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

Atmospheric Pressure

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

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 www.thomsonedu.com/login. Stepped Art Fig. 6-2, p. 143

Measuring air Pressure

Barometers mercury barometer aneroid barometer altimeter barograph

Barometers aneroid barometer

Barometers Altimeter

Pressure Readings station pressure and sea-level pressure isobars

Surface and Upper Air Charts

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.

Why the Wind Blows

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

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

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.

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

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.

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

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.

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

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.

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

Surface Winds

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

Winds and Vertical Motions

Winds and Vertical Motions divergence and convergence hydrostatic equilibrium

Measuring and Determining Winds

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.

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