17.1 – Air Masses and Fronts
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An air mass is a huge
body of air that has
similar temperature,
humidity, and air pressure
throughout.
Types of Air Masses
Scientists classify air
masses according to
temperature and humidity.
► There are four major types
of air masses that influence
weather in N. America.
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1. Maritime
Tropical –
► Warm, humid air masses
that form over the oceans
and Gulf of Mexico
► They travel up from the
south bringing warm,
humid air.
►Bring
showers and
thunderstorms in the
summer.
► Bring snow in the winter.
2. Maritime Polar
Cool, humid air masses
that form over the North
Pacific and Atlantic Oceans.
► They affect the West Coast
more than the East Coast.
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They bring rain, and cool
temps to the west coast.
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3. Continental Tropical
Hot, dry air masses that
form only in summer over
areas in the SW U.S. and N.
Mexico.
► They move NE, bringing
hot dry weather.
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4. Continental Polar
Large air masses that form
over N. Canada and Alaska.
► Bring cool, dry air to much
of North America.
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FRONTS
As huge masses of air
move across land and
water, they bump into each
other.
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►When
2 masses with
different temps and density
collide, they do not mix.
► The area where the air
masses meet and do not
mix becomes a “front”
A front forms because one
air mass is less dense than
the other air mass.
► There are 4 types of
fronts: cold fronts, warm
fronts, stationary fronts,
and occluded fronts.
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Cold Fronts
Cold fronts form when a
rapidly moving cold air mass
runs into a slowly moving
warm air mass.
► Often results in the
formation of clouds, rain,
and/or snow.
► Also, thunderstorms.
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Warm Fronts
Forms when a rapidly
moving warm air mass
collides with a cold air
mass.
► Can cause rain or fog.
► Slower moving than cold
front.
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Stationary Fronts
Forms when a warm air
mass and a cold air mass
meet and no movement
occurs.
► Neither air mass has
enough force to move the
other.
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Occluded Front
Forms when a warm air
mass is caught between two
cooler air masses.
► The warm air mass gets
lifted and cut off from the
ground by the two cooler air
masses.
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17.2 - Storms
► A swirling center of low air
pressure is called a cyclone.
► As warm air at the center of the
cyclone rises, cooler air blows
toward this from areas with higher
pressure.
► The winds swirl in a counter-
clockwise fashion b/c of the
Coriolis Effect.
► Cyclones and low pressure
areas are associated with
storms and precipitation.
Anticyclones are the opposite
of cyclones.
► They are areas of high
pressure that are associated
with dry and clear weather.
► Air blows from high pressure
areas to low pressure areas.
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Kinds of Storms
A storm is a violent
disturbance in the atmosphere.
► Storms involve sudden
changes in air pressure, which
cause rapid air movements.
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Hurricanes
A hurricane is a tropical storm
that has winds of 74mph or
greater.
► They begin over warm water
as a low pressure area.
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The center of a hurricane is a
ring of clouds surrounding a
quiet “eye”
► When the eye arrives, the
winds grow calm and the sky
may clear.
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Hurricanes can last for over a
week.
► They get their energy from
the warm, humid air at the
ocean’s surface.
► When they get over land, they
begin to lose energy.
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Tornadoes
► Tornadoes form in low, heavy
cumulonimbus clouds.
► They form when cold, dry air
mass from Canada collides with
a warm, humid air mass from
the Gulf of Mexico.
During a tornado, the safest
place to be is in the basement
of a well-built building.
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