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AIR PRESSURE AND WINDS
Chapter 8
Atmospheric Pressure
 What causes air pressure to change in the horizontal?
 Why does the air pressure change at the surface?
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
 Daily Pressure Variations
 Thermal tides in the tropics
○ Driven by heating and cooling air cycles, strongest at
equator
 Mid-latitude pressure variation are driven more by transitory
pressure cells
 Pressure Measurements
 Barometer, barometric pressure
○ Standard atmospheric pressure 1013.25 mb
○ 1013.25 mb = 1013.25 hPa = 29.92 in. Hg = 76 torr = 76 cm
Hg = 14.7 psi
 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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 Altitude corrections: high altitude add pressure, 10mb/100m
above sea level
○ Adjusted reading: Sea-level Pressure
Surface and Upper Level Charts
 Sea-level pressure chart: constant height
 This is the most common pressure chart you see on weather
maps.
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Surface and Upper Level Charts
 Upper level or isobaric chart: constant pressure surface (e.g.
500mb)
 High heights correspond to higher than normal pressures at
a given latitude and vice versa
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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
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Newton’s Law of Motion
 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.
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 a short distance is a strong
pressure gradient and vice versa.
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 Large change in pressure over a short distance is a strong
pressure gradient and vice versa.
 This is 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 influences 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
 Why parallel? The pressure gradient force is in equilibrium
with the Coriolis force.
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Forces that Influence Winds
 Gradient Winds Aloft
 Cyclonic: counterclockwise (LOW)
 Anticyclonic: clockwise (HIGH)
 These rotations are opposite in southern hemisphere
 Gradient wind (aloft, above the level of frictional influence):
parallel to curved isobars
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parallel to curved isobars
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Forces that Influence Winds
 Winds on Upper-level Charts
 Winds parallel to contour lines and flow west to east
○ What about southern hemisphere? East to west?
 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
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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
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Homework for Chapter 8
 Chapter 8 Questions for Review, p. 219
 #1, 6, 8-10, 12, 16, 19
 Chapter 8 Questions for Thought, p. 220
 #8, 14
 Chapter 8 Problems and Exercises, p. 221
 #1, 4
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Project for Chapter 8
 Download Project 5 from the course webpage.
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