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Weather Forecasting
How can weather be predicted by looking at
changing cloud types and frontal systems?
How are weather observations and
measurements combined to produce weather
maps?
How can the data for a specific location at one
point in time be displayed in a station model?
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Weather Observations
 Weatherunderground.com Put in your zip
code.
 Weather.com
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Weather Observations
 Based on what is happening NOW!
– First things first—look outside!
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Barometer
 Gives
barometric or
air pressure
 General rule
– High-pressure
area will be
clear
– Low-pressure
area will be
cloudy and
rainy.
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Thermometer
 How warm or cold is
it?
 Is it rising?
 Is it falling?
 What can this tell
you?
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Wind Vane
 What direction is the wind
coming from?
 Knowing where the wind is
coming from gives clues to
the temperature and the
amount of water in the air
moving into an area.
– Winds from the south are
often warmer and carry
more moisture than winds
from the north
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Anemometer
 Determines wind
speed
 How hard is it
blowing?
 Is it gusting?
 Rated using
Beaufort Scale
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Beaufort Scale
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Another View
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Forecasting
 Uses the speed and direction of
movement for fronts, high and low
pressure centers, and areas of clouds and
precipitation.
 Can predict where those features will be at
some future time.
– Example: if a storm system is 1000 miles west
of your location and moving to the east at 250
miles per day, you would predict it to arrive in
your area in 4 days.
Image/Text/Data from the University of Illinois WW2010 Project."
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Image/Text/Data from the University of Illinois WW2010 Project."
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Air Mass
 An air mass is a large
body of air that has
similar temperature
and moisture
properties throughout.
Image/Text/Data from the University of Illinois WW2010 Project."
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Air Mass
 Once an air mass moves out of its source region,
it is modified as it encounters surface conditions
different than those found in the source region.
 For example, as a polar air mass moves
southward, it encounters warmer land masses and
is heated by the ground below.
 Air masses typically clash in the middle latitudes,
producing some very interesting weather.
Image/Text/Data from the University of Illinois WW2010 Project."
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Cold Air Mass
Cold Temperature, Little Moisture
 Those who live in northern
United States expect cold
weather during the winter
months.
 This is because of the invasion
of cold arctic air masses that
originate from the snow covered
regions of northern Canada.
 The overlying air becomes very
cold and very stable.
 The longer this process
continues, the colder the
developing air mass becomes,
until changing weather patterns
transport the arctic air mass
southward.
Image/Text/Data from the University of Illinois WW2010 Project."
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Warm Air Masses
Warm Temperature and Rich in Moisture
 Originate over the warm waters
of the tropics and Gulf of Mexico,
where heat and moisture are
transferred to the overlying air
from the waters below.
 The northward movement of
tropical air masses transports
warm moist air into the United
States, increasing the potential
for precipitation.
Image/Text/Data from the University of Illinois WW2010 Project."
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High Pressure
 The positions of high and low
pressure centers can greatly
influence a forecast.
 Fair weather generally
accompanies a high pressure
center and winds flow clockwise
around a high.
 This means that winds on the
back (western) side of the high
are generally from a southerly
direction and typically mean
warmer temperatures.
 On the front (eastern) side of a
high, winds are generally from the
north and this typically results in
colder temperatures.
Image/Text/Data from the University of Illinois WW2010 Project."
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High Pressure
Forecast Tip:
 If a city is expected to be located west of a
high pressure center then warmer
temperatures are likely.
 However, if the city is expected to be in the
northerly winds of a high pressure center,
then forecast colder temperatures.
 Cities under the influence of high pressure
centers can expect generally fair weather
with little or no precipitation.
Image/Text/Data from the University of Illinois WW2010 Project."
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Low Pressure
 In contrast, clouds and
precipitation generally
accompany a low pressure
center and winds flow
counterclockwise around lows.
 This means that winds on the
back (western) side of the low
are generally from a northerly
direction and typically mean
colder temperatures.
 On the front (eastern) side of a
low, winds are generally from
the south and this typically
results in warmer temperatures.
Image/Text/Data from the University of Illinois WW2010 Project."
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Low Pressure
 Forecast Tip:
– If a city is expected to be located west of a low
pressure center then colder temperatures are
likely.
– However, if the city is expected to be in the
southerly winds of a high pressure center, then
forecast warmer temperatures.
– Cities under the influence of low pressure
centers can expect generally cloudy conditions
with precipitation.
Image/Text/Data from the University of Illinois WW2010 Project."
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Clouds
 Clouds are classified into a system that uses
Latin words to describe the appearance of
clouds as seen by an observer on the
ground.
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Cirrus Clouds
High Level Clouds
 The most common form of high-level clouds are thin
and often wispy cirrus clouds.
 Typically found at heights greater than 20,000 feet
(6,000 meters), cirrus clouds are composed of ice
crystals that originate from the freezing of supercooled water droplets.
 Cirrus generally occur in fair weather and point in the
direction of air movement at their elevation.
Image/Text/Data from the University of Illinois WW2010 Project."
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Altocumulus
Mid Level Clouds
 Altocumulus may appear
as parallel bands or
rounded masses.
 Special characteristic: a
portion of an altocumulus
cloud is shaded.
 Usually form by convection
in an unstable layer aloft,
which may result from the
gradual lifting of air in
advance of a cold front.
 Presence on a warm and
humid summer morning is
commonly followed by
thunderstorms later in the
day.
Image/Text/Data from the University of Illinois WW2010 Project."
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Fair Weather Cumulus
 Appearance of floating
cotton and have a
lifetime of 5-40 minutes.
 Exhibit only slight
vertical growth, with the
cloud tops designating
the limit of the rising air.
 Fair weather cumulus
can later develop into
towering cumulonimbus
clouds associated with
powerful thunderstorms
under the right
conditions.
Image/Text/Data from the University of Illinois WW2010 Project."
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Cumulonimbus Clouds
 Much larger and more
vertically developed than fair
weather cumulus.
 Exist as individual towers or
form a line of towers called a
squall line.
 Can be ten or more miles in
height, extending through all
the levels of altitude.
 Form when a front of cool air
meets a body of hot, moist air.
 Can produce rain showers,
snow showers, hail, or even
thunderstorms.
Photograph by: NOAA
Image/Text/Data from the University of Illinois WW2010 Project."
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Nimbostratus Clouds
Low Level clouds
 Dark, low-level clouds
with precipitation
 Low clouds are primarily
composed of water
droplets since their
bases generally lie
below 6,500 feet (2,000
meters).
 When temperatures are
cold enough, these
clouds may also contain
ice particles and snow.
Image/Text/Data from the University of Illinois WW2010 Project."
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Cloud Cover
 At night cloud cover
has the opposite
effect. If skies are
clear, heat emitted
from the earth's
surface freely escapes
into space, resulting in
colder temperatures.
Image/Text/Data from the University of Illinois WW2010 Project."
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Cloud Cover
However, if clouds are
present, some of the
heat emitted from the
earth's surface is
trapped by the clouds
and reemitted back
towards the earth. As a
result, temperatures
decrease more slowly
than if the skies were
clear.
Image/Text/Data from the University of Illinois WW2010 Project."
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Effects of Cloud Cover
 During the day, the earth
is heated by the sun.
 If skies are clear, more
heat reaches the earth's
surface (as in the
diagram below).
 This leads to warmer
temperatures.
Image/Text/Data from the University of Illinois WW2010 Project."
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Effects of Cloud Cover
 However, if skies are
cloudy, some of the sun's
rays are reflected off the
cloud droplets back into
space.
 Therefore, less of the
sun's energy is able to
reach the earth's surface,
which causes the earth to
heat up more slowly.
 This leads to cooler
temperatures.
Image/Text/Data from the University of Illinois WW2010 Project."
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Forecasting Based on Cloud Cover
 Forecast Tip:
When forecasting nighttime temperatures, if
cloudy skies are expected, forecast warmer
temperatures than you would predict if clear
skies were expected.
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