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Wind
• Horizontally moving air
• Air/wind naturally flows
from areas of high
pressure to low pressure.
• Wind is natures attempt to
balance out inequalities in
atmosphere
Direction
• Winds are always labeled by the direction
from which they come from
– A north wind comes from the north and blows
towards the south
• Wind vane
– Always points into the wind
– Prevailing winds are winds blowing from more
than one direction
– Changes in wind direction tend to mean changes
in temperature
Speed
• Measure speed using a cup anemometer
• Read from a computer similar to a speedometer
What effects wind?
• Three factors:
– Pressure-Gradient Force
– Coriolis Effect
– Friction
1. Pressure-Gradient Force
• Differences in pressure cause wind; greater
the pressure, the greater the wind speed
• Isobars show pressure differences on maps
• Spacing of isobars indicates the amount of
pressure change occurring over a given
distance. Closer the isobars, the greater the
pressure gradient. Widely spaced isobars
indicate a weak pressure gradient.
2. Coriolis Effect
• Air moves out of regions with high pressure
into regions of low pressure.
• Coriolis effect: winds are deflected due to
rotation of the Earth; winds turn right in the
N. Hemisphere and to the left in the S.
Hemisphere.
• Effects direction, not speed
• Deflection was made clearly evident in WWII
by the US Navy
• http://www.classzone.com/books/earth_scien
ce/terc/content/visualizations/es1904/es1904
page01.cfm
3. Friction
• Friction on wind is only important within a few km of the
surface.
• Acts to slow air movement by about 20% and thus alter
wind directions.
• Two types of friction:
– Molecular friction: friction between individual air molecules
– Friction between two surfaces: air and land
• Friction between two surfaces: Air rubbing against Earth’s
surface causes friction. It is surface friction that causes the
phenomenon of "gusty winds" at the surface.
• Upper level winds (jet stream) tend to be stronger than
surface winds.
Jet Stream
• Jet streams are a narrow, variable band of very
strong, predominantly westerly air currents
encircling the globe several miles above the earth
• River-like currents of fast moving air traveling 75-150
mph in a west to east direction
• Located in lower stratosphere
• Typically cover long distances and can be thousands
of miles long
• Can be discontinuous and meander across the
atmosphere but they all flow east at a rapid speed
• Separated into:
– Polar jet stream
– (Sub) tropical jet stream
Jet Stream and El Niño/La Niña
• Jet stream helps carry El Niño and La Niña
El Niño
• An irregularly occurring climatic changes
affecting the equatorial Pacific region every
few years.
• Characterized by the appearance of unusually
warm, nutrient-poor water off northern Peru
and Ecuador, typically in late December.
• Two strongest events on record occurred
between 1982-83 and 1997-98
• 1997-98: Particularly bad for United States
– Huge storms struck west coast and gulf causing
large-scale beach erosion and floods
• When El Niño happens: air pressure drops
in southeastern pacific and rises in western
pacific.
• When event ends (hurricane/storms), the
pressure “swings” in the opposite direction;
rises in southeast and decreases in west.
• Called Southern Oscillation.
La Niña
• Characterized by the appearance of unusually
cooler surface temperatures in eastern Pacific
• Typical winter blows colder than normal over
the pacific northwest and the great plains.
• Winter 1998-1999:
– Record snowfall in Washington state
– Greater hurricane activity than El Nino
Global Atmospheric Circulation
• Atmospheric Circulation: large scale
movement of air
• Underlying cause of wind is unequal heating
(difference in pressure).
• Scientist made models to help understand
circulation
Single Cell – Non-Rotating Earth
• First and most basic
model of
atmospheric
circulation
• Circulation is known
as a Hadley Cell
Rotating Earth
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Equatorial Low/Doldrums
Trade Winds
Subtropical High
Westerlies
Polar Front
Polar Easterlies
Polar Highs
Equatorial Low/Doldrums
• Place of inactivity/low pressure
• Abundant precipitation
Trade Winds
• Wind blowing steadily towards the equator
from the northeast (N. hemisphere) or the
southeast (S. hemisphere)
• Prevailing and most persistent winds of the
tropics
Subtropical High
• Belt of high pressure located at latitudes 30° N
and S
• Semi-permanent
Westerlies
• Move in western direction, towards poles
• Govern most of air flow/weather in North
America
• Move from high to low pressure
• Prevailing surface winds
• Mid-latitudes, between 30° and 60°
Polar Front
• Fronts are boundary between two different
air masses
• Stormy front
• Difference in temperature and density
• Do not readily mix
– Results in the formation of the polar front
– The polar front is a zone of strong convergence
and rapidly rising air
Polar Easterlies
• Cool, dry, prevailing winds
• Move east, towards areas of low pressure
Polar Highs
• Cold air is moving south, but does not go
beyond polar front
• Sinks
• Circulation results in high pressure
Local Air Circulation
• Land and Sea breezes
• Mountain and Valley breezes
• Chinook and Santa Ana winds
Land and Sea Breezes
• Water, unlike air and land, does not change
temperature quickly.
• Water absorbs or loses a lot of heat energy
before the temperature changes.
• During the day the sun heats the land quickly. As
land heats, the air above is heated. The hot air
rises, become less dense creating low pressure.
The air over the sea is cooler because it takes
much longer to heat water. The air over the sea
keeps its high pressure
• The differences in pressure creates wind or a
breeze.
• Winds blow from the sea to the land to try to
equalize the pressure. This is called a sea
breeze.
• At night land cools rapidly. Because the air of
the sea is warmer, the wind's direction
changes.
• The cooler heavier air over the land forms a
high pressure causing the air to move towards
the sea where the air pressure is lower. This is
called a land breeze.
• http://www.classzone.com/books/earth_scien
ce/terc/content/visualizations/es1903/es1903
page01.cfm
Mountain and Valley Breezes
• Localized wind that occurs one right after the
other in a daily cycle
• Characterized by upslope flow during the day,
and down slope flow during the night
• During the day, the air along the valley wall
becomes warmer
• The cooler air sinks into the valley as the rest of
the air (warm air) blows up slope creating a valley
breeze.
• No as it gets darker, the air over the valley is
forced to ascend or rise into the atmosphere
while the cooler air blows down slope.
• This creates a mountain breeze.
Santa Ana Winds
• Strong, dry winds that blow westward
• Can bring hot to cold temperatures
• Sweep through Southern California in fall and
winter
• Sometimes travels 80-90 mph
• Contribute to forest and brush fires
Chinook Winds
• Name comes from the Chinook Indians who
lived in the region
• Warm, dry wind
• Strong and gusty (reach 100 mph or higher)
• Causes temperature to rise rapidly
– January 1966, in 4 minutes time, temperature
rose 38°F