Download - Meteorology Pressure
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Sep 2012
Lesson 4.3
Meteorology
Pressure
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Reference
From the Ground UpChapter 6.3:PressurePages 127 - 130
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Introduction• Air pressure differences around the
Earth cause many different pressure systems which in turn cause different weather phenomena.
• Pilots need to recognize these pressure systems in order to know what weather will result.
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Outline• Atmospheric Pressure• Pressure Systems• Pressure Gradient• Coriolis Force
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Atmospheric Pressure• Pressure measured with a Mercury Barometer,
expressed in inches of mercury (“ Hg)
• Standard pressure= 29.92” Hg= 1 bar = 1013.2 mb (millibars)= 1013.2 hPa (hectopascals)= 101.3 kPa (kilopascals)
• Station Pressure (actual pressure at observing station) is corrected to Mean Sea Level (MSL) Pressure to become Altimeter Setting
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Altimeter Pressure Error• Low to high pressure, altimeter reads lower• “Low to high, watch the sky”
HL
Intended Path
Actual Path
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Altimeter Pressure Error• High to low pressure, altimeter reads higher• “High to low, look out below”
HL Intended Path
Actual Path
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Pressure Systems
• Weather maps show areas of pressure with different symbols/lines
• Lines that join places of equal pressure are Isobars
• Isobars in hectopascals, in intervals of 4, above and below 1000 hPa
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Low Pressure Areas• Low Pressure Areas
– Called cyclone, depression or low– Pressure lowest in centre– Wind flows counterclockwise, inwards towards centre, then
up– Inward airflow called Convergence– Creates unstable, poor weather
• Secondary Low– Smaller low within a low
• Trough (of Low Pressure)– Elongated U-shaped are of low pressure
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Low Pressure Areas• Example of hurricane (low pressure system)
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High Pressure Areas• High Pressure Area
– Called anticyclone or high– Pressure highest in centre– Wind flows down through centre, clockwise, then outwards– Outward airflow called Divergence– Creates stable, good weather
• Ridge (of High Pressure)– Elongated U-shaped are of high pressure
• Col– Neutral area between two highs and two lows
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Pressure Gradient• Air moves from high pressure to low pressure
(creating wind)
• Pressure gradient is rate of change of pressure over distance at right angles to isobars
• Closer isobars = steeper pressure gradient = faster winds
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Coriolis Force• Created by Earth’s rotation, causes air to move
parallel to isobars
• In Northern Hemisphere, air spirals clockwise into low and counterclockwise out of high
• Buys Ballot’s Law– If you stand with your back to the wind, low pressure area is
on your left
• On ground, Surface Friction slows down air and reduces coriolis force
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Next Lesson
4.4 – MeteorologyWinds
From the Ground UpChapter 6.4:WindsPages 130 - 136