class #5: air pressure and winds chapter 8 1class #5 tuesday, july 13, 2010

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Class #5: Air pressure and winds

Chapter 8

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Air pressure and winds

Chapter 8

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Atmospheric Pressure

• What causes air pressure to change in the horizontal?

• Why does the air pressure change at the surface?

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Atmospheric Pressure

• Horizontal Pressure Variations– It takes a shorter column of dense, cold air to

exert the same pressure as a taller column of less dense, warm air

– Warm air aloft is normally associated with high atmospheric pressure and cold air aloft with low atmospheric pressure

– At surface, horizontal difference in temperature = horizontal pressure in pressure = wind

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Atmospheric Pressure

• Special Topic: Gas Law

P is proportional to T x ρ

P = pressureT = temperatureρ = density

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Atmospheric Pressure

• Daily Pressure Variations– Thermal tides in the tropics– Mid-latitude pressure variation driven by

transitory pressure cells• Pressure Measurements– Barometer, barometric pressure• Standard atmospheric pressure 1013.25mb

– Aneroid barometers• Altimeter, barograph

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Atmospheric Pressure

• Pressure Readings– Instrument error: temperature, surface tension– Altitude corrections: high altitude add pressure,

10mb/100m above sea level

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Surface and Upper Level Charts

• Sea-level pressure chart: constant height • Upper level or isobaric chart: constant

pressure surface (i.e. 500mb)– High heights correspond to higher than normal

pressures at a given latitude and vice versa

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Class #5 Tuesday, July 13, 2010 23Table 8-1, p. 203

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Surface and Upper Level Charts

• Observation: Constant Pressure Surface– Pressure altimeter in an airplane causes path

along constant pressure not elevation– May cause sudden drop in elevation– Radio altimeter offers constant elevation

Class #5 Tuesday, July 13, 2010 26Fig. 2, p. 204

Class #5 Tuesday, July 13, 2010 27Fig. 3, p. 204

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Newton’s Law of Motion

• AN object at rest will remain at rest and an object in motion will remain in motion as long as no force is executed on the object.

• The force exerted on an object equals its mass times the acceleration produced.– Acceleration: speeding up, slowing down, change

of direction of an object.

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Forces that Influence Winds

• Pressure Gradient Force: difference in pressure over distance– Directed perpendicular to isobars from high to

low.– Large change in pressure over s short distance is a

strong pressure gradient and vice versa.– The force that causes the wind to blow.

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Forces that Influence Winds

• Coriolis Force– Apparent deflection due to rotation of the Earth– Right in northern hemisphere and left in southern

hemisphere– Stronger wind = greater deflection– No Coriolis effect at the equator greatest at poles.– Only influence direction, not speed– Only has significant impact over long distances

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Forces that Influence Winds

• Geostrophic Winds– Earth turning winds– Travel parallel to isobars– Spacing of isobars indicates speed; close = fast,

spread out = slow• Topic: Math & Geostrophic Winds

Vg = 1 x Δpfρ d

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Class #5 Tuesday, July 13, 2010 42Fig. 4, p. 211

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Forces that Influence Winds

• Gradient Winds Aloft– Cyclonic: counterclockwise– Anticyclonic: clockwise– Gradient wind parallel to curved isobars– Cyclostrophic near Equator

• Observation: Estimates Aloft– Clouds indicate direction of winds, place pressure

in location consistent with cloud location.

Class #5 Tuesday, July 13, 2010 44Fig. 5, p. 212

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Class #5 Tuesday, July 13, 2010 49Stepped Art

Fig. 8-29, p. 214

Class #5 Tuesday, July 13, 2010 50Fig. 6, p. 215

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Forces that Influence Winds

• Winds on Upper-level Charts– Winds parallel to contour lines and flow west to east– Heights decrease from north to south

• Surface Winds– Friction reduces the wind speed which in turn

decrease the Coriolis effect.– Winds cross the isobars at about 30° into low

pressure and out of high pressure– Buys-Ballots Law

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Class #5 Tuesday, July 13, 2010 54Fig. 8-32, p. 217

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Winds and Vertical Motion

• Replacement of lateral spreading of air results in the rise of air over a low pressure and subsidence over high pressure

• Hydrostatic equilibrium and equation• Topic: Hydrostatic equation

Δp = -ρgΔz

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Class #5 Tuesday, July 13, 2010 58Fig. 7, p. 218

Class #5 Tuesday, July 13, 2010 59Fig. 8-35, p. 220

Class #5 Tuesday, July 13, 2010 60Fig. 8-36, p. 221

Class #5 Tuesday, July 13, 2010 61Fig. 8-CO, p. 192