Download 08_WindWeather - davidmlawrence.com

NAS 125: Meteorology
Wind and Weather
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.
Rev. 30 March 2006
Wind and Weather
2
Rev. 30 March 2006
Wind and Weather
3
Rev. 30 March 2006
Wind and Weather
4
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.
Rev. 30 March 2006
Wind and Weather
5
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.
Rev. 30 March 2006
Wind and Weather
6
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.
Rev. 30 March 2006
Wind and Weather
7
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.
Rev. 30 March 2006
Wind and Weather
8
Rev. 30 March 2006
Wind and Weather
9
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
Rev. 30 March 2006
Wind and Weather
10
Rev. 30 March 2006
Wind and Weather
11
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.
Rev. 30 March 2006
Wind and Weather
12
Rev. 30 March 2006
Wind and Weather
13
Rev. 30 March 2006
Wind and Weather
14
Rev. 30 March 2006
Wind and Weather
15
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)
Rev. 30 March 2006
Wind and Weather
16
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
Rev. 30 March 2006
Wind and Weather
17
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.
Rev. 30 March 2006
Wind and Weather
18
Rev. 30 March 2006
Wind and Weather
19
Rev. 30 March 2006
Wind and Weather
20
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.
Rev. 30 March 2006
Wind and Weather
21
Rev. 30 March 2006
Wind and Weather
22
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.
Rev. 30 March 2006
Wind and Weather
23
Rev. 30 March 2006
Wind and Weather
24
The Coriolis effect, part 1
• The Coriolis effect can significantly influence longrange 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;
Rev. 30 March 2006
Wind and Weather
25
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.
Rev. 30 March 2006
Wind and Weather
26
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.
Rev. 30 March 2006
Wind and Weather
27
Rev. 30 March 2006
Wind and Weather
28
Rev. 30 March 2006
Wind and Weather
29
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
Rev. 30 March 2006
Wind and Weather
30
Rev. 30 March 2006
Wind and Weather
31
Rev. 30 March 2006
Wind and Weather
32
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.
Rev. 30 March 2006
Wind and Weather
33
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.
Rev. 30 March 2006
Wind and Weather
34
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
Rev. 30 March 2006
Wind and Weather
35
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.
Rev. 30 March 2006
Wind and Weather
36
Rev. 30 March 2006
Wind and Weather
37
Rev. 30 March 2006
Wind and Weather
38
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)
Rev. 30 March 2006
Wind and Weather
39
Rev. 30 March 2006
Wind and Weather
40
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)
Rev. 30 March 2006
Wind and Weather
41
Rev. 30 March 2006
Wind and Weather
42
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.
Rev. 30 March 2006
Wind and Weather
43
Rev. 30 March 2006
Wind and Weather
44
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
Rev. 30 March 2006
Wind and Weather
45
Rev. 30 March 2006
Wind and Weather
46
Rev. 30 March 2006
Wind and Weather
47
Rev. 30 March 2006
Wind and Weather
48
Rev. 30 March 2006
Wind and Weather
49
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.
Rev. 30 March 2006
Wind and Weather
50
Rev. 30 March 2006
Wind and Weather
51
Rev. 30 March 2006
Wind and Weather
52
Rev. 30 March 2006
Wind and Weather
53
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.
Rev. 30 March 2006
Wind and Weather
54
Rev. 30 March 2006
Wind and Weather
55
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
Rev. 30 March 2006
Wind and Weather
56
Rev. 30 March 2006
Wind and Weather
57