Atmospheric Pressure and
Wind
Atmospheric pressure:
– force exerted by a column of air per
unit area
– Normal atmospheric pressure at sea
level =
1013 millibars
Air pressure patterns
controlled by:
1. Temperature changes
2. Rotation of earth
1.Temperature changes:
• When air is heated:
– air expands and PRESSURE DROPS
• When air is cooled:
– air compresses and PRESSURE
INCREASES
Result:
• WARM surfaces develop thermal
LOWS
• COLD surfaces develop thermal
HIGHS
THERMAL HIGHS
THERMAL LOW
2. Rotation of earth:
• Earth’s rotation causes air to
accumulate in certain latitudes and to
be deflected away from certain
latitudes
• accumulation : HIGH pressure
• deflection: LOW pressure
Highs and Lows in cross-section:
• HIGHS:
– clear skies
• rising barometer
means good
weather
• LOWS:
– cloudy skies
• falling barometer
means bad
weather
Global Patterns of High and Low
Pressure
Equatorial Low
• 5o N - 5 o S
• Intertropical Convergence Zone
(ITCZ)
• thermal Low
– high sun angles, long days, available
energy
– ascending air
– heavy precipitation
– cloud cover
Subtropical Highs
•
•
•
•
•
•
•
25o - 40o N & S
rotation-induced Highs
air deflected to subtropics
descending air
clear skies
hot dry air
great deserts here
Subpolar Lows
• 55o - 70o N & S
• rotation-induced Lows
• warm air from low latitudes is lifted
as it meets cold polar air
• ascending air
• storm centers here
Polar Highs
•
•
•
•
90o N & S
thermal Highs
cold polar temps at high latitudes
descending air
Note: all pressure belts shift seasonally
What causes wind?
Wind is air moving from High to Low
pressure.
Wind is named after direction it
comes FROM.
(a “west wind” comes out of the west; flows
eastward)
Two components of wind
1.Speed
2. Direction
1.Wind Speed is determined by:
a. Steepness of pressure gradient
• Steep gradient: closely spaced isobars
• Gradual gradient: widely spaced isobars
b. Friction
• Friction from surface lowers wind speed
2. Wind Direction is determined by:
a. Direction of pressure gradient
b. Coriolis force
c. Friction
a. Direction of pressure
gradient
• from High to Low
• makes wind would blow perpendicular
to isobars
2. Wind Direction is determined by:
a. Direction of pressure gradient
b. Coriolis force
c. Friction
b. Coriolis force
• apparent deflection of moving things
(like the wind) on a rotating surface
(like the earth)
• Imagine tossing a ball across a rotating
room…
the ball’s
direction
Ball appears to be deflected to the right, but it has been going in
the same direction all along.
Airplanes, rockets, migrating birds, ocean
currents, air are deflected from their
paths of motion because the earth is
rotating.
in Northern Hemisphere, deflection to
RIGHT of movement
in Southern Hemisphere, deflection to LEFT
of movement
Watch this animation…
Deflection increases with latitude:
• no Coriolis at equator;
• greatest deflection at poles
• Imagine sitting on a chair on a platform at
varying latitudes….
If you are sitting
on the north pole,
how many degrees
will the room
rotate/spin in one
day?
YOU!
360°
If you are on the
equator, how many
degrees will the room
rotate/spin in one day?
0!
If you are between the poles
and the equator, how many
degrees will the room
rotate/spin in one day?
Between 0 and 360,
depending on latitude
If Coriolis effect were only influence on wind
direction, wind would blow parallel to isobars
2. Wind Direction is determined by:
a. Direction of pressure gradient
b. Coriolis force
c. Friction
c. Friction
the “drag” produced by earth’s surface
– applied opposite direction of motion
– reduce angle of Coriolis deflection
Pressure gradient
Northern Hemisphere
Northern Hemisphere
OUT and CLOCKWISE
Southern Hemisphere
Southern Hemisphere
OUT and COUNTERCLOCKWISE
Northern Hemisphere
Northern Hemisphere
IN and COUNTERCLOCKWISE
Southern Hemisphere
Southern Hemisphere
IN and CLOCKWISE
Winds in Upper Atmosphere
no friction
only the pressure gradient and Coriolis
effect
– wind is parallel to isobars:
GEOSTROPHIC WIND
Northern Hemisphere
Northern Hemisphere
CLOCKWISE
Southern Hemisphere
COUNTERCLOCKWISE
Northern Hemisphere
COUNTERCLOCKWISE
Southern Hemisphere
CLOCKWISE
Trade Winds
5o - 25o N & S
– NE, SE
– steady,
persistent
Global Wind Systems
(Surface Winds)
Westerlies
35o - 60o N & S
– not steady or
persistent
Polar Easterlies
65o - 80o N & S
– more prevalent in
Southern,
variability in
Northern
Equatorial Belt of Variable
Winds and Calm
5oN - 5o S
ITCZ
“Doldrums”
Subtropical Belt of
Variable Winds and Calm
30o - 35o N & S
“Horse Latitudes”
Polar Front Zone
60o - 65o N & S
zone of conflict
between differing
air masses
Polar Zone of Variable
Winds and Calm
80o - 90o N & S
Hadley Cells
Winds Aloft
• Upper Level
Westerlies (25o 90o)
• Polar Low
• Tropical High
Pressure Belt (15o 20o N & S)
• Equatorial
Easterlies
Jet Streams
• Narrow zones of
extremely high wind
speeds
• occur where there are
strong temp contrasts
• Polar Jet (westerly)
• Subtropical Jet
(westerly)
Summary!