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Air Pressure and Wind
Atmospheric pressure
 Force
exerted by the weight of the air
above
 Decreases with increasing altitude
 Units of measurement


Millibar (mb) – standard sea level pressure is
1013.2 mb
Inches of mercury – standard sea level
pressure is 29.92 inches of mercury
Wind
 Horizontal


Out of areas of high pressure
Into areas of low pressure
 Controls

movement of air
of wind
Pressure gradient force
• Isobars – lines of equal air pressure
• Pressure gradient – pressure change over
distance
Wind
 Controls


of wind
Coriolis effect
Friction
Wind
 Upper


air winds
Generally blow parallel to isobars – called
geostrophic winds
Jet stream
• "River" of air
• High altitude
• High velocity (120-240) kilometers per hour
Rotating Air Bodies

Low Pressure Zone
Formation



Warm air rises
Creates a low
pressure zone
At the Earth’s surface,
air “feeds” the low
pressure zone, moves
counterclockwise

High Pressure Zone
Formation



Cool air sinks
Creates a high
pressure zone
At the Earth’s surface,
winds blow clockwise
Rotating Air Bodies
 Bends
in the polar jet create troughs and
ridges
 Forms cyclones and anticyclones
Rotating Air Bodies

Cyclones




Low pressure zone in polar
jet trough
Winds at surface flow
counterclockwise towards
the core
Air is updrafted and cooled
Forms clouds, rain and
upper level outflow of air
Rotating Air Bodies
 Anticyclones





High pressure zone at
ridge of polar jet
Air converges in upper
atmosphere
Descends towards the
ground
Flows outward at surface
Dry, windy conditions
Cyclones and Anticyclones
Cyclonic and anticyclonic winds
in the Northern Hemisphere
General atmospheric circulation

Idealized global
circulation

Equatorial low
pressure zone
• Rising air
• Abundant precipitation
General atmospheric circulation

Idealized global
circulation

Subtropical high
pressure zone
• Subsiding, stable, dry air
• Location of great deserts
• Air traveling equatorward
from the subtropical high
produces the trade winds
• Air traveling poleward
from the subtropical high
produces the westerly
winds
General atmospheric circulation

Idealized global
circulation

Subpolar low
pressure zone
• Warm and cool winds
interact
• Polar front – an area of
storms
General atmospheric circulation

Idealized global
circulation

Polar high pressure
zone
• Cold, subsiding air
• Air spreads
equatorward and
produces polar easterly
winds
• Polar easterlies collide
with the westerlies
along the polar front
General atmospheric circulation
 Influence

of continents
Seasonal temperature differences disrupt the
• Global pressure patterns
• Global wind patterns

Influence is most obvious in the Northern
Hemisphere
General atmospheric circulation
 Influence

of continents
Monsoon
• Seasonal change in wind direction
• Occur over continents


During warm months
• Air flows onto land
• Warm, moist air from the ocean
Winter months
• Air flows off the land
• Dry, continental air
Local winds
 Produced
from temperature differences
 Small scale winds
 Types



Land and sea breezes
Mountain and valley breezes
Chinook and Santa Ana winds
Illustration of a sea breeze
and a land breeze
The Santa Ana Winds
Wind measurement
 Two

basic measurements
Direction
• Winds are labeled from where they originate
• Direction indicated by either


Compass points (N, NE, etc.)
Scale of 0º to 360º
• Prevailing wind comes more often from one
direction

Speed
• often measured with a cup anemometer
Wind measurement
 Changes

in wind direction
Associated with locations of
• Cyclones
• Anticyclones

Often bring changes in
• Temperature
• Moisture conditions
El Niño
Normal conditions in the tropical
Pacific Ocean
 Surface
winds move from east to west
 From high pressure in S. America to low
pressure in Australia
 Drags water westward
 Warm water pools in the western Pacific
Normal conditions
Every 3 – 8 years, system
reverses
 Called
the Southern Oscillation
 Trade winds weaken or reverse
 Warm water migrates from Australia to S.
America
 Arrives in time for Christmas – Corriente
del Niño
El Niño
What is El Niño?
 Basically,
it's a giant puddle (or pod) of
heated water that sloshes across the
Pacific Ocean
 Similar to an iceberg



Bulge on the surface
Most of “pod” beneath the surface
Due to difference in density
 National
Geographic’s Model
ENSO - El Niño-Southern
Oscillation
 Typically
lasts 1 year
 May last up to 3
 In multi-year events, first year not as
affected
 Affects both hemispheres
Recognizing an El Niño
 Sea
Surface Temperatures (SST)
 Normal: 6-8° C warmer in the western
tropical Pacific than in the eastern tropical
Pacific
 Check SST to see if in “normal” range
La Niña
Return to “normal” conditions from an El Niño
strong
 Produces:





Strong currents
Powerful upwelling
Chilly and stormy conditions along S. American coast
Eastern Pacific cools rapidly, Western Pacific
warms rapidly
 Renewed Trade Wind activity spreads the cooler
eastern Pacific waters westward
Global distribution of
precipitation
 Relatively
complex pattern
 Related to global wind and pressure
patterns

High pressure regions
•
•
•
•
Subsiding air
Divergent winds
Dry conditions
e.g., Sahara and Kalahari deserts
Global distribution of
precipitation
 Related
to global wind and pressure
patterns

Low pressure regions
•
•
•
•
Ascending air
Converging winds
Ample precipitation
e.g., Amazon and Congo basins
Average annual precipitation
in millimeters
Global distribution of
precipitation
 Related


to distribution of land and water
Large landmasses in the middle latitudes
often have less precipitation toward their
centers
Mountain barriers also alter precipitation
patterns
• Windward slopes receive abundant rainfall from
orographic lifting
• Leeward slopes are usually deficient in moisture
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