NAS 125: Meteorology

Wind and Weather

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The Fitz, part 1

• In 1976, Gordon Lightfoot, a Canadian singer/songwriter, released a song, The Wreck of the Edmund Fitzgerald, that recounts the November 10, 1975, sinking of the largest ore carrier on the Great Lakes. The song, which became a huge hit, has a haunting melody which seared the wreck of the ship, with the loss of 29 lives, into the consciousness of many who have heard Lightfoot’s recording.

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The Fitz, part 2

• The 222-meter-long ship, carrying 26,000 tons of iron ore, departed the Duluth-Superior harbor on the afternoon of November 9. The Fitz’s destination was a plant at Zug Island on the Detroit River.

• At 0600 CST on November 9, a low-pressure system began developing over central Kansas. It rapidly intensified as it tracking toward the northeast.

• The storm passed near La Crosse, Wis., at 0600 on November 10.

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The Fitz, part 3

• The storm was centered just west of Marquette, Mich., at noon, with a central pressure of 982 mb.– Gale-force northeast winds swept the eastern end of Lake

Superior, gusting to 115 km (71 mph) at Sault Ste Marie, Mich.

• At 0100 on November 10, the Fitz reported northeast winds of 97 km with winds to 3 m.

• At 0700, with the ship about 73 km north of Copper Harbor, Mich., the ship reported northeast winds of 67 km.

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The Fitz, part 4

• The Fitz’s captain, Ernest McSorley, then chose a course that took the ship through waters that sheltered it from the strong northeast winds.

• The storm passed over the ship during the afternoon.• The storm’s center approached Moosonee, Ontario, a

town on the shore of James Bay, that evening.• By that time, winds over Lake Superior had shifted

from the northeast to the north, then to the northwest and west.

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The Fitz, part 5

• The longer fetch (length of open lake surface exposed to the wind) of the northwest and west wind fueled the development of higher waves.

• A nearby ship, the Arthur M. Anderson, reported winds of 95 km, gusting to 137 km, with waves of 3.5 m to 5 m.

• Sometime between 0615 and 0625, the Fitz disappeared from the Anderson’s view as well as radar, sinking in 163 m of water within 27 km of Whitefish Point, Mich., and shelter from the wind.

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Wind, part 1

• Wind is the horizontal movement of air relative to the Earth’s surface.– The velocity of wind is a vector quantity, with a magnitude

(speed), and direction.

– Wind has both horizontal and vertical components, but the horizontal components are usually the most significant, except in localized systems such as thunderstorms.

• Measurement of wind direction– Wind vane

– Windsock

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Wind, part 2

• Measurement of wind direction (continued):– Direction is always given as that from which the wind

blows.

• Measurement of wind speed– Wind speed can be estimated using the Beaufort scale, a

graduated sequence ranging from 0 (calm conditions) to 12 (for hurricane-strength) winds.

• Named for Sir Francis Beaufort, a Royal Navy ship captain who wanted to standardize ways of describing sea conditions.

– Wind speed can be measured directly using anemometers.

– Continuous wind speed and direction data are useful.

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Physics

• Force: a push or pull that can cause an object at rest to move, or can affect the motion of an object already in motion.– The terms force and acceleration can be used

interchangeably, according to Newton’s second law of motion:

• force = (mass) x (acceleration)

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Forces

• Forces that affect the motion of air parcels:– Air pressure gradients

– Centripetal forces (actually occurs as a consequence of other forces)

– Coriolis effect

– Friction

– Gravity

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Pressure gradient force

• The pressure gradient force results from the difference in air pressure between two locations.– The steeper the gradient, the greater the force, and vice

versa.

– Air flows from where the pressure is greatest to where it is lowest.

– Horizontal pressure gradients at the surface can be denoted by the spacing of isobars (lines of equal pressure).

– Pressure gradients are measured along lines perpendicular to the isobars.

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Centripetal force

• Isobars plotted on weather maps are curved, thus the wind blows in curved paths.

• The centripetal force is a force that confines an air parcel (or any object to a curved path).– If the force dissipates, the parcel flies off in a straight line

in keeping with Newton’s first law of motion (that an object in motion stays in motion and an object at rest stays at rest until being acted upon by some force.

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Coriolis effect

• The Coriolis effect is the apparent deflection of free moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, in response to the rotation of Earth.– The objects, which are moving in a straight line in the

atmosphere, appear to move a long a curved path as the Earth’s surface rotates out from under the object.

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The Coriolis effect, part 1

• The Coriolis effect can significantly influence long-range movements.

• There are four basic points to remember:– A free moving object appears to deflect to right in Northern

Hemisphere and to left in Southern Hemisphere;

– The apparent deflection is strongest at the poles and decreases progressively toward the equator where there is zero deflection;

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The Coriolis effect, part 2

• Four basic points to remember (continued):– Fast-moving objects seem to be deflected more than slower

ones because the Coriolis effect is proportional to the speed of the object;

– The Coriolis effect influences direction only, not speed.

• The Coriolis effect influences winds and ocean currents, in particular serving as important component of general circulation of oceans.

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The Coriolis effect, part 3

• It does not affect the circulation pattern of water draining out of a washbowl – the time involved is too short and water speed so slow; instead draining direction is determined by the characteristics of the plumbing system, shape of washbowl, and pure chance.

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Friction, part 1

• Friction is the resistance an object encounters as it moves against another object.

• Viscosity: Friction of fluid flow– Molecular viscosity: results from the random motion of

molecules in a liquid or gas

– Eddy viscosity: results from the large, irregular motions that develop within fluids

• Example: Effect of rocks in a fast-moving stream

• Snow fences demonstrate effects of frictional slowing of wind.

– Eddy velocity most important in meteorological processes

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Friction, part 2

• The rougher the surface of the Earth, the greater is the eddy viscosity of the wind.– Forest-covered landscapes have more eddy viscosity than

grass-covered ones.

• Horizontal wind speed increases with altitude, up to about 1,000 m above the surface.– The portion of the atmosphere below 1,000 m is called the

atmospheric boundary layer (ABL).

• Turbulence is fluid flow caused by eddy motion.– Turbulence is often demonstrated as gusts of wind.

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Gravity

• All air parcels are subject to gravity.• Gravity results from the interaction of two forces, the

centripetal force, and gravitation.– Gravitation is the force of attraction between two objects.

• The magnitude of gravitation is the directly proportional to the masses of the two objects and inversely proportional to the distance between their centers of mass.

– The force (acceleration) of gravity is about 9.8 m/sec2

– Gravity acts directly downward (toward the heaviest object’s center of mass.

– Gravity does not modify the horizontal wind.

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Joining forces, part 1

• The five forces (pressure gradient, centripetal, Coriolis effect, friction, and gravity) interact to control the horizontal and vertical motions of the atmosphere.– Hydrostatic equilibrium

– Geostrophic wind

– Gradient wind

– Surface winds

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Joining forces, part 2

• The hydrostatic equilibrium is point at which the gravitational force is equals the vertical pressure gradient force, such that the net vertical acceleration of a parcel of air is zero.

• The geostrophic wind is a wind above the ABL that moves parallel to isobars as a result of balance between the pressure gradient force and the Coriolis effect.– Air parcels move in an oscillatory pattern which dampens as

they approach geostrophic equilibrium, this is called an inertial oscillation.

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Joining forces, part 3

• Gradient wind is similar to geostrophic wind except that it blows in a curved path as a result of interactions among the pressure gradient force, Coriolis effect, and centripetal forces.– Centripetal forces prevent equilibrium, however.

– Gradient winds blow around anticyclones and cyclones

– Idealized anticyclone (Northern Hemisphere)• Pressure gradient force: away from center of cyclone

• Coriolis effect: inward, slightly greater than pressure gradient force, leading to inward centripetal force

• Clockwise flow (above ABL, parallel to isobars)

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Joining forces, part 3

• Gradient wind (continued):– Idealized cyclone (Northern Hemisphere)

• Pressure gradient force: in toward center of cyclone, slightly greater than Coriolis effect, leading to inward centripetal force

• Coriolis effect: outward

• Counterclockwise flow (above ABL, parallel to isobars)

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Surface winds, part 1

• Geostrophic and gradient winds are frictionless (they occur at altitudes above the ABL).

• In the ABL, friction combines with the Coriolis effect to balance the horizontal pressure gradient force.– Friction works in a direction opposite to the wind direction.

– Coriolis effect operates at an angle perpendicular to the wind direction.

– Friction slows wind velocity, thus weakening the Coriolis effect, with the result that wind direction shifts toward low pressure.

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Surface winds, part 2

• The effect of friction decreases with altitude to the point where it is nil at the top of the ABL.

• Friction also affects winds flowing around anticyclones and cyclones, shifting wind directions toward low pressure.– Anticyclone (Northern Hemisphere): clockwise circulation

that spirals outward

– Cyclone (Southern Hemisphere): counterclockwise circulation that spirals inward

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Continuity, part 1

• Air is a continuous fluid, with links between the horizontal and vertical components of wind.– Wind follows topography.

• In anticyclonic circulation, air diverges from the center of the high-pressure cell, but the air dispersing at the surface is replaced by descending air from converging currents aloft.

• In cyclonic circulation, air converges and rises at the surface, but, instead of building up, diverges aloft.

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Continuity, part 2

• The roughness of the surface can induce vertical motions in the air.– When wind blows form land to sea, it accelerates (low

roughness), thus begins to stretch, which induces downward motion of air; this is known as speed divergence.

– When wind blows from a smooth to a rough surface, it slows and piles up, thus inducing upward motion; this is known as speed convergence.

• This contributes to lake-effect snows.

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Scale

• Meteorologists and climatologists subdivide atmospheric circulation phenomena into discrete systems that operate at various spatial and temporal scales.– Planetary scale: global in scale; includes polar easterlies,

midlatitude westerlies, and trade winds

– Synoptic scale: continental or oceanic in scale; includes migrating cyclones, hurricanes, air masses

– Mesoscale: thunderstorms and sea and lake breezes

– Microscale: small systems such as tornadoes

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