factors affecting wind - robertshultz.weebly.com

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



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



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



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



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



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



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



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



• 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



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



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



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



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



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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