Unit 2: Surface Processes and the
Hydrosphere
Lesson 2: Wind and the Coriolis effect
(Heath Earth Science – Pg. 522-536)
Today’s Objectives
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Explain the characteristics and significance of
the atmosphere, including:
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Outline the complex wind circulation patterns over
the Earth
Describe wind deflection due to the Coriolis effect
Bill Nye Video – Wind
Understanding Air Pressure
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Last lesson, we noted that air pressure is simply the
pressure exerted by the weight of the air above
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Quick experiment – what is the pressure currently being
exerted on your desk?
Answer:
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Avg. air pressure at sea level is about 1kg/cm2 (roughly the
same pressure as a column of water 10m in height)
Approximately 5000 kg (the weight of a 50 passenger school
bus!)
Why doesn’t the desk collapse?
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Air pressure is exerted in all directions! (down, up, and
sideways)
Measuring Air Pressure
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Meteorologists measure atmospheric pressure in
millibars (mb or mbar) or kilopascals (kPa)
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A barometer is used to measure atmospheric pressure
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Standard sea level pressure is 1013.2 mb or 101.325 kPa
1 mb = 100 Pa, or 0.1 kPa (1 Pa = 1kg·m/s2)
Bar = pressure, meter = measure)
There are several kinds of barometers, including:
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Mercury barometers
Aneroid barometers
Measuring Air Pressure
Mercury Barometer
Aneroid Barometer
Mapping Air Pressure
• An isobar is a line drawn on a map connecting places of equal sea-level
pressure – the closer the isobars, the faster the air pressure is changing
• Using the wind speed symbols, what do you notice about the relationship
between wind speed and isobars?
What is Wind?
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Wind is the horizontal movement of air from areas of higher
pressure to areas of lower pressure
The greater the difference in air pressure between two locations,
the greater the speed of the wind
 What happens when somebody uncorks a bottle of wine?
 That sound is made by air rushing into the bottle
Wind is nature’s attempt to balance inequalities in air pressure
Since unequal heating of Earth’s surface generates pressure
differences, solar radiation is the ultimate energy source for
most wind
Factors Affecting Wind
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If the Earth didn’t rotate, and if there were no friction
between moving air, and Earth’s surface, air would flow in
a straight line from HL pressure
BUT, because these and other factors exist, wind is
controlled by a combination of forces, including:
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1) Pressure-gradient force (PGF)
2) Coriolis effect
3) Friction
Factors Affecting Wind – Pressure-gradient force
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Pressure differences create wind
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The greater these differences, the greater the wind speed
On weather maps, places of equal pressure are connected
using isobars
The spacing of isobars indicates the amount of pressure
change occurring over a given distance, expressed as the
pressure gradient
Similar to the slope of a hill:
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A steep pressure gradient, like a steep hill, causes greater
acceleration of air
A shallow pressure gradient, like a gentle hill, causes a smaller
acceleration of air
Factors Affecting Wind – Pressure-gradient force
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Closely spaced isobars indicate a steep pressure gradient
and high winds
Widely spaced isobars indicate a weak pressure gradient
and light winds
The pressure gradient is the driving force of wind, and has
both magnitude and direction
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Its magnitude is determined by the spacing of isobars
Its direction is always from areas of higher pressure to areas of
lower pressure, and at right angles to the isobars
Once the air starts moving, the Coriolis effect and
friction take effect, but only to modify the movement, not to
produce it
Factors Affecting Wind – Coriolis effect
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On the weather map we looked at, the
wind does not cross the isobars at right
angles as the pressure gradient force
directs it
This is a result of Earth’s rotation and
has been named the Coriolis effect,
after the French scientist who first
described it
All free-moving objects or fluids, including
wind, are deflected to the right of their path
of motion in the Northern Hemisphere and
to the left in the Southern Hemisphere
(video)
Gustave Coriolis
Factors Affecting Wind – Coriolis effect
• We attribute the apparent
shift in wind direction to the
Coriolis effect. This
direction is:
• 1) Always directed at right
angles to the direction of air
flow
• 2) affects only wind
direction, not wind speed
• 3) is affected by wind
speed (the stronger the
wind, the greater the
deflection)
• 4) is strongest at the poles
and weakens toward the
equator where it is non
existent
Factors Affecting Wind - Friction
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The effect of friction on wind is only
important within a few kilometers of
Earth’s surface
Friction acts to slow the movement of air
As a result, wind direction is also effected
(see diagram)
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Coriolis effect is stronger with increasing wind
speed; because friction slows wind, the
Coriolis effect is weakened and wind direction
changes
The result is movement of air at an angle
across the isobars towards the area of lower
pressure
Geostrophic Wind
• When the Coriolis effect balances with the PGF, the wind will blow parallel to the
isobars
• Upper-air winds generally take this path and are called geostrophic winds
• Due to lack of friction, geostrophic winds travel at higher speeds than surface
winds
Highs and Lows
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Among the most common
features on any weather
map are areas called
pressure centers:
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Lows, or cyclones
Highs, or anticyclones
• In a low, the pressure decreases from the outer isobars toward the center of the
low and air rises
• In a high, the pressure increases from the outer isobars toward the center of
high and air sinks
• Due to the PGF and Coriolis effect, winds blow outward and to the right from a
high – called an anticyclone, or anticyclonic flow
• Winds blow inward and to the left into a low – called a cyclone, or cyclonic flow
Highs and Lows
Global Wind Patterns
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As noted, the primary cause of wind is unequal heating
of Earth’s surface
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In tropical regions, more solar radiation is received than is
radiated back to space
In polar regions, less solar energy is received than is radiated
back to space
Attempting to balance these differences, the atmosphere
acts as a giant heat-transfer system, moving warm air
towards the poles, and cool air toward the equator
This system is very complex, but we can develop an
understanding by first considering how air would circulate
on a non-rotating Earth with a uniform surface
What if Earth didn’t Rotate?
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Two large thermally produced
cells would form:
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Heated equatorial air would
rise up into the troposphere, and
be forced towards the poles
The air would cool, sink, and
begin moving back towards the
equator
This hypothetical circulation
would include upper-level air
moving poleward, and surface
air flowing equatorward
Idealized Global Circulation
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Pressure Belts
Wind Circulation Cells
If we add the effect of rotation,
this simple convection system will
break down into smaller cells
The Coriolis effect would
deflect the air’s poleward or
equatorward motion, causing it to
rise or sink earlier
This would create three pairs of
latitude wind circulation cells
on each side of the equator, that
are divided by zones called
pressure belts
Idealized Global Circulation –
Equatorial Low
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Rising air at the equator is
associated with a pressure
belt known as the
Equatorial low or the
Doldrums
Since surface air arrives here
from both the northern and
southern hemispheres, this
region is also called the
intertropical convergence
zone (ITCZ)
This region is associated with
abundant precipitation, and
weak winds
Idealized Global Circulation –
Hadley Cell
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Warm air rising from the
equator begins moving
poleward
However, soon the air is
turned by the Coriolis effect
By the time it reaches 30
degrees latitude, the air is
cooled and sinks into a zone
of relatively high pressure at
the surface – known as a
Subtropical high, or the
Horse Latitudes
This cell of air circulation is
called the Hadley cell, and
the winds are the trade
winds
Idealized Global Circulation –
Polar Highs
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Surface high-pressure areas lie at
both poles – Polar highs
Cold air flows toward the
equator at the surface, turned
westward by the Coriolis effect,
called the easterlies
At higher levels above the poles,
the air that comes from lower
latitudes sinks to replace this air
Rising air at about 60 degrees
latitude completes the circulation
– Subpolar lows
This air is forced to rise because
the cold air from the poles collides
with warmer air coming from
lower latitudes
Idealized Global Circulation –
Polar Fronts
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Where cold surface air
from the poles meets
warmer surface air from
lower latitudes at about
60 degrees latitude*, is
the Polar Front
 *Average latitude
Circulation between 30
and 60 degrees latitude is
harder to find
 Surface air in this cell
flows toward the poles
from the subtropical
high are called the
westerlies
Summary of Idealized Global
Circulation
• Wind Circulation Cells:
• 0-30 degrees = Trade Winds
(Hadley Cell)
• 30-60 degrees = Westerlies
• 60-90 degrees = Polar Easterlies
• Pressure Belts:
• 0 degrees = Equatorial low
(Doldrums, ITCZ)
• 30 degrees = Subtropical high
(Horse Latitudes)
• 60 degrees = Subpolar low
(Polar front)
• 90 degrees = Polar high
Influence of Continents
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One final thing to consider when looking at global
circulation patterns is the effect of continents
1) Land heats more in summer and cools more in
winter compared to the oceans
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This causes unequal heating of the surface
Hottest points on Earth not always at the equator
2) Generally greater friction above land than above
water
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Friction affects the speed and direction of air, thus changing
global wind patterns
Average
Global
Pressure