Lecture:Local Winds

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Transcript Lecture:Local Winds

How much force does the wind
• Wind Force=wind pressure x area
– =v2 (ft/sec) x 0.0024 x area (ft2)
• The wind produces how much force on an 8 x 6
foot fence panel during a weak hurricane
– Force=0.0024 x (88ft/sec)2 X 48ft2= 890 lbs
• Katrina (130mph—189ft/sec)=4110 lbs!
• This principle affects everything from
building design to car fuel efficiency;
aerodynamic lift in aircraft
Wind and Exposed Soil
Microscale winds bounce and skip sand grains, expose desert
pavement, erode patterns into rocks, and shape sand dunes
with ripples revealing wind direction.
Wind and Snow Surfaces
. Turbulent eddies of wind can trigger rolling of moist snow to
create cylindrical snow rollers.
Snow fences are used to accumulate snow so that it does not
drift over roads
Wind and Vegetation
Wind speeds increase with elevation above earth's frictional
surface, where stronger winds sculpt and denude the branches
of trees.
Shelter belts may protect downwind property, but may also
create unwanted turbulent eddies.
Wind and Waves
-Water waves are generated by surface friction and eddies that
force winds and water upward and downward.
-Wave height increases with wind speed, wind duration, and
fetch (the length) of open water.
Wind Direction and Origin
-Wind direction is typically given as the origin from which it
blows, such as "westerlies" coming out of the west.
-Counter-examples use the adverbs 'on' and 'up' to indicate
direction, such as 'onshore wind', and "upslope wind" for wind
moving uphill.
Charting Prevailing Winds
-Winds shift in direction, noted by degrees or compass direction, and
of prevailing wind directions are useful in planning.
-A wind rose of likely directions might be used to orient runways and
pollution sources
Wind Rose
• Monthly or annual average speed and
directions represented graphically
Wind Measurements
-Pilot balloons observed
with theodolites, and
rawinsondes are useful
ways to detect winds in the
lower atmosphere.
-Doppler radar detects
changes in wind speed.
Also very helpful in
identifying developing
storms and tornadoes.
Thermal Wind Circulations
Thermal Circulations
-Solar heating and radiational cooling of earth's surface generates
cold-core thermal highs and warm-core thermal lows.
-Winds can circulate between these two systems.
Thermal Winds
Changes in air temperature
causing warm air to rise and
cool air to sink can also
generate horizontal winds.
Rising warm air creates a
surface low and upper level
Sinking cool air creates a
surface high and upper level
Land/Sea Breeze
• Forms when large scale pressure
differences are small (large scale winds
are weak), or when large scale winds are
blowing in direction of breeze
– There must be strong temperature
differences between land and water.
• Leading edge of the sea breeze front
frequently will cause lifting and trigger
Sea/Land Breezes
-Land heats more quickly than water, creating land-water
temperature differences along a coastline.
- During the day the land's warm-core thermal low draws a sea
breeze, while at night, the warmer sea draws a land breeze.
Sea and Land Breezes
Opposing breezes
may converge on
an isthmus of land,
and this rising
moist unstable air
will trigger
The leading edge of
land-water breezes
can bring rapid
changes in humidity
or pollutants.
Sea Breeze Animation
Asian Monsoon
Seasonal reversal of monsoon
winds in southern Asia is
explained by continental
temperature shifts.
Summer monsoon depressions of
low pressure and rains are
enhanced by solar insolation,
latent heat of condensation, and
jet stream patterns.
El Nino-Southern Oscillation is
also known to affect monsoon
Southwest US Monsoon
Monsoon-like winds
develop in July
across the
southwestern U.S.
As the continental
interior heats and
rises, humid Gulf
air sweeps in
creating instability
and thunderstorms
Mountain and Valley Winds
-Solar heating of hill slopes creates air that is warmer and less
dense than air at the same elevation over the valley, and
generates rising valley breezes.
-Nighttime radiational cooling of the mountain slope creates
relatively denser air that sinks as a mountain breeze, or gravity
Mountain and Valley Breezes
Daily warming of mountain slopes that trigger valley breezes
of moist air may result in cumulus clouds above mountain
Katabatic Wind
• Elevated plateaus with snow cover may foster development of a
thin layer of high pressure cold air.
– Pressure gradient winds are triggered due to lower pressure
above the adjacent valley, pushing cold air into the lower
valley…can be as strong as 100 kts
• Common in Columbia River Gorge in Washington state; in the
former Yugoslavia, and France
Chinook Winds
• Known as the “snow eater”, or
Chinook winds
• Common in US Great Plains
• Strong westerly winds force air
• Moisture condenses and
precipitates when ascending
high mountains
• The air is then warmed upon
• Air is much warmer and much
drier on the lee side of the
The drying/warming process happens any time the air rises and then descends over
high mountain ranges, but Chinook winds form when upper level winds are strong
and perpendicular to the mountain range
Chinook Wall Cloud
Chinook wall clouds are an indicator of the presence of
Chinook conditions.
Chinook—Rapid Temperature
Santa Ana Winds
Easterly winds that descend from southern California's elevated
desert plateau are heated by compression and lowered in
Santa Ana Story
• Named after Southern California's Santa Ana
Canyon and a fixture of local legend and
literature, the Santa Ana is a blustery, dry and
warm (often hot) wind that blows out of the
• In Raymond Chandler's story Red Wind, the title
being one of the offshore wind's many
nicknames, the Santa Anas were introduced as
"those hot dry [winds] that come down through
the mountain passes and curl your hair and
make your nerves jump and your skin itch.
– On nights like that every party ends in a fight. Meek
little wives feel the edge of the carving knife and study
their husbands' necks. Anything can happen."
Haboob Winds
Al Asad, Iraq
-Desert winds with dust and sand may form from cold downdrafts
along the leading edge of a thunderstorm.
-These winds are most commonly found in the African Sudan and
Arizona Sonora deserts.
Dust Devils
-Not a tornado
-On a hot, dry day the sun strongly heats the surface of the
ground, causing strong thermals….
Dust Devils
-Winds near the surface can
cause the thermal to spin
-The dust devil then creates a
rising column of dust and
(light) debris.