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Weather Changes
Chapter 12, Section 2
I.
Weather Constantly Changes!
A.
Air masses – large body of air that has
properties similar to the part of the Earth’s
surface over which it develops
1. Can be over water or land from tropical or polar
regions
a. If air mass forms over land, the air mass is dry
b. If air mass forms over water, the air mass is moist
c. If air mass forms in polar regions, air mass is cold
d. If air mass forms in tropical regions, air mass is
warm
2. Temperature and humidity conditions in air masses
are important to weather when these air masses
move
3. Can cover thousands of square kilometers
I. Weather Constantly Changes!
B.
High Pressure
1.
Winds blow from high pressure to
low pressure
A. High-pressure areas are associated with
fair weather
B. Rotation of Earth causes these winds to
spiral in a clockwise direction in northern
hemisphere
I. Weather Constantly Changes!
C. Low Pressure
1. Winds blow from high pressure to low pressure
A. Low-pressure areas are associated with cloudy weather
II. Fronts
Front is a boundary between 2 air masses of
different density, moisture, or temperature
B. Cloudiness, precipitation, and storms occur at
frontal boundaries
C. 4 types of fronts
A.
1.
2.
3.
4.
Cold front
Warm front
Stationary front
Occluded front
Cold Front

Dense, cold air mass
pushes into warmer air
mass and lifts it, causing
the warm air to rise, cool,
and condense & form
clouds

When the temperature
difference is large
between cold front and
warm air, thunderstorms
& tornadoes may form.

Snowstorms & blizzards
can occur if temperature
is low enough
Warm Front

Lighter, warmer
air advances and
slides over
denser, colder air
 Light, steady
precipitation,
stratus clouds
Stationary Front

Stalemate between
air masses
 Both air masses can
remain in same place
for several days
 Can produce
precipitation and light
wind
Occluded Front

Occurs with 3 air masses: cold,
cool, and warm air

Cold air mass moves toward
cool air mass, warm air between
them (this is a rare front)

Cold air pushes warm air up
away from Earth’s surface

Can create violent weather
Front Summary
Presentation Express animations!
III. Severe Weather
a.
Thunderstorm
1. storm with thunder, lightning, heavy rains and strong
winds; form within large cumulonimbus clouds;
usually form along a cold front but can form within
an air mass.
b.
Tornado
1. rapidly whirling, funnel-shaped cloud that extends
down from a storm cloud; the very low pressure and
strong winds can cause great damage to people
and property; are likely to form within the frontal
regions where strong thunderstorms are also
present.
III. Severe Weather
A. Hurricane -- low pressure tropical storm
that forms over warm ocean water; winds
form a spinning circular pattern around the
center, or eye, of the storm; the lower the
air pressure at the center, the faster the
winds blow toward the center of the storm.
IV. Severe Weather Safety
A.
Watch
1.
B.
When conditions are favorable for
severe thunderstorms, tornadoes,
hurricanes, floods, and blizzards
Warning
2.
Severe weather conditions already
exist and action should be taken
Severe Weather
Caught Ya Being Brilliant!
According to the map, the most likely areas of heavy
precipitation are:
a.
in the Western United States.
b.
in the Eastern United States.
c.
across the entire country.
d.
nowhere--there is no precipitation anywhere in
the country.
Caught Ya Being Brilliant!
According to the map, which type of weather situation is
more likely in the Western part of the United States
than in the Eastern part?
a. high pressure systems
b. low visibility
c. heavy precipitation
d. low pressure systems
Caught Ya Being Brilliant!
A meteorologist who is looking at the above map might be
concerned that:
a. the Western part of the United States is going to see flooding
in certain areas.
b. the Eastern part of the United States is going to be struck by a
hurricane.
c. the Western part of the United States is going to have heavy
snow storms.
d. the Eastern part of the United States is going to suffer from a
drought.