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Air Pressure & Wind (1) Factors Affecting Wind
Shultz revision of Schott ‘04 1
Understanding Air Pressure
Average air pressure at sea level is about 1 kg per cm2 (14.7 lbs/in2)
Roughly the same pressure that is produced by a column of water 10 m (33 ft)
high
The pressurized suits used by astronauts on space walks are designed to
duplicate the atmospheric pressure experienced at Earth’s surface
o Without these protective suits to keep body fluids from boiling away,
astronauts would perish in minutes
Factors Affecting Wind
Wind→ air flowing horizontally with respect to
Earth’s surface
Results from differences in air pressure
o Air flows from areas of higher pressure to areas of lower pressure
Wind is nature’s attempt to balance inequalities in air pressure
Unequal heating of Earth’s surface generates these differences
o Solar radiation is the ultimate energy source for most wind
If Earth did not rotate, and there was no friction, air would flow in a straight
line
Wind is controlled by
1) Pressure-gradient force
2) Coriolis Effect
3) Friction
Pressure-Gradient Force
Air Pressure & Wind (1) Factors Affecting Wind
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Pressure differences create wind, and the greater these differences, the
greater the wind speed
Variations in air pressure are determined from readings taken at hundreds of
weather stations
o Pressure data are shown on a weather map using isobars lines that
connect places of equal air pressure
iso = equal; bar = barometer
Pressure gradient- the amount of pressure change occurring over a given distance
o Closely spaced isobars indicate a steep pressure gradient and high winds
and vice versa
o Driving force of wind, and it has both
magnitude and direction
Once air starts to move the Coriolis effect and
friction come into play, but only to modify the
movement, not to produce it
Air Pressure & Wind (1) Factors Affecting Wind
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Coriolis Effect
Wind does not cross the isobars at right angles as the pressure-gradient force
directs it
o This deviation is the result of Earth’s rotation
o Coriolis Effect (force) → the deflective force of Earth‘s rotation on all
free-moving objects, including the atmosphere and oceans
Named after the French scientist
who first thoroughly described it
Free-moving objects are deflected to the right
in the Northern Hemisphere
This deflection:
o 1) is always directed at right angles to the
direction of air flow
o 2) affects only wind direction, not speed
o 3) is affected by wind speed (stronger
speed = greater deflection)
o 4) is strongest at the poles and weakens equatorward, becoming
nonexistent at the equator
Friction
Only important near the surface
Acts to slow the axis movement
Air Pressure & Wind (1) Factors Affecting Wind
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Surface Winds vs. Winds Aloft
Eventually, the Coriolis Effect will balance the pressure-gradient force, and the
wind will blow parallel to the isobars
o Upper-air winds generally take this path and are called geostrophic winds
Usually above 600 m (2000 ft)
Travel at higher speeds than surface winds
Most prominent feature of upper-level flow are jet streams
o First encountered by high-flying bombers during WWII
o Fast-moving rivers of air that travel between 120 and 240 km (75-150 mph)
per hour in a west-to-east direction
o Can have “Zonal” or “Meridional” Flow
Below 600 m, friction complicates the airflow
o Friction lowers the windspeed and reduces the Coriolis effect
Upper air flow is nearly parallel to the isobars, whereas the effect of friction
causes the surface winds to move more slowly and cross the isobars at an angle
Highs & Lows
One of the most common features on any weather maps are areas designated as
pressure centers.
o Low, or cyclones, are centers of low pressure
Air Pressure & Wind (1) Factors Affecting Wind
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Kyklon = moving in a circle
o Highs, or anticyclones, are centers of high pressure
Cyclones
o Centers of low pressure
o Pressure decreases toward the center
o Wind blow inward and counter-clockwise due to friction
Anticyclones
o Centers of high pressure
o Pressure increases toward the center
o Winds blow outward & clockwise
Divergence
Weather Generalizations
Rising air is associated with cloud formation and precipitation
Subsidence produces clear skies
In a cyclone, the net inward transport of air causes a shrinking of the area
occupied by the air mass
o Horizontal convergence
o Whenever air converges horizontally, it must pile
up or increase in height to allow for the
decreased area it now occupies
Generates a taller, and therefore heavier air
column
Divergence aloft must occur at a rate equal to the inflow
below to maintain a low pressure center
Convergence aloft accompanies divergence at the surface and general
subsidence of the air column
Air Pressure & Wind (1) Factors Affecting Wind
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o Descending air is compressed and warmed
Fair Weather!
General Circulation of the Atmosphere
Underlying cause of wind is unequal surface heating
o The atmosphere acts as a giant heat-transfer system, moving warm air
poleward and cool air equatorward
Circulation on a Nonrotating Earth
On a hypothetical nonrotating planet with a smooth surface of either all land or
all water, two large thermally produced cells would form
o Heated equatorial air would rise until it reached the tropopause, which
would deflect the air poleward
o Reaches the poles and sinks, spreads out in all directions at the surface and
moves back toward the equator
This hypothetical circulation system has upper-level air flowing poleward and
surface air flowing equatorward
If we add the effect of rotation, this
simple convection system will break
down into small cells
Idealized Global Circulation
Near the equator, the rising air is
associated with the pressure zone known as the equatorial low a region marked
by abundant precipitation
As air reaches 20° or 30° N or S, it sinks back to the surface
Air Pressure & Wind (1) Factors Affecting Wind
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o This subsidence and associated adiabatic heating produce hot, arid
conditions
Subtropical High – center of this zone of subsiding dry air which
encircles the globe near 30°
• Location of Great deserts
At the surface, airflow is outward from the center of the subtropical high
o Some of the air travels equatorward and is deflected by the coriolis
effect.
Creates the Trade Winds
• Steady easterly winds are called TRADE winds because sailors
relied on them to carry cargoes from Europe to the West
Indies and South America.
o Some travels poleward and is deflected by the coriolis effect
Generates the Prevailing Westerlies of the mid-latitudes
• Because they blow from the West to the East, they are called
Prevailing Westerlies.
As the Westerlies move poleward, they encounter the cool Polar Easterlies in
the region of the subpolar low
o The interaction of these warm and cool winds produces the stormy belt
known as the polar front
Mixing of warm & cold air along polar front has a major effect on
weather changes in the U.S.
The source region for the variable polar easterlies is the Polar High and here,
cold air is subsiding and spreading equatorward.
Doldrums
Surface at equator is strongly heated warm air rises steadily = LOW Pressure
Air Pressure & Wind (1) Factors Affecting Wind
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o Cool air moves in and is warmed rapidly and rises.
Little motion = weak winds
Regions near the equator with little to no winds are called the doldrums
Horse Latitudes
Warm air that rises at the equator divides and flows both north and south.
o 30°N & S, air stops moving toward the poles and sinks
Forms a belt of calm air
Name is from 100’s of years ago, sailors stuck in these waters ran
out of food and water for their horses and had to throw them
overboard.
Influence of Continents
The only truly continuous pressure belt is the subpolar low in the Southern
Hemisphere
o Ocean is uninterrupted by landmasses
Where the landmasses break-up the ocean surface, large seasonal temperature
differences disrupt the:
1. Global pressure patterns
2. Global wind patterns
The circulation over the oceans is dominated by semi-permanent cells of high
pressure in the subtropics & cells of low pressure over the subpolar regions
Large landmasses become cold in the winter and develop a seasonal high
pressure system from which surface flow is directed off-land
o Seasonal changes in wind direction are known as the monsoons
During warm months air flows onto land
Air Pressure & Wind (1) Factors Affecting Wind
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• Warm, moist air from the ocean = rainy, summer monsoon
Dry continental air blows toward the ocean = winter monsoon
The Westerlies
Does not fit the convection system proposed for the tropics
Between 30° & 60°, the general west –to-east flow is interrupted by migrating
cyclones and anticyclones
A close correlation exists between the paths taken by these surface pressure
systems and the position of the upper-level air flow, → upper air strongly
influences the movement of these systems
o Steep temp gradient across the middle latitudes in the winter months
corresponds to a stronger flow aloft.
o Polar jet stream fluctuates seasonally such that its average position
migrates southward in winter & northward in summer
The # of cyclones generated is also seasonal
o Most in cooler months when temp. gradients are greatest
Local Winds
Winds than influence much smaller areas
Produced from pressure differences that arise from temperature differences
→caused by unequal heating of earth’s surface
Local winds are simply small scale winds produced by a locally generated
pressure gradient
Land & Sea Breezes
Land heats more quickly than water
o Air above the land surface heats up, expands & rises
Air Pressure & Wind (1) Factors Affecting Wind
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Area of Low Pressure
Creates a sea breeze because cooler air over water (higher pressure) moves
toward the warmer land (lower pressure)
o Sea breezes can be a significant moderation influence
At night, the reverse may take place
o Land Breeze
Land cools more rapidly than the sea
Mountain & Valley Breezes
During daylight hours, the air along the slopes of mountains is heated more
intensely than air at the same elevation over the valley floor
o Warmer, less dense air glides up along the slope
Valley Breeze
• Indentified by cumulus clouds the develop on adjacent
mountain peaks
• Common during the warm air season
Reverses at night →mountain slope cools more rapidly and drains downslope into
the valley
o Mountain Breeze
Dominant in cold season
Air Pressure & Wind (1) Factors Affecting Wind
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Chinook & Santa Ana Winds
Chinook winds are warm, dry winds common on the eastern slopes of the Rockies
o “Snow-Eaters”
o Created when air descends the leeward side of a mountain and warms by
compression
o Occur mostly in the winter and spring
A Chinook like wind that occurs in southern California is the Santa Ana
o Increase the threat of fire
How Wind Is Measured
Direction and speed are two important basic measurements
o Winds are always labeled by the direction from which they blow
Wind Vane – instrument used to determine wind direction
o Always points into the wind
Cup Anemometer – used to determine wind speed
o Anemo=wind, metron=measuring instrument
Because changes in wind direction often bring changes in temp. and moisture
conditions, the ability to predict winds can be very useful
El Niño & La Niña
The cold Peruvian current flows equatorward along the coast of Ecuador and
Peru
o Encourages upwelling of cold, nutrient filled waters
Near the end of each year a warm current that flows southward along the
coasts of Ecuador and Peru replaces the cold Peruvian current
Air Pressure & Wind (1) Factors Affecting Wind
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o Called “El Niño”= The child (Christmas season)
Every 3-7 years these counter currents become unusually strong and replace
the cold off shore waters with warm equatorial water
El Niño devastates the fishing industry
Some inland areas that are normally arid receive an abnormal amount of rain
Two of the strongest El Niño Events on record occurred between 1982-83 &
1997-98
o ’97-‘98 El Niño brought ferocious storms to California and heavy rains and
floods to the southern U.S. and destroyed hurricanes in the Atlantic
Each time an El Niño occurs, the barometer pressure drops over large portions
of the SE Pacific, whereas in the Western Pacific, near Indonesia and North
Australia, the pressure rises
o When El Niño comes to an end, the pressure difference swings back
This see-saw pattern of atmospheric pressure between the East and West
Pacific is called the Southern Oscillation
o ENSO→El Niño/ Southern Oscillation
The steady westward flow of the trade winds creates a warm surface current
that moves east to west along the equator and results in a “piling up“ of a thick
layer of warm surface water that produces higher sea level (by 30 cm) in the W.
Pacific
o Eastern Pacific is characterized by a strong Peruvian current, upwelling of
cold water and lower sea levels
When the Southern Oscillation occurs, this normal situation changes
o Pressure rises in Indonesia causing a weakened or reversed pressure
gradient along the equator
Trade Winds diminish or change direction
Air Pressure & Wind (1) Factors Affecting Wind
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During most El Niños, warmer than normal winters occur in the Northern U.S.
and Canada
Drought conditions are generally observed in Indonesia, Australia and the
Philippines
La Niña
Opposite of El Niño
o Surface temps in the eastern Pacific are colder than average
A typical La Niña winter blows colder than normal air over the Pacific NW and
the northern Great plains while warming the rest of the U.S.
Greater precip. is expected in the NW
Usually results in greater hurricane activity
o Cost of hurricane damage is 20x greater in La Niña years
Air Pressure & Wind (1) Factors Affecting Wind
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Global Distribution of Precipitation
In general regions influenced by high pressure, with its associated subsidence
and diverging winds, experience relatively dry conditions
Regions under the influence of low pressure and its converging winds and
ascending air receive ample precipitation.
o Rainest region→equator (low pressure)
Latitudinal variation in precip due to air temp and capacity for water
o Cold air and low latitudes =less precip
Distribution of Land & Water affects precip as well
o Large landmasses in the mid-latitudes commonly experience decreased
precipitation toward their interiors
o Mountain barriers also affect precipitation!
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