Forecasting Weather
http://www.usatoday.com/weather/basics/w
works0.htm
What are Fronts?
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A boundary between two air masses is
called a __Front_________
The shape of the _Front_______ depends
on the density of the air masses and their
speed
When a front forms:
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Pressure falls when a front approaches (low
pressure edges of air masses move in)
Changes in wind direction occur. High
pressure air moves Clockwise, low pressure
air moves Counterclockwise
Temperature changes occur
Precipitation occurs and can be in the form
of strong storms
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Low pressure is associated with the
formation of clouds and precipitation.
Warm air rises and when it reaches a certain
elevation it cools and become saturated.
Clouds form and when they are saturated, it
precipitates
There are four main types of
fronts
Warm Front:
Warmer air invades a cold air
mass and is lifted over and
replaces cooler air.
Cirrus and stratus clouds are
associated with warm
fronts
Gentle rain for days, slow
clearing, and rising
temperatures follow
Cold Front
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Cooler air invades a warm
air mass and moves under
the warm air and replaces
it
Cumulus clouds and
thunderstorms produce
HEAVY RAIN for a short
period of time
Cooler temperatures and
clear and fair weather
invade the area once the
storms have passed
Stationary Front
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Occurs when pressure
differences cause a warm
front or cold front to stop
moving
It may remain in the
SAME area for a few
days.
Weather conditions
include light winds and
precipitation across the
entire frontal region.
Occluded Front
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Results from two cool air masses merging
and forcing warmer air between them to rise
Strong winds and heavy precipitation may
occur
How do weather systems
move?
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Weather systems move across North
America from West to East.
Weather forecasting
Making predictions about future weather
based on weather data.
Weather Maps
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Provide data from stations all over the earth
Help meteorologists forecast weather
Data included:
– Temperature, air pressure, change in air
pressure during the last three hours, wind
speed, direction, dew point, visibility, cloud
cover, cloud types and precipitation.
Isobars
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Isobars - pressure lines
drawn on weather maps to
connect places having
equal air pressure.
Isobars that are close
together indicate a large
pressure difference over a
small area
Isotherms
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Isotherms are lines drawn to connect places
having equal temperatures
High and Low Pressure
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Pressure is indicated in the middle of the
isobars
H stands for a high pressure center
L stands for a low pressure center.
Winds develop and move from high to low
pressure areas.
Technology and Weather
Forecasting
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Satellites photograph sections of the earth’s
surface and show cloud coverage
Air Mass
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A large body of air that has the same
properties as the surface over which it
develops
An air mass over the GULF of MEXICO
would be warm and moist
An air mass over CANADA would be cool
and dry
Air masses
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Cool and dry --> high pressure, low
humidity CANADA
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Warm and moist --> low pressure, high
humidity GULF of MEXICO
Thunderstorms
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Occur inside warm moist
air masses and fronts
Result when warm moist
air rises quickly and cools
and condenses rapidly
Heavy raindrops fall,
dragging air with them,
creating strong winds
Produced by
cumulonimbus clouds
Lightning
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Atoms of air which are
caught in the strong
winds lose and gain
electrons, creating
cloud regions which
are positively or
negatively charged
Currents flow between
these regions resulting
in lightning
Thunder
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Rapid heating and cooling of air due to
lightning (rapid expanding and contracting)
Tornado
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Forms along fronts
Wind at different
heights blows in
different directions
and at different speeds
This along with a
strong updraft
produces a rotation
Hurricanes
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A low pressure area
develops where the ocean
water is very warm
Trade winds blowing in
opposite directions meet
causing a swirl of air that
is rotating
As moist air rises over the
low pressure system, it
cools and condenses and
begins to descend
This pattern is repeated
over and over
Clouds - condensed water vapor
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Recipe for a Cloud:
– Water vapor
– Condensation nuclei (dust, smoke, ice, salt)
– Relative humidity of 100% - saturation point or
dew point
Types of Clouds: Stratus
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fair weather or precipitation
Layers of sheets of clouds, can
be dull gray blanket of clouds
Low altitude clouds
See pages 425-427
Types of Clouds:
Cumulus
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White puffy clouds with
flat bases
Formed when moist air
rises
Fair weather or
thunderstorms
See pages 425-427
Types of Clouds: Cirrus
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Wispy
High altitude
Contain ice crystals
Fair weather
See pages 425-427