chapter8estrada.cune.edu/facweb/brent.royuk/sci381/docs/chapter8.pdf3/9/11 1 Chapter 8 Concordia...

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Chapter 8

1 Concordia University Geog/Sci-381 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

3 Concordia University Geog/Sci-381 Chapter 8 4 Concordia University Geog/Sci-381 Chapter 8

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 ○  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


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.

  This is the force that causes the wind to blow.


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What is the pressure gradient?



 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 29 Concordia University Geog/Sci-381 Chapter 8 30 Concordia University Geog/Sci-381 Chapter 8

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 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




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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Fig. 6, p. 215 44 Concordia University Geog/Sci-381 Chapter 8


Buys-Ballots Law –noun Meteorology . the law stating that if one stands with one's back to the wind, in the Northern Hemisphere the atmospheric pressure will be lower on one's left and in the Southern Hemisphere it will be lower on one's right: descriptive of the relationship of horizontal winds to atmospheric pressure.

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



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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