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

3. Atmospheric Pressure air pressure - definition
air pressure and temperature
pressure gradient force
Air pressure is, quite literally, the weight of the atmosphere above us.

4. 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
Stepped Art
Fig. 6-2, p. 143

5. Measuring air Pressure

6. Barometers
mercury barometer
aneroid barometer
altimeter
barograph

7. Barometers
aneroid barometer

8. Barometers
Altimeter

9. Pressure Readings
station pressure and sea-level pressure
isobars

10. Surface and Upper Air Charts

11. Surface and Upper Air Charts
isobaric maps
contour lines
ridges
troughs
Color-filled contour maps are the same as ordinary contour maps, except that the area between adjacent lines is filled in with color.

12. Why the Wind Blows

13. Newton’s Laws of Motion
Newton’s first law
Newton’s second law

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

15. Pressure Gradient Force
strength and direction of the pressure gradient force
The horizontal (rather than the vertical) pressure gradient force is responsible for causing air to move.

16. Figure 6.11: The pressure gradient between point 1 and point 2 is 4 mb per 100 km. The net force directed from higher toward lower pressure is the pressure gradient force.
Fig. 6-11, p. 151

17. Coriolis Force real and apparent forces Coriolis force
strength and direction of the Coriolis force
factors that affect the Coriolis force
It is sometimes claimed that “water swirls down a bathtub drain in opposite directions in the northern and southern hemispheres”. This is not true.

18. 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.
Fig. 6-14, p. 153

19. Straight-line Flow Aloft
combination of the pressure gradient and Coriolis forces
geostrophic wind
Geostrophic winds can be observed by watching the movement of clouds.

20. Curved Winds Around Lows and Highs Aloft
cyclonic and anticyclonic flow
centripetal force
gradient wind

21. Winds on Upper-level Charts
gradients in contour lines
meridional and zonal winds
Height contours on upper-level charts are interpreted in the same way as isobars on surface charts.

22. Figure 6.19: An upper-level 500-mb map showing wind direction, as indicated by lines that parallel the wind. Wind speeds are indicated by barbs and flags. (See the blue insert.) Solid gray lines are contours in meters above sea level. Dashed red lines are isotherms in °C.
Stepped Art
Fig. 6-19, p. 158

23. Surface Winds

24. Surface Winds planetary boundary layer friction
frictional effects on the wind
Most people rarely venture out of the planetary boundary layer.

25. Winds and Vertical Motions

26. Winds and Vertical Motions
divergence and convergence
hydrostatic equilibrium

27. Measuring and Determining Winds

28. The Influence of Prevailing Winds
characterization of wind direction
prevailing winds
wind rose
Wind direction is defined in the opposite way as ocean currents: a southerly current means water is moving towards the south.

29. Wind Instruments wind vane anemometer aerovane radiosonde
wind profiler
By observing flags and smoke plumes, our eyes are also effective wind instruments.