The Winds

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The Winds
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What Wind Is
Wind is the movement of air relative to the
Earth. It is named by the direction it comes
from. Thus, a northwest wind blows from the
northwest and moves toward the southeast. Blue
Northers are the cold winds that sweep down the
Great Plains and invade Texas from the north.
Easterlies are winds that blow from the east and
a NorEaster is a violent extratropical cyclone
named for the direction of the wind in the sector
of the storm where blizzard conditions are most
likely. Wind speed is the distance the air moves
divided by the time interval. Thus, for example,
if air moves a distance of 100 meters in 50
seconds, the wind speed is 2 m/s (?4 mph).
Meteorologists use several different units for
speed. The SI measure is m/s. The most common
measure is miles per hour (mph) in the United
States and kilometers per hour outside the USA.
The wind barbs on weather maps use the mariners
favorite measure, knots or nautical miles per
hour. Nautical miles are measured by the Earth
60 nautical miles 1? of latitude (69 miles or
111 km). A wind speed of 60 knots means that the
air would move 60 nautical miles or a distance
equal to 1? of latitude in an hour.
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When wind speed exceeds about 20 knots the wind
can pick up dust and produce dust storms. Dust
storms raged across the Great Plains during the
droughts of the Dust Bowl in the early to mid
1930s.
Dust Storm Approaching Stratford, Texas in 1935
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A dramatic dust storm (sometimes called a haboob)
engulfing Phoenix, AZ on 05 July 2011
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Power from the Winds
Power from windmills is proportional to v3. Its
theoretical maximum value is given by Betzs Law
For a windmill at sea level with blades 50 m long
(A pr2 ? 7900 m2) in a wind speed of 10 ms-1
(?20 mph) the maximum possible power is about 3
Megawatts.
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Wind Systems and the Scales of Motions The
worlds winds can be classified according to
their size or scale. 1. Planetary scale motions
such as the Jet Stream, the Trade Winds and
monsoons are about 10,000 km - roughly the
Earths radius. 2. Synoptic scale phenomena are
roughly 1000 km. They include moving high
pressure areas, tropical and extratropical
cyclones - the grand storm systems. 3. Mesoscale
winds such as thunderstorms and sea breezes are
between 4 and 400 km. 4. Microscale motions such
as tornadoes are less than 4 km across. Earths
rotation plays a major role in the character of
planetary and synoptic scale motions, which have
lifetimes of days. For smaller scale motions with
a lifetime much shorter than a day, the impact of
Earths rotation is small, so that it has almost
no direct effect on microscale motions. Some
wind systems defy easy classification. Both
fronts and streaks in the jet stream are long,
narrow zones that are synoptic scale in length
but mesoscale in width.
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Driving the Winds Sea Breeze by Day, Land Breeze
by Night Temperature differences produce density
and pressure differences that drive the winds.
One classic example is the sea breeze and land
breeze system. During the day, the land quickly
gets hotter than the sea and air rises over the
land. Cooler air over the sea then blows in from
the sea near the surface while after rising 500
or so meters the wind aloft blows from land to
sea and sinks over the ocean to complete a
circulation cell. At night the land gets colder
than the sea so the direction of circulation
reverses, but is generally weaker. The sea breeze
typically penetrates 10 km or more inland.
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Sea Breeze Yucatan Peninsula 15 June 2005 1625
UTC
00 UTC 18 July 1998
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Valley Breeze by Day, Mountain Breeze by Night
During the day the sun heats the mountainsides.
This heats the air right over the slopes more
than the air at the same level over the valley.
The warm, light air then slides up the slopes and
up the valley. If it rises enough it will produce
cumulus or even cumulonimbus clouds over the
summits while over the valley the cooler air
sinks, making the sky clear. At night the
mountainsides radiate heat to space and cool.
This chills the air above the slopes, which then
slides down the slopes and down the valley, often
in spurts. The sky clears over the summits and
stratus or fog may form in the chilled air of the
valleys.
Cu
Stratus, Fog
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Fog fills the Imperial Valley of California 13
Jan 2004 This fog sometimes gets so thick that it
does not burn off during the day but can persist
for days on end. Lines in the fog are flow lines
(click to highlight them) showing (humid) air
blowing into the valley from the Pacific Ocean
through the Golden Gate, just north of San
Francisco (SFO). At the same time, snow covers
the Sierra Nevada Mountains, which enclose the
Valley on its Eastern side. Sinuous breaks in the
snow identify narrow river valleys.
SFO
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Newtons Laws of Motion govern the motions of all
objects including air. Newtons First Law of
Motion If no force is exerted on an object, its
velocity will not change. Newtons Second Law of
Motion F ma The force on an object equals its
mass times its acceleration. The first law
represents a fundamental change from the mistaken
notion that the natural state of things is be
still. Any object resists attempts to change its
speed or its direction of motion. This resistance
to change is called inertia. Speed is the
distance traversed divided by the time interval.
Velocity is speed in a particular direction. If
the direction of a moving object changes, its
velocity also changes, even if the speed remains
constant. Acceleration is the change of velocity
divided by the time interval. Objects accelerate
when their speed or direction of motion changes.
Run Program FREEFALL
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Forces and their Accelerations Newtons Second
Law (F ma) enables us to predict changes in
motion (i. e., acceleration, a) provided we know
the forces acting on air. The four primary forces
on air (and water) and their resulting
accelerations are, 1. Weight (Gravity), produces
a downward acceleration, g 10 m s-2 at the
surface of the Earth. 2. The Pressure Gradient
Force produces an acceleration, apg from a point
of high pressure to a point of low pressure that
is proportional to the pressure difference
between the two points divided by the distance
between the points 3. Friction (viscosity for
fluids) produces an acceleration, af in the
direction opposite the motion and is proportional
to the speed of the wind or current. Friction
acts to reduce all relative motions (e. g.,
shear). 4. The Coriolis Force (a consequence of
Earths Rotation) produces an acceleration, acor
to the right of the motion in the North
Hemisphere, that is proportional to the speed of
the wind or current (and to the sine of the
latitude). Buoyancy is a residual force that
results when density differences cause an
imbalance of weight and pressure in air or any
fluid.
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Pressure differences accelerate air directly from
high to low pressure. The motive force is called
the pressure gradient force because it is
proportional to the pressure gradient, the
pressure difference between two points divided by
the distance between the points.
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Earths Rotation and the Coriolis Force
Coriolis force accelerates moving objects to
their right in the NH and to their left in the SH
because the sense of Earths rotation is opposite
in the two hemispheres.
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Earth as a Coriosel The cannon ball moves in a
straight line, but curves relative to a viewer on
the coriosel, who is turning to his left
(counterclockwise)
Run Program CORIOLIS
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Combinations of the Forces Most air motions
result from combinations of several forces. Some
classical situations result from the balance of
two or more forces. They include Hydrostatic
balance Geostrophic wind Gradient wind
Cyclostrophic wind Ekman Spiral
1. Hydrostatic Balance and Buoyancy The sky does
not fall because the downward force of gravity
almost exactly cancels the upward pressure
gradient force. The hydrostatic equation
expresses the balance and is more than 99
accurate in the atmosphere except for extreme
cases such as tornadoes. When the atmosphere is
not perfectly hydrostatic, the difference between
the upward pressure gradient force and the force
of gravity is the buoyancy force. A parcel or
balloon x less (more) dense than the
surroundings will have an upward (downward)
acceleration due to buoyancy x that of gravity.
Thus, a parcel 10 denser than the surroundings
will accelerate downward at 1 m/s2 or 10 of g.
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Archimedes' Principle Any object, wholly or
partly immersed in a fluid, is buoyed up by a
force equal to the weight of the fluid displaced
by the object.
renvVg
renv
rparVg
Hot air balloons were invented in China. Zhuge
Liang (?250 AD) used Kongming lanterns for
military signaling. The first documented balloon
flight in Europe was made on August 8, 1709, in
the Portuguese Court in Lisbon using a small
paper burning balloon built by Bartolomeu de
Gusmão, a priest. The first recorded balloon
flight with humans took place on October 19, 1783
in Annonay, France in a balloon built by the
brothers Joseph-Michel and Jacques-Etienne
Montgolfier, paper manufacturers.
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2. The Geostrophic Wind Normally, water flows
downhill--directly to low pressure. But water
spinning in a pail, flows around the pail even
though the water slopes down toward the center.
Similarly, large-scale winds do not blow directly
towards low pressure, but instead blow with low
pressure to their left in the Northern Hemisphere
and to their right in the Southern Hemisphere.
Thus, If you stand with your back to the wind,
then low pressure is on your left (in the
Northern Hemisphere). This strange behavior of
the large scale winds is called Buys Ballots
Law after the Dutch meteorologist, Christopher
Buys-Ballot. Above the atmospheric boundary
layer (more than about 1 km above ground level)
there is almost no friction, so low pressure is
almost exactly 90? to the left of the wind. If,
the wind also flows in a straight line at a
steady pace there is no net acceleration and it
is called the geostrophic wind. The geostrophic
wind blows parallel to the isobars with low
pressure exactly 90 to its left (right) in the
North (South) Hemisphere. Only two forces act
on the geostrophic wind. The pressure gradient
force pulls wind toward low pressure. It is
exactly cancelled by the Coriolis force, which
pulls the wind to its right. The geostrophic wind
represents a perfect balance between the pressure
gradient force and the Coriolis force. The
geostrophic wind speed is inversely proportional
to the distance between isobars
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Buys Ballots Law and the Geostrophic Wind
The Forces Pressure Gradient Force points
towards Low Pressure Coriolis Force points to
the right of the wind (North Hemisphere). When
there is no friction (at least 1 km above the
ground) and the wind is steady, the two forces
are equal and opposite so they cancel. The
result The Geostrophic Wind Buys Ballots Law
Stand with your back to the wind in the North
Hemisphere and Low Pressure is on your
left. Forecasting Application If high level
clouds such as cirrus move from left to right
when your back is to the wind then low pressure
is approaching.
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The Geostrophic Wind and Constant Pressure
Charts Weather above the ground is depicted using
charts at constant pressure. Standard (mandatory)
levels for constant pressure charts are 850, 700,
500, 300, 250, and 200 hPa (1 hPa 100 Pascals).
Constant pressure charts have hills and valleys
that correspond to high and low pressure areas.
All contain contour lines of height. Other
contour lines include T (850), vertical velocity
and RH (700), vorticity (500) and wind speed
(300, 250, and 200). They are used because,
1. High and Low pressure areas are displayed as
hills and valleys. 2. Wind speed is
inversely proportional to the distance between
contours. 3. Temperature patterns are free
of compression effects. The next slide shows why
highs appear as hills on constant pressure
surfaces and relates the pressure surfaces to the
geostrophic wind and to geostrophic currents. If
sea level pressure is 1020 hPa at the center of a
high pressure area, the 1000 hPa surface lies
directly above the high center because p
decreases with height. Lower pressure at sea
level surrounds the high. If the pressure at sea
level at some distance from the high pressure
center is 1000 hPa, we can find the 3-dimensional
1000 hPa surface by connecting all points where
pressure is 1000 hPa. This produces a dome-like
surface that arches over the high and intersects
sea level at the 1000 hPa isobar. To see how
geostrophic winds and currents develop, release a
ball on a constant p surface. As the ball slides
down, the Coriolis force will deflect it to its
right until it finally moves so that the
surfaces downslope side is on its left. This
explains why the Gulf Stream, doesnt flood NYC
even though sea level is more than 1 m higher at
Bermuda!
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Forecasting Temperature by Advection
(Wind) Temperature changes over time when the air
upwind is colder or warmer. This is seen on
constant pressure charts when Isotherms
(typically solid lines) cross Contours (typically
dashed lines). Directions for making a 12 hour
Temperature Forecast 1. Estimate average wind
speed and direction upwind from forecast city. 3.
Calculate distance air travels (each 5 knots 1
latitude per 12 hours). 4. Pinpoint upwind source
of air arriving at forecast city. 5. Future T at
forecast city is current T at upwind
source. Assumptions No heating or cooling, no
vertical motions, no change of wind. Problems 1
When the atmosphere is stable, rising air causes
cooling. 2 Weather systems also tend to move
from west to east and change shape so that wind
changes both speed and direction.
In the drawing to the right, cold air in the
Northwest (NW) moves to the SE while warm air in
the SE moves toward the N.
COLD
The next slide shows how T changed as the
snowstorm of 26-27 Dec 2010 moved up the East
Coast of the USA.
WARM
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The Strange Tilt of Weather Systems Outside the
tropics, it is often observed that highs and lows
and troughs and ridges are not vertically aligned
but tilt upward to the West. In fact, 1 Lows
and troughs always tilt upward toward the coldest
air. 2 Highs and ridges always tilt upward
toward the warmest air. 3. Systems with
symmetrical Temperature patterns do not tilt with
height.
L
L
COLD
COLD
COLD
WARM
WARM
L
L
Note how the trough (red dashed lines on next
slide) slopes up to the West (where air is much
colder) for the major snowstorm of 26-27 December
2010.
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