wind & global circulation winds.pdf · wind & global circulation • air is an invisible...

Wind & Global Circulation

• Wind is moving air

• Wind is the result of various external forces placed on air

• An external force is one that changes speed and/or direction

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• There are five forces that control wind – we will discuss three in this topic 1. Pressure Gradient Force (PGF) 2. Coriolis Force (CF) 3. Friction (F)

• These three forces make up the geostrophic wind

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• Air is an invisible gas but exhibits fluid flow

• In the above image, consider the water in each tube to be comparable to the amount of air above each city

What relative values of air pressure would you assign to each city?

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• City B has higher air pressure at surface than City A because there is more air above B

• The “door” is preventing air from flowing

If the door is removed which way does the air flow?

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• The PGF will move air away from higher air pressure and toward lower air pressure

What will the pressure be in both cities over time?

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Summary: • Pressure Gradient Force (PGF) was the initial force that caused air to move

• Air moved DIRECTLY AWAY FROM HIGHER PRESSURE and DIRECTLY TOWARD LOWER

PRESSURE in order for the atmosphere to achieve a pressure balance

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• Assuming no other forces, PGF moves air directly away from higher isobars and toward lower isobars in a straight path

Is this the flow you observe when weather forecasting? • No. The real wind direction is influenced by more than just PGF

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• Because of the earth's rotation, there is a second force known as the Coriolis force (CF) that affects the direction of wind flow

• Named after Gustav-Gaspard Coriolis, a French scientist (1835)

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• In the northern hemisphere, the CF-induced deflection is 90o to the right of the intended path

• In the southern hemisphere, the CF-induced deflection is 90o to the left of the intended path

• This results in wind flow parallel to isobars in a friction-free environment (above 5,000 ft.)

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• Air around the center of a low pressure system in the northern hemisphere flows counter-clockwise

• Air around the center of a high pressure system in the northern hemisphere flows clockwise

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• Air around the center of a low pressure system in the southern hemisphere flows clockwise

• Air around the center of a high pressure system in the southern hemisphere flows counter-clockwise

• This geostrophic wind does not allow pressure differences to balance

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• Buys Ballot Law:

Place your back to the wind and stick left arm out – left arm points toward lower pressure (assumes NO FRICTION)

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𝐶𝐹 = 2𝛺𝑉𝑠𝑖𝑛Ø 𝛺 = rotation rate of Earth per second (.0000116) 𝑉 = wind speed Ø = latitude

• CF is a function of 1) wind speed and 2) latitude

• CF is always a very weak force due to the very small value of 𝛺

• The Earth can only put one spin on any object per day

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• As with any force that causes an object to be displaced (moved), the amount of displacement will be a function of the time that that force has been exerted on the object.

• At zero time, there would be no movement due to the force. As time increases, there

would be a more noticeable displacement. • The Coriolis force is a VERY WEAK force so it takes a fairly long time for one to notice its

effects! • Therefore, the Coriolis Effect only occurs when a freely moving object travels a large

distance (i.e. travels a long time.) An example would be air that is moving over hundreds of miles which takes many hours to do.

• Contrary to public belief, water draining in a sink is NOT affected by CF because there is

not enough time for the Coriolis force to cause a noticeable displacement.

• Read the TRUTH ABOUT CORIOLIS

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http://geography.about.com/cs/corioliseffect/index.htm?terms=coriolis+effect


• Friction slows air that moves across the surface

• Because CF is a function of wind speed, a slower wind reduces the strength of the CF

• A weaker CF cannot deflect air the full 90o

• Instead, the weaker CF only deflects air approx. 60o away from the PGF direction

• Now wind will flow at an angle across isobars toward lower pressure

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Newmediastudio.org

• High pressure - air moves around clockwise and away from the center (NH) • Low pressure - air moves around counter-clockwise and toward the center (NH) 16


• Lord Hadley in 1735 proposed the single-cell global circulation model

• Warm, humid air near equator is less dense and tends to rise resulting in low pressure

• Cold, dry air at poles is more dense and tends to sink resulting in high pressure 17


• A hemispheric PGF causes surface air to begin moving away from the poles and toward the tropics

• The result is a single hemispheric wind flow known as the Hadley Cell

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• The Hadley single-cell model was NOT correct: • CF would deflect air toward the west causing easterly winds at all latitudes. We

observe westerly winds in the United States

• Easterly winds would slow the rotation of the Earth – we know this is not happening

• The atmosphere is “attached” to the rotating Earth so the entire atmosphere cannot move against this west to east rotational direction

• Hadley’s model only featured one semi-permanent high pressure and one semi-permanent low pressure in each hemisphere – observations show there are others

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• William Ferrel (1856) devised the model appearing above known as the three cell global circulation model

• Semi-permanent Low Pressures are located near 0o and 60oN&S and are featured by rising air, converging winds, and wetter climates

• Semi-permanent High Pressures are located near 30oN&S and 90oN&S and are featured by sinking air, diverging winds, and drier climates

sfsu.edu

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• Semi-permanent Low Pressures are located near 0o and 60oN&S and are featured by rising air and wetter climates

• Semi-permanent High Pressures are located near 30oN&S and 90oN&S and are featured by sinking air and drier climates

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• Surface wind flow from the three-cell model appears above

• Left image is for a smooth planet with no land masses

• Right image is the “real world” average surface wind flow

vsc.edu

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• 0o – 30oN&S featured by easterly winds (east to west) known as Trade Winds

• 30o – 60oN&S featured by westerly winds (west to east) known as Prevailing Westerlies

• 60o – 90oN&S featured by easterly winds (east to west) known as Polar Easterlies

vsc.edu

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• During winter, surface pressure systems tend to move southward 24


• During summer, surface pressure systems tend to move northward 25


𝑃 =1

2 𝐷 𝑉3

𝐷 = air density 𝑉 = wind speed

• The power of wind equation is above and units are Watts per meter squared (W/m2)

• Even small changes in wind speed greatly increase the power

• Ex. Doubling wind speed increase power eight-fold

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wind & global circulation winds.pdf · wind & global circulation • air is an invisible...

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