Air Pressure and Winds
Atmospheric Pressure
 What causes air pressure to change in the horizontal?
 Why does the air pressure change at the surface?
 Horizontal Pressure Variations
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It takes a shorter column of dense, cold air to exert the same pressure as a taller column of less
dense, warm air
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Warm air aloft is normally associated with high atmospheric pressure
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Cold air aloft with low atmospheric pressure
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At surface, horizontal difference in temperature = horizontal pressure in pressure = wind
The atmospheric air density remains constant with height. The air pressure at the surface is related to the number
of molecules above. When air of the same temperature is stuffed into the column, the surface air pressure rises.
When air is removed from the column, the surface pressure falls.
(a) Two air columns, each with identical mass, have the same surface air pressure.
(b)It takes a shorter column of cold air to exert the same pressure as a taller column of warm air, as column 1
cools, it must shrink, and as column 2 warms, it must expand.
(c) Because at the same level in the atmosphere there is more air above the H in the warm column than above the L
in the cold column, warm air aloft is associated with high pressure and cold air aloft with low pressure. The
pressure differences aloft create a force that causes the air to move from a region of higher pressure toward a
region of lower pressure. The removal of air from column 2 causes its surface pressure to drop, whereas the
addition of air into column 1 causes its surface pressure to rise.
 Daily Pressure Variations
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Thermal tides in the tropics
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Mid-latitude pressure variation driven by transitory pressure cells
 Pressure Measurements
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Barometer, barometric pressure
○
Standard atmospheric pressure 1013.25mb
 Pressure Readings
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Instrument error: temperature, surface tension
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Altitude corrections: high altitude add pressure, 10mb/100m above sea level
Surface level charts are modified to reflect atmospheric pressures AS IF they were at sea level (approximate
adjustment: 10mb per 100 meters.
a) Pressure at 4 cities.
b) Pressure modified to one level (sea level)
c) Shows isobars based on one level
Surface and Upper Level Charts
 Sea-level pressure chart: constant height
 Upper level or isobaric chart: constant pressure surface (i.e. 500mb)
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High heights correspond to higher than normal pressures at a given latitude and vice versa