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

Atmospheric Pressure air pressure - definition air pressure - definition air pressure and temperature air pressure and temperature P = ρRT P = ρRT at constant P, cold parcel is denser; at constant P, cold parcel is denser; at constant T, higher surface P means denser air; at constant T, higher surface P means denser air; at constant density, higher surface P means higher air T at constant density, higher surface P means higher air T pressure gradient force pressure gradient force Air pressure is, quite literally the weight of the atmosphere above us.Air pressure is, quite literally the weight of the atmosphere above us.

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

Measuring air Pressure Standard atmospheric pressure: mb = hPa = in.Hg

Barometers mercury barometer mercury barometer aneroid barometer aneroid barometer altimeter altimeter barograph barograph

Pressure Readings station pressure station pressure after corrections of after corrections of temperature, gravity, and temperature, gravity, and instrument error (surface instrument error (surface tension of mercury) tension of mercury) sea-level pressure sea-level pressure after corrections of after corrections of altitude (1mb/10m) altitude (1mb/10m) isobars isobars

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

Figure 2, p. 150

Why the Wind Blows Newton’s Laws of Motion Newton’s first law Newton’s first law An object at rest (or in motion) will remain at rest (or in motion) as long as no force is exerted on the object 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 Newton’s second law F = ma (force = mass times the acceleration) F = ma (force = mass times the acceleration) acceleration could be change of speed or direction 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 Four forces include pressure gradient force, Coriolis force, centripetal force (or its opposite, centrifugal force), and friction

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

Pressure Gradient Force pressure gradient (pressure difference/distance) pressure gradient (pressure difference/distance) pressure gradient force (from high to low pressure) pressure gradient force (from high to low pressure) strength and direction of the 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.The horizontal (rather than the vertical) pressure gradient force is responsible for causing air to move.

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

Fig. 6-14, p. 153

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

Curved Winds Around Lows and Highs Aloft cyclonic and anticyclonic flow cyclonic and anticyclonic flow centripetal force (opposite to centrifugal force) centripetal force (opposite to centrifugal force) gradient wind gradient wind

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

Figure 4, p. 157

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

Fig. 6-21, p. 160

Winds and Vertical Motions divergence and convergence divergence and convergence hydrostatic equilibrium (vertical PGF = gravity) hydrostatic equilibrium (vertical PGF = gravity)

Measuring and Determining Winds

The Influence of Prevailing Winds characterization of wind direction characterization of wind direction prevailing winds prevailing winds wind rose wind rose Wind direction is defined in the opposite way as ocean currents: a southerly current means water is moving towards the south.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 wind vane anemometer anemometer aerovane aerovane radiosonde radiosonde wind profiler wind profiler By observing flags and smoke plumes, our eyes are also effective wind instruments.By observing flags and smoke plumes, our eyes are also effective wind instruments.

Fig. 6-29, p. 163 Wind Power