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GEOG. 1710 EARTH SCIENCE
LECTURE 6. ATMOSPHERIC PRESSURE AND WINDS
As mentioned earlier, atmospheric pressure is generated by the
weight of overlying air; due to compression, the air becomes less
dense with increasing height and the rate of pressure reduction
decreases with increasing height.
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Air pressure is usually measured by a barometer - such as
the old mercury barometer; air pressure forces the mercury to
rise up the tube containing a vacuum. The higher the pressure,
the higher the column of mercury supported. Average sea level
pressure is about 30 inches of mercury, which is equivalent to
about 15 lbs per square inch; however, the most-widely accepted
measure is the millibar (a measure of force per unit area) average sea level pressure is about 1013 mb.
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The pressures are shown by isobars - lines of equal pressure. This is
similar to the contour map, except the highs and lows are of air
pressure rather than elevation. Note that a linear zone of low
pressure is a trough; a linear zone of high pressure is a ridge; the
steepness of the pressure slope is a pressure gradient.
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Wind
Air will naturally flow from high pressure to low pressure
areas; the driving force is the pressure gradient force (pgf) - the
larger the pgf, the faster the air will move. It is this movement of
air that we call wind.
Factor Controlling Wind Direction
1. PGF - acts perpendicular across the isobars from high to low.
2. The Coriolis effect is caused by the rotation of the Earth; it
causes moving objects (including air) to be deflected always to
the right in the northern hemisphere and always to the left in
the southern hemisphere. The magnitude of the Coriolis effect
increases with increasing velocity.
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In the absence of other forces, for example, in the upper
atmosphere above 5000 feet, PGF and Coriolis force balance out
and produce a GEOSTROPHIC WIND which blows parallel to
the isobars.
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Below 5000 feet friction becomes a factor - it slows down the wind. This doesn’t
affect PGF but it does reduce Coriolis force - the result is a wind blowing across
the isobars towards the low
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If the air is converging onto the low pressure center, where does
it all go? The answer is "up".
The air converges and rises in a
low and descends and diverges in a high. The correct
terminology for the pressure centers - a low is referred to as a
CYCLONE with CYCLONIC CIRCULATION; a high is
referred to as an ANTICYCLONE with ANTICYCLONIC
CIRCULATION.
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General Circulation of the Atmosphere
The principles outlined above can be used to understand the
large scale global circulation of winds - both horizontally and
vertically.
The Intertropical Convergence Zone (ITC)
Imagine vertical circulation on a non-rotating Earth; a simple
convection cell might result between the hot low at the equator
and the cold high at the pole. However, the Earth is rotating and
this simple cell breaks down by about 30 degrees north and
south of the equator, due to the deflection of the upper air flow
by Coriolis and radiative colling of the upper air flow causing it
to become denser and sink down through the atmosphere. The
result is the HADLEY CELLS - the equatorial component of
global circulation.
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Associated features:
1. The INTERTROPICAL CONVERGENCE ZONE between
them, marked by low pressure, rising air and cloud development
(explained later).
2. The SUBTROPICAL HIGHS (STHs) around 30 degrees
north and south, where the air descends causing high pressure
and clear skies.
3. The surface TRADE WINDS, blowing from the STHs
towards the ITC.
Note that the trade winds are deflected by Coriolis, forming the
Northeast trades in the northern hemisphere and the southeast
trades in the southern hemisphere. The ITC and STHs also shift
north and south with the seasons.
4. The surface WESTERLIES, blowing poleward from the
STHs across the mid-latitudes and deflected from west to east by
Coriolis.
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The S.E. Asia Monsoon, caused by the shifting
trade winds and ITCZ.
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At the pole, cold sinks down forming the POLAR HIGH. Air
diverging from this high is deflected to the west by Coriolis
forming the POLAR EASTERLIES, which blow to about 60
degrees latitude.
The POLAR EASTERLIES meet the WESTERLIES at about
60 degrees latitude; this converging air rises, forming the
POLAR FRONT or SUBPOLAR LOW.
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Jet Streams And Rossby Waves
There are two places in the upper troposphere where air
converges strongly and is being deflected from west to east by
Coriolis - above the Tropics and above the Polar Front; these
zones of high velocity winds are subtropical and polar front JET
STREAMS - velocities can exceed 200 miles per hour.
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ROSSBY WAVES
The polar front jet is particularly important, firstly because it
tends to "guide" upper and lower air flow in the mid-latitudes,
and secondly, because it often undulates (due to mountain
barriers and migrating pressure system) producing large waves
known as ROSSBY WAVES. As these develop, lobes of cold
polar air can push down into mid-latitudes and lobes of warm
subtropical air can push poleward - this makes mid-latitude
weather the most variable on Earth.
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Ocean Currents
Much of the world's oceanic circulation simply reflects the
movement of ocean water by the global wind patterns
Note particularly the effect of the subtropical highs and southern
hemisphere westerlies.
Global Heat Transfer: the westerlies and ocean currents transfer
surplus heat from the tropics towards the poles.
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