density, higher P means higher air T Q: Because P = ρRT, higher T always leads to higher P a) true, b) false Q: 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 Figure 6.2: (a) Two air columns, each with identical mass, have the same surface air pressure. (b) Because it takes a shorter column of cold air to exert the same surface pressure as a taller column of warm air, as column 1 cools, it must shrink, and as column 2 warms, it must expand. (c) Because at the same level in the atmosphere there is more air above the H in the warm column than above the L in the cold column, warm air aloft is associated with high pressure and cold air aloft with low pressure. The pressure differences aloft create a force that causes the air to move from a region of higher pressure toward a region of lower pressure. The removal of air from column 2 causes its surface pressure to drop, whereas the addition of air into column 1 causes its surface pressure to rise. (The difference in height between the two columns is greatly exaggerated.) Watch this Active Figure on ThomsonNow website at

4 Q: Which statement is correct?
a) Warm air leads to high pressure in the mid-troposphere; b) Cold air lead to high pressure in the mid-troposphere 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 Q: 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: mb = hPa = 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 Q: If 1 mb change corresponds to 10 m in height change near surface, what would 1 mb change correspond to in mid-troposphere? a) > 10 m, b) 10 m, c) < 10 m

7 Q: If surface pressure is 952 mb at 600 m above sea level, its sea level pressure is: a) 892 mb, b) 952 mb, c) 1012mb, d) 1552 mb Q: if surface pressure is 1032 mb at 100 m below sea level, what is the sea level pressure? Q: if sea level pressure is 1009 mb, what is the surface pressure at 300 m above sea level?

8 Q: Can two isobars drawn on a surface weather map ever intersect?
yes, no

9 Surface and Upper Air Charts
Surface map: isobars, high (H), low (L), cross-isobar flow (note: sea level pressure is shown) 500 mb map: height contour lines, ridges, troughs, flow parallel to height contours (note: height above sea level at constant 500 mb is shown)

10 Q: Since the height at 500 mb is higher in the south than in the north, the pressure in the south is: a) great than that in the north, b) equal that in the north, c) less than that in the north The thickness between two pressure levels (or the height above sea level at a given pressure) is proportional to the average temperature of this layer: the higher the temperature, the greater the height. Figure 2: The area shaded gray in the diagram represents a surface of constant pressure. Because of the changes in air density, a surface of constant pressure rises in warm, less-dense air and lowers in cold, more-dense air. These changes in elevation of a constant pressure (500-mb) surface show up as contour lines on a constant pressure (isobaric) 500-mb map.

11 Q: 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 Q: 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 Q: Why do height contours decrease in value from south to north? B A

12 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 Q: 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)

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

14 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. Q: how to increase PGF? a) increasing pressure difference; b) decreasing distance between isobars; c) both a) and b)

15 Q: where is the wind strongest in the right figure (A, B, C, or D)?
Q: What is the wind speed at point A? a) 40 knots; b) 40 miles/hour; c) 40 km/hour A

16 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 (or more exactly the sine function of latitude) Its direction: perpendicular to wind, to the right-hand side over Northern Hemisphere (NH), and to the left over SH Coriolis force changes the direction only (but not the wind magnitude)

17 Q: The claim that “water swirls down a bathtub drain in
opposite directions in the northern and southern hemispheres” a) is true; b) is false Q: The Coriolis effect is stronger if a) wind speed is faster; b) latitude is higher; c) both a) and b) Q: What are sin(30o) and sin(0o)? Figure 6.14: On nonrotating platform A, the thrown ball moves in a straight line. On platform B, which rotates counterclockwise, the ball continues to move in a straight line. However, platform B is rotating while the ball is in flight; thus, to anyone on platform B, the ball appears to deflect to the right of its intended path.

18 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.

19 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 Co > PGF

20 Q: 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 Q: 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 Q: what is the direction of centrifugal force? a) always outward; b) always inward; c) depending on NH or SH Q: what is the balance of PGF, Co, and Centrifugal forces for SH cyclonic flow? a) PGF = Co + Cen; b) Co = PGF + Cen

21 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.

22 West wind over midlatitudes in NH and SH
Q: What is the wind direction for a cyclone over southern hemisphere? a) clockwise, b) anticlockwise, c) either way Figure 4: Upper-level chart that extends over the Northern and Southern hemispheres. Solid gray lines on the chart are isobars.

23 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

24 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 Figure 6.21: (a) Surface weather map showing isobars and winds on a day in December in South America. (b) The boxed area shows the idealized flow around surface-pressure systems in the Southern Hemisphere.

25 Q: draw the three force (PGF, Co, Centrifugal) balance and wind direction for a NH low pressure center. Q: draw the three force (PGF, Co, Centrifugal) balance and wind direction for a SH low pressure center. Q: if surface wind is southwesterly in Tucson, the wind at 2000 m would be a) southerly; b) westerly; c) southwesterly; d) northeasterly

26 Winds and Vertical Motions
divergence and convergence (right-hand rule) hydrostatic equilibrium (vertical PGF = gravity) Q: Vertical PGF is much larger than horizontal PGF. a) true; b) false Q: why does vertical PGF usually not result in upward motion?

27 Determining Wind Direction and Speed
wind direction: the direction where wind comes from prevailing wind: wind direction that occurs most frequently wind rose Q: If the wind is southwesterly, the wind direction is a) 45o; b) 135o; c) 225o; d) 315o

28 Wind Instruments wind vane cup anemometer aerovane rawinsonde
wind profiler By observing flags and smoke plumes, our eyes are also effective wind instruments. Q: The arrow of the vane points a) into the wind b) away from the wind

29 Q: at 14:00 local time, the near-surface wind is a) westerly;
b) southerly; c) southwesterly; d) northeasterly Figure 6.29: A profile of wind direction and speed above Hillsboro, Kansas, on June 28, 2006. Wind Power

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