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Bell work
How would you describe the air you are
breathing right now? Is it warm or cool?
Humid or dry? Is it stale, sweet, or salty?
The air you are breathing right now was
hundreds of miles away yesterday. Do
you know how or why air moves from
one place to another?
Fronts
Objectives
• Identify the four kinds of air masses that
influence weather in the United States.
• Describe the four major types of fronts.
• Explain how fronts cause weather changes.
Air Masses
 Changes
in weather are caused by the
movement and interaction of air
masses.
 An air mass is a large body of air
where temperature and moisture
content are similar throughout.
 Air masses are characterized by their
moisture content and temperature.
Air Masses
 The
moisture content and temperature
of an air mass are determined by the
area over which the air mass forms.
 These areas are called source regions.
 There are many types of air masses,
each of which is associated with a
particular source region.


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The characteristics of
these air masses are
represented on maps by
a two-letter symbol.
The first letter indicates
the moisture content
that is characteristic of
the air mass.
The second letter
represents the
temperature that is
characteristic of the air
mass.
Air Masses
Air Mass Symbols
M= Maritime, forms over the water; wet
 C= Continental, forms over land; dry
 T= Tropical, forms near the equator; warm
 P= polar, forms near the poles; cold

mT=wet and warm
 mP=wet and cold
 cT=dry and warm
 cP=dry and cold

Fronts
 The
area in which two types of air
masses meet is called a front.
 The four kinds of fronts—cold fronts,
warm fronts, occluded fronts, and
stationary fronts
Objectives
• Identify the four kinds of air masses that
influence weather in the United States.
• Describe the four major types of fronts.
• Explain how fronts cause weather changes.
Cold Front




Cold dense air pushes warm air out of
the way
Cold fronts move very quickly and bring
short periods of rain/thunderstorms
Lower temperatures are behind the front
SYMBOL – the direction of the “arrows”
points towards the direction the front is
MOVING
Cold Front:
 Warm air is
abruptly
pushed
upward,
cooling,
condensing
moisture into
cumulus or
cumulonimbus
clouds
Cold Front
 Notice how
steep the angle
is between the
two air masses
 Typically brings
sudden, heavy
rains and
storms
Objectives
• Identify the four kinds of air masses that
influence weather in the United States.
• Describe the four major types of fronts.
• Explain how fronts cause weather changes.
Warm Front




Warm air moves up the cold front as it
slowly displaces the cold air
Warm fronts move slowly, and bring
many days of steady precipitation
Higher temperatures are behind the front
SYMBOL – direction of “half-moons” is
the direction the front is moving
Warm Front
 WARM FRONT:
when a warm air
mass moves into
a colder,denser
air mass.
 Warm air rides up
and over the
colder air
 Notice the angle
of slope between
the two air
masses.
Warm Front
 The weather
during a WARM
FRONT starts
with cirrus
clouds about
24-48 hours
before the rain
begins
 Cirrus clouds
are “at the front
of the front”
Warm Front
 As more warm
air is pushed
upward, more
moisture
condenses
forming
cirrostratus
clouds
Warm Front
 As warm more
warm air is
pushed up,
heavier clouds
form mid-way up
over the cold air
 Altostratus and
stratus
Warm Front
 The final
cloud type in
a warm front
is the
nimbostratus
 “nimbo” =
“rain”
 Warm
front: rain
or snow
is steady
over
several
hours or
days
Objectives
• Identify the four kinds of air masses that
influence weather in the United States.
• Describe the four major types of fronts.
• Explain how fronts cause weather changes.
Occluded Fronts
 An occluded front forms when a warm air mass
is caught between two colder air masses.
 The coldest air mass moves under and pushes
up the warm air mass.
 The coldest air mass then moves forward until it
meets a cold air mass that is warmer and less
dense.
Occluded Fronts
 The colder of these two air masses moves
under and pushes up the warmer air mass.
 Sometimes, though, the two colder air masses
mix. An occluded front has cool temperatures
and large amounts of rain and snow.
Stationary Front



The air from the warm front and cold
front meet, but do not move
These fronts have the same weather as
warm fronts
SYMBOL – warm and cold fronts are
moving in opposite directions, thus
making a stationary condition
Stationary front
 A stationary front forms when a cold air
mass meets a warm air mass.
 Both air masses do not have enough
force to lift the warm air mass over the
cold air mass.
 The two air masses remain separated.
 A stationary front often brings many days
of cloudy, wet weather.
Objectives
• Identify the four kinds of air masses that
influence weather in the United States.
• Describe the four major types of fronts.
• Explain how fronts cause weather changes.