Download Factors Affecting Wind Pressure

Air Pressure & Wind
(1) Factors Affecting Wind
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
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Air Pressure & Wind
(1) Factors Affecting Wind
 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
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Air Pressure & Wind
(1) Factors Affecting Wind
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
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Air Pressure & Wind
(1) Factors Affecting Wind
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
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Air Pressure & Wind
(1) Factors Affecting Wind
 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
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Subsidence produces clear skies
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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
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Air Pressure & Wind
(1) Factors Affecting Wind
o Descending air is compressed and warmed

Fair Weather!
General Circulation of the Atmosphere
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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
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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
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If we add the effect of rotation, this
simple convection system will break
down into small cells
Idealized Global Circulation
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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
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Air Pressure & Wind
(1) Factors Affecting Wind
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.
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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
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Air Pressure & Wind
(1) Factors Affecting Wind
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
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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
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The only truly continuous pressure belt is the subpolar low in the Southern
Hemisphere
o Ocean is uninterrupted by landmasses
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Where the landmasses break-up the ocean surface, large seasonal temperature
differences disrupt the:
1. Global pressure patterns
2. Global wind patterns
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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

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During warm months air flows onto land
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Air Pressure & Wind
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• Warm, moist air from the ocean = rainy, summer monsoon
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Dry continental air blows toward the ocean = winter monsoon
The Westerlies
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Does not fit the convection system proposed for the tropics
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Between 30° & 60°, the general west –to-east flow is interrupted by migrating
cyclones and anticyclones
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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
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The # of cyclones generated is also seasonal
o Most in cooler months when temp. gradients are greatest
Local Winds
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Winds than influence much smaller areas
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Produced from pressure differences that arise from temperature differences
→caused by unequal heating of earth’s surface
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Local winds are simply small scale winds produced by a locally generated
pressure gradient
Land & Sea Breezes
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Land heats more quickly than water
o Air above the land surface heats up, expands & rises
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Air Pressure & Wind
(1) Factors Affecting Wind
 Area of Low Pressure
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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
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At night, the reverse may take place
o Land Breeze
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Land cools more rapidly than the sea
Mountain & Valley Breezes
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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
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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

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Dominant in cold season
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Air Pressure & Wind
(1) Factors Affecting Wind
Chinook & Santa Ana Winds
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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
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A Chinook like wind that occurs in southern California is the Santa Ana
o Increase the threat of fire
How Wind Is Measured
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Direction and speed are two important basic measurements
o Winds are always labeled by the direction from which they blow
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Wind Vane – instrument used to determine wind direction
o Always points into the wind
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Cup Anemometer – used to determine wind speed
o Anemo=wind, metron=measuring instrument
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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
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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
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Air Pressure & Wind
(1) Factors Affecting Wind
o Called “El Niño”= The child (Christmas season)
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Every 3-7 years these counter currents become unusually strong and replace
the cold off shore waters with warm equatorial water
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El Niño devastates the fishing industry
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Some inland areas that are normally arid receive an abnormal amount of rain
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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
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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
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This see-saw pattern of atmospheric pressure between the East and West
Pacific is called the Southern Oscillation
o ENSO→El Niño/ Southern Oscillation
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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

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Trade Winds diminish or change direction
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Air Pressure & Wind
(1) Factors Affecting Wind
 During most El Niños, warmer than normal winters occur in the Northern U.S.
and Canada
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Drought conditions are generally observed in Indonesia, Australia and the
Philippines
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La Niña
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Opposite of El Niño
o Surface temps in the eastern Pacific are colder than average
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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.
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Greater precip. is expected in the NW
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Usually results in greater hurricane activity
o Cost of hurricane damage is 20x greater in La Niña years
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Air Pressure & Wind
(1) Factors Affecting Wind
Global Distribution of Precipitation
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In general regions influenced by high pressure, with its associated subsidence
and diverging winds, experience relatively dry conditions
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Regions under the influence of low pressure and its converging winds and
ascending air receive ample precipitation.
o Rainest region→equator (low pressure)
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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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