Ms. Cawsey
The Elements of Climate: Pressure and Wind
Geog 12
Since the direction of the winds is of such great importance in controlling the climate of a place, it is
necessary to study winds and wind systems in greater detail.
Wind: a horizontal movement of air from one place to another above the earth’s surface
A CALM: when no wind… but that doesn’t mean no air moving… just may be moving upward in
vertical currents… therefore, just an absence of HORIZONTAL AIR MOVEMENT
Some technical stuff:
Air measured in MILLIBAR…air is a mass, and exerts pressure on the ground… the AVERAGE mass
of a column of air on a square cm of land = 1040 gm. A column of mercury covering the same square
area and same mass would only rise 75cm…. Therefore measurement is in cm of mercury. 1 millibar =
0.80 mm of mercury
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Lows/ Depressions (cyclones): Air masses having a pressure significantly below average sea-level
pressures
Highs/ Anticyclones: Air masses having pressure of +1000mb (100kPa)
Air ALWAYS moves from HIGHS to LOWS
THE PLANETARY WIND SYSTEM
Why the winds of the world blow as they do:
If world surface was same all over and no axis toward the sun:
Air at equator would be very hot
Air at poles would be very cold
Cold air is heavy and would come down from poles (forming high pressure area at poles)
Cold polar air would flow toward equator and force hot air there to rise (forming lowpressure area)
The equator hot air would then go towards the poles at high altitudes to complete
circulation
Therefore, simple convection current
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The effect of rotation is to cause the winds to be deflected according to a definite physical law
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particles on the earth’s surface)
To understand deflection, we must understand the following:
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If calm air existed all over the earth, the atmosphere above any point on the earth would move at the
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It “ lags ” behind and is deflected form its southerly course
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the movement
On the other hand, the air moving from the equator to the north
poles as high altitudes will go from an area of maximum speed
rotation into areas that have less speeds of rotation
It “ moves ahead” and is deflected from its northerly course
toward the east, but once again to the right in relation to its
direction of movement
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The air flowing poleward at great heights, being deflected toward the east in both
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2 great belts of high pressure known as the SUBTROPICAL HIGHS, or the HORSE
LATITUDES
In each hemisphere the air then proceeds in 2 great streams, the one toward the equator and
the other one to the poles
The equatorial stream completes 1 cell of circulation, and the surface winds are
deflected to become the famous TRADE WINDS,
The poleward streams from the subtropical highs meet the cold air masses moving from
the polar highs at about latitudes 60↑N & 60↑S
At these latitudes, air is forced to rise to form the low-pressure areas known as the belt
of subpolar lows
Some of the rising air flows back to the poles, and some flows back to the subtropical
highs.
The surface winds at these higher latitudes become the westerlies and the polar
easterlies
Now we have 3 cells in each hemisphere, meshing like the gears of a machine
Unequal heating and the rotation of the earth have combined to form what is known as the
PLANETARY WIND SYSTEM the system of pressure and winds that might be expected
to exist on a planet covered entirely with land or sea and with the sun overhead at the
equator.
The Swinging of the WIND BELTS
The apparent movement of the sun causes the region of greatest heat to move between the
latitudes 12↑N and 8↑S.
The distance above and below the equator is not the same because of the greater amount of land in
the Northern Hemisphere and the slightly longer period during which the sun is north of the
equator
Since the heat of the sun is one of the main causes of the distribution of pressure, the pressure and
wind belts also move with this migration of thermal equator
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A little hint to help witn low pressure and high pressure:
If you have a lot of air in a little balloon, you will have HIGH PRESSURE
If you have the same air in a big ballown, you will have LOW PRESSURE
When air sinks, its pressure INCREASES
When air rises, its pressure DECREASES
Hot air rises, cold air sinks (think slippers in winter)
Ms. Cawsey
The Elements of Climate: Pressure and Wind
Geog 12
Since the direction of the winds is of such great importance in controlling the climate of a place, it is
necessary to study _____________ and wind systems in greater detail.
Wind: __________________________________________________________________________
___________: when _______… but that doesn’t mean no air moving… just may be moving upward in
vertical currents… therefore, just ____________________________________________________
Some technical stuff:
Air measured in MILLIBAR…air is a mass, and exerts pressure on the ground… the AVERAGE mass
of a column of air on a square cm of land = 1040 gm. A column of mercury covering the same square
area and same mass would only rise 75cm…. Therefore measurement is in cm of mercury. 1 millibar =
0.80 mm of mercury
!
"
#
! $% &
_______________________________________: Air masses having a pressure significantly below
average sea-level pressures
___________________________________: Air masses having pressure of +1000mb (100kPa)
______________________________________________________________________________
THE PLANETARY WIND SYSTEM
Why the winds of the world blow as they do:
If world surface was _________________ and ___________ toward the sun:
Air at ______________ would be ________________________
Air at __________ would be _________________
______________ and would come ______________ (forming high pressure area at poles)
Cold polar air would ___________________ and __________ there to ___ (forming lowpressure area)
The equator ____________would then go _______________ at high altitudes to complete
_____________________
Therefore, ___________________________
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The effect of rotation is to cause the winds to be deflected according to a definite physical law
(first noted by American meteorologist, William Ferrel, who applied “Ferrel’s Law” to all moving
particles on the earth’s surface)
To understand deflection, we must understand the following:
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So, for example:
If there is a particular flow of air moving toward the equator
from the north pole:
It is moving into regions with a higher velocity
It “ lags ” behind and is deflected form its southerly course
toward the west OR to the right in relation to the direction of
the movement
On the other hand, the air moving from the equator to the north
poles as high altitudes will go from an area of maximum speed
rotation into areas that have less speeds of rotation
It “ moves ahead” and is deflected from its northerly course
toward the east, but once again to the right in relation to its
direction of movement
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The deflection becomes ____________ with increasing _______________________because the
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The air flowing poleward at great heights, being _____________________________ in both
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_________________ of high pressure known as the ______________________, or the _______
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In each hemisphere the ______________________________, the one toward the equator and
the other one to the poles
The equatorial stream completes 1 cell of circulation, and the surface winds are
deflected to become the famous _________________________________,
The poleward streams from the subtropical highs meet the cold air masses moving from
the polar highs at about latitudes _________________________________
At these latitudes, air is forced to rise to form the low-pressure areas known as the belt
of ________________________
Some of the rising air flows back to the poles, and some flows back to the subtropical
highs.
The surface winds at these higher latitudes become the _________ and the _________
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Now we have _______ in ____ hemisphere, meshing like the
______________________
Unequal heating and the rotation of the earth have combined to form what is known as the
_________________________ the system of pressure and winds that might be expected
to exist on a planet covered entirely with land or sea and with the sun overhead at the
equator.
The Swinging of the WIND BELTS
The apparent _______________________ causes the region of _______________ to move
between the latitudes 12↑N and 8↑S.
The distance above and below the equator is _______________ because of the greater amount of
land in the Northern Hemisphere and the slightly longer period during which the sun is north of the
equator
Since the heat of the sun is one of the main causes of the distribution of pressure,
__________________________________________ with this migration of thermal equator
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