Wind as a Natural ResourceWind as a Natural Resource

• Temperature differences in the atmosphere cause pressure and Temperature differences in the atmosphere cause pressure and density variations, which results in wind.density variations, which results in wind.

• Since other sources of heat for the Earth such wind as Since other sources of heat for the Earth such wind as volcanism and tidal friction are very small in comparison to volcanism and tidal friction are very small in comparison to the sun, wind is really a form of solar energy. the sun, wind is really a form of solar energy.

• Between one and two percent of the incident solar energy is Between one and two percent of the incident solar energy is converted into wind.converted into wind.– http://www.windpower.org/en/tour/wres/index.htm– The total solar energy incident on the Earth is about 1.74 hundred The total solar energy incident on the Earth is about 1.74 hundred

quadrillion watts. (1.74x10quadrillion watts. (1.74x101717 Watts). Watts).– One or two percent of a few hundred quadrillion watts is a lot of One or two percent of a few hundred quadrillion watts is a lot of

power!power!– Obviously, wind is a significant resource if we can tap it. Obviously, wind is a significant resource if we can tap it.

Winds on EarthWinds on Earth

• It is colder near the poles because:It is colder near the poles because:– The surface is tilted from the direction of the sun, resulting The surface is tilted from the direction of the sun, resulting

in less energy per area.in less energy per area.• The reduction (averaged over a year) goes as the cosine of the The reduction (averaged over a year) goes as the cosine of the

latitude. latitude. – The path that sunlight takes through the atmosphere is The path that sunlight takes through the atmosphere is

longer, resulting in less energy reaching the ground.longer, resulting in less energy reaching the ground.• How does this phenomena effect the dangers emanating from the How does this phenomena effect the dangers emanating from the

well known polar ozone hole? When does it become really well known polar ozone hole? When does it become really dangerous?dangerous?

• These temperature gradients cause pressure and These temperature gradients cause pressure and density differences that lead to atmospheric motion.density differences that lead to atmospheric motion.

Wind Patterns on EarthWind Patterns on Earth

• General wind patterns are caused by the shape General wind patterns are caused by the shape of the Earth, its rotational motion, and the of the Earth, its rotational motion, and the amount of heating incident on the planet. amount of heating incident on the planet. – These “average” winds are not influenced by These “average” winds are not influenced by

surface features, etc.surface features, etc.

• Local winds are influenced by geographical Local winds are influenced by geographical features such as shorelines, mountain ranges, features such as shorelines, mountain ranges, and so on.and so on.

General Wind Patterns on EarthGeneral Wind Patterns on Earth

• Averaged over a year, the sun is directly overhead on an East-Averaged over a year, the sun is directly overhead on an East-West line at the equator.West line at the equator.

• Because of buoyant effects, air that is heated rises to “high” Because of buoyant effects, air that is heated rises to “high” altitude. (About 10 km altitude) altitude. (About 10 km altitude) – At the equator, warm air rises and is replaced by cooler air moving in At the equator, warm air rises and is replaced by cooler air moving in

from the north and south.from the north and south.– Winds are light at the equator – the so called “doldrums” that were Winds are light at the equator – the so called “doldrums” that were

feared by sailors.feared by sailors.– The atmospheric pressure is also low near the equator because of the The atmospheric pressure is also low near the equator because of the

rising air.rising air.

• The warm air moves toward the poles at high altitude and The warm air moves toward the poles at high altitude and cools as it flows north or south. cools as it flows north or south.

• When the equatorial heated air reaches about 30° When the equatorial heated air reaches about 30° latitude, it descends back to the surface.latitude, it descends back to the surface.– This creates a band of generally high pressure, dry weather This creates a band of generally high pressure, dry weather

and low winds in these regions – which is where most of and low winds in these regions – which is where most of the Earth’s deserts are located.the Earth’s deserts are located.

– These are called the horse latitudes because becalmed These are called the horse latitudes because becalmed sailors had to toss horses overboard to preserve water.sailors had to toss horses overboard to preserve water.

• Some of the sinking air moves back toward the Some of the sinking air moves back toward the equator, with the rest circulating toward the poles.equator, with the rest circulating toward the poles.

• The circulatory cells (between 0 and 30° latitudes) are The circulatory cells (between 0 and 30° latitudes) are called Hadley cells.called Hadley cells.

General Wind Patterns on Earth General Wind Patterns on Earth (continued)(continued)

• A similar cell occurs between latitudes of 30° and 60°.A similar cell occurs between latitudes of 30° and 60°.– The sinking air at 30° moves toward the poles until it meets The sinking air at 30° moves toward the poles until it meets

cold air from the poles, which forces the warm air up.cold air from the poles, which forces the warm air up.– Some of this air then circulates back toward the equator at Some of this air then circulates back toward the equator at

high altitude, completing another circulatory cell between high altitude, completing another circulatory cell between latitudes of 30 and 60°.latitudes of 30 and 60°.

– These are called the Ferrell cells.These are called the Ferrell cells.

• The rest of the air moves poleward until it sinks back The rest of the air moves poleward until it sinks back to the surface near the pole, forming a third pair of to the surface near the pole, forming a third pair of circulatory cells called the polar Hadley cells.circulatory cells called the polar Hadley cells.

General Wind Patterns on Earth General Wind Patterns on Earth (continued)(continued)

• The result is a series of three large circulatory The result is a series of three large circulatory cells between the equator and each pole.cells between the equator and each pole.

• This is illustrated on the next slide.This is illustrated on the next slide.– These winds are not North/South winds, however. These winds are not North/South winds, however.

They are turned by the Coriolis effect into They are turned by the Coriolis effect into predominantly East/West winds. predominantly East/West winds.

General Wind Patterns on Earth General Wind Patterns on Earth (continued)(continued)

The Coriolis EffectThe Coriolis Effect

• In the frame of reference of the center of the Earth, we are on a the surface In the frame of reference of the center of the Earth, we are on a the surface of a sphere that is spinning about an axis extending through the Earth’s of a sphere that is spinning about an axis extending through the Earth’s poles.poles.

• Our velocity in this frame of reference is rOur velocity in this frame of reference is r, where r is the distance from , where r is the distance from the pole to the surface, and the pole to the surface, and is the angular speed of the Earth, which is 2 is the angular speed of the Earth, which is 2 radians in 23.934 hours, or 23 hours, 56 minutes and 4 seconds. radians in 23.934 hours, or 23 hours, 56 minutes and 4 seconds. – This is 86,164 seconds.This is 86,164 seconds.– Why not 24 hours/revolution?Why not 24 hours/revolution?– =7.292x10=7.292x10-5-5 rad/sec. rad/sec.

• The distance from the axis to the surface coincides with the Earth’s radius, The distance from the axis to the surface coincides with the Earth’s radius, RREE, only at the equator., only at the equator.– RREE=6356.7 km = 6.3567x10=6356.7 km = 6.3567x1066 m = 3950 miles = 20.856x10 m = 3950 miles = 20.856x1066 feet feet

• The distance r is given by r=RThe distance r is given by r=REEcos(latitude).cos(latitude).

• For example, at a latitude of 45°, the rotational velocity is:For example, at a latitude of 45°, the rotational velocity is:– V=(6.3567x10V=(6.3567x1066 m)(7.292x10 m)(7.292x10-5-5 rad/sec)(cos(45))=327.8 m/sec. rad/sec)(cos(45))=327.8 m/sec.

• At different latitudes, the speeds vary as follows:At different latitudes, the speeds vary as follows:

The Coriolis Effect (continued)The Coriolis Effect (continued)

0

50

100

150

200

250

300

350

400

450

500

0 10 20 30 40 50 60 70 80 90

Latitude

Vel

ocit

y (m

/sec

)

Latitude(°) Velocity (m/sec)

0 463.5310 456.4920 435.5830 401.4340 355.0950 297.9560 231.7770 158.5480 80.4990 0.00

• The calculations show that as air at the surface moves toward the equator The calculations show that as air at the surface moves toward the equator from a latitude of 30° to 20°, it must accelerate by approximately 35 m/sec from a latitude of 30° to 20°, it must accelerate by approximately 35 m/sec (about 80 mph) in the tangential direction.(about 80 mph) in the tangential direction.– This is the Coriolis acceleration.This is the Coriolis acceleration.

• In both Hemispheres, this results in a westward deflection in these latitudes In both Hemispheres, this results in a westward deflection in these latitudes – these are the “trade winds” which blow out of the East.– these are the “trade winds” which blow out of the East.

• On the other hand, if the surface winds are moving away from the equator, On the other hand, if the surface winds are moving away from the equator, as in the Ferrel cells, the air is deflected Eastward. These winds are called as in the Ferrel cells, the air is deflected Eastward. These winds are called “westerlies.” “westerlies.”

• Near the poles, the effect is again reversed in the polar Hadley cells, Near the poles, the effect is again reversed in the polar Hadley cells, resulting in “polar easterlies.”resulting in “polar easterlies.”

The Coriolis Effect (continued)The Coriolis Effect (continued)

Wind Speed due to Longitudinal Wind Speed due to Longitudinal Movement (neglecting friction, etc.) Movement (neglecting friction, etc.)

• West to East Rotational speed at 29° North:West to East Rotational speed at 29° North:– V=(6.3567x10V=(6.3567x1066 m)(7.292x10 m)(7.292x10-5-5 rad/sec)(cos(29))=405.4 m/sec. rad/sec)(cos(29))=405.4 m/sec.

• West to East Rotational speed at 23° North:West to East Rotational speed at 23° North:– V=(6.3567x10V=(6.3567x1066 m)(7.292x10 m)(7.292x10-5-5 rad/sec)(cos(23))=426.7 m/sec. rad/sec)(cos(23))=426.7 m/sec.

• If a particle at 29° North is somehow picked up If a particle at 29° North is somehow picked up and suddenly deposited at 23° North:and suddenly deposited at 23° North:– It is moving 21.3 m/sec “slower” in the west to east direction It is moving 21.3 m/sec “slower” in the west to east direction

than the surrounding air.than the surrounding air.

– This will seem like a wind blowing out of the WestThis will seem like a wind blowing out of the West

North 24°

North 29°

Summary of General Wind Patterns on Summary of General Wind Patterns on EarthEarth

Local WindsLocal Winds

• There are several features that leads to localized There are several features that leads to localized winds. winds.

• These winds include:These winds include:– Sea BreezesSea Breezes

– Mountain-Valley WindsMountain-Valley Winds• ChinooksChinooks

– Santa-Ana WindsSanta-Ana Winds

– Monsoon WindsMonsoon Winds

Sea BreezesSea Breezes• Sea Breezes occur because of different heat absorption and Sea Breezes occur because of different heat absorption and

transfer characteristics between land and water.transfer characteristics between land and water.

• The daytime behavior is illustrated below:The daytime behavior is illustrated below:

• This behavior reverses at night, when the land is cooler than This behavior reverses at night, when the land is cooler than the water.the water.

Mountain-Valley WindsMountain-Valley Winds

• In mountain regions, during daylight hours air that is In mountain regions, during daylight hours air that is heated by the sun rises along the upslope of the heated by the sun rises along the upslope of the mountains and valleys.mountains and valleys.

• During the night, an opposite flow occurs as cool air During the night, an opposite flow occurs as cool air descends into the valley. descends into the valley.

• Monsoon means wind that changes direction with the season.Monsoon means wind that changes direction with the season.

• The Asian monsoons are the most famous:The Asian monsoons are the most famous:– In the summer, the interior of Asia heats up more than the ocean, In the summer, the interior of Asia heats up more than the ocean,

creating low pressure.creating low pressure.

– In the winter, the opposite effect occurs.In the winter, the opposite effect occurs.

• Therefore, monsoons can be thought of as colossal sea-breezesTherefore, monsoons can be thought of as colossal sea-breezes

MonsoonsMonsoons

Summer Winter

ChinooksChinooks

• Chinooks are warm, dry winds on the leeward side of mountain rages.Chinooks are warm, dry winds on the leeward side of mountain rages.– Chinook means “snow eater” in some Native American dialects.Chinook means “snow eater” in some Native American dialects.

• They are caused as winds blow over the mountain range, then are They are caused as winds blow over the mountain range, then are compressed adiabatically as they descend the leeward slopes.compressed adiabatically as they descend the leeward slopes.– The adiabatic compression results in a temperature rise – and a drop in relative The adiabatic compression results in a temperature rise – and a drop in relative

humidity.humidity.

– Consequently, these dry winds can melt a lot of snow!Consequently, these dry winds can melt a lot of snow!

• Chinooks are very common on the east slope of the northern Rockies.Chinooks are very common on the east slope of the northern Rockies.– These winds coupled with certain geographical features, such as occur near These winds coupled with certain geographical features, such as occur near

Boulder Colorado and Livingston Montana, can result in occasional very high Boulder Colorado and Livingston Montana, can result in occasional very high winds. (>100 mph)winds. (>100 mph)

Santa-Ana WindsSanta-Ana Winds

• These winds occur when desert plateaus are heated in the These winds occur when desert plateaus are heated in the summer resulting in high pressuresummer resulting in high pressure– The high pressure forces the hot air off the plateaus and down through The high pressure forces the hot air off the plateaus and down through

mountain valleys to lower elevations. mountain valleys to lower elevations.

– The air temperature further increases due to adiabatic compression as it The air temperature further increases due to adiabatic compression as it descends the slopes.descends the slopes.

• Since the temperature increases and no moisture is added, the relative Since the temperature increases and no moisture is added, the relative humidity further decreases – resulting in very dry winds.humidity further decreases – resulting in very dry winds.

• They are (informally) named after the hot winds that occur in They are (informally) named after the hot winds that occur in Southern California in the summer.Southern California in the summer.

Jet StreamsJet Streams• The discussions in this presentation have focused on winds near the surface.The discussions in this presentation have focused on winds near the surface.

– These winds occur at altitudes less than about 10 km, or about 35,000 feet.These winds occur at altitudes less than about 10 km, or about 35,000 feet.

– Surface winds are what we are concerned with in wind energy applications.Surface winds are what we are concerned with in wind energy applications.

• At higher altitudes, similar effects occur, primarily at the boundaries between the At higher altitudes, similar effects occur, primarily at the boundaries between the Hadley and Ferrel cells, but there is limited friction from the surface.Hadley and Ferrel cells, but there is limited friction from the surface.

• The result is high-altitude high-velocity winds known as the jet streams.The result is high-altitude high-velocity winds known as the jet streams.

• These winds are important in determining the motion of large air masses and These winds are important in determining the motion of large air masses and weather systems.weather systems.– Consequently, they are of indirect importance in wind energy applications.Consequently, they are of indirect importance in wind energy applications.