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CHAPTER 19.3
Regional Wind Systems
LOCAL WINDS
 In the middle latitudes (where we live)
the wind circulation is complex.
 Local winds are small-scale winds
produced by locally generated
pressure gradients.
 They are caused by topographic
effects or by variation in surface
composition (land and water) in the
immediate area.
LAND AND SEA BREEZES
 At costal areas the land will heat up
more during the water. That makes
the air above the land heat up,
expand, and rise making an area of
low pressure.
 A sea breeze will happen because
cooler air over the water (that is at a
higher pressure) moves toward the
warmer land and low pressure air.
 A sea breeze will usually start around
noon and blow at the greatest
intensity mid- to late afternoon.
 At night the opposite may take place,
the land cools off faster than the
water. The cooler air (high pressure)
over the land will move to the sea,
where the air is warmer (lower
pressure).
 Large lakes can also simulate these
conditions.
VALLEY AND MOUNTAIN BREEZES
 During the day, the air along the
slopes of the mountains is heated
more than the air at the same
elevation on the valley floor. Because
of this warmer air on the mountain is
less dense, it move up along the slope
and makes a valley breeze.
 Again after sunset the reverse may
happen. The rapid cooling of the air
along the mountain slope produces
cooler air next to the ground, this air
is dense so it will move downslope
into the valley. This is called a
mountain breeze.
HOW WIND IS MEASURED
 2 basic wind measurements –
direction and speed.
 Winds are labeled based on the
direction from which they blow. So a
northern wind blows from the north
to the south.
 A wind vane will help determine
which way the wind is blowing.
WIND DIRECTION
 Wind will constantly blow more often
from one direction – this is called a
prevailing wind.
 In the United States, the westerlies
consistently move weather from the
west to the east across the continent.
WIND SPEED
 An anemometer is used to measure
wind speed.
EL NINO AND LA NINA
 There is a cold ocean current that
moves toward the equator along the
coast of Ecuador and Peru. Near the
end of the year, a warm current that
flows southward along the coasts of
Ecuador and Peru replaces the cold
current. During this time the local
residents named this warm current El
Nino.
 These warm currents only usually last
a few weeks. Then the cold current
comes back.
EL NINO
 Every 3 to 7 years, the warm current
become unusually strong and replace
the cold offshore waters with warm
waters.
 El Nino is these periods of time where
the ocean warming affects the
eastern tropical Pacific.
 This brings about abnormal weather
patterns. They will receive an
abnormal amount of rain. The warm
water will move the jet streams. This
brings different weather conditions,
mostly more rain in areas that
normally don’t get a lot of rain.
LA NINA
 This is when surface temperatures in
the eastern Pacific are colder than
average. This also gives a unique
weather pattern.
 Cold air will blow over the Pacific
Northwest and the northern Great
Plains. The rest of the United States
will be warm. The Northwest will also
get more precipitation.
 La Nina can cause more severe
hurricanes.
GLOBAL DISTRIBUTION OF PRECIPITATION
 Tropical areas get the most rain. This
is the area where we will find rain
forests.
 Subtropical areas contain deserts.