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Weather Patterns
Air Masses
• Large body of air
that has
properties similar
to the part of
Earth’s surface
over which it
developed
Air Pressure
Winds blow from areas of high pressure
to areas of low pressure
Low Air Pressure
Air rises in areas of
low pressure
When air rises in low
pressure areas it cools,
condenses and makes clouds
High Air Pressure
Air sinks in high pressure
areas and makes it difficult
to form clouds
Winds blow away from the
center of high pressure
FRONTS
A boundary between two air
masses of different density,
moisture, or temperature.
Cold Front
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Colder air advances toward
warm air
Cold air wedges under warm air
As warm air is lifted up, it cools
and condenses, forming clouds
If temperature difference
between air masses is large,
severe weather may occur
Cold fronts are drawn on
weather maps as blue lines with
triangles going in the directions
the front is moving
Warm Front
• Lighter, warmer air
advances over heavier,
colder air
• Can lead to hours or
days of wet weather
• Drawn on weather maps
as red lines with
semicircles
OCCLUDED FRONT
• Three air masses of different temperatures
• Warm air mass is trapped between and above two
cold air masses
• Drawn on weather maps as purple lines with
triangles and semicircles
Warm
STATIONARY FRONT
• Neither air mass moves
• Can stay in same place for
several days
• Light wind, and precipitation
• Drawn on weather map as
alternating red and blue line
with red semicircles pointing
in the direction the warm air
is moving and blue triangles
in the direction the cold air is
moving
Thunderstorms
• Occur in warm moist air masses
along fronts
• Warm air is forced upward by cool
air
• Air cools and condenses to form
cumulonimbus clouds
• Sinking cool air and strong
updrafts of warm air cause strong
winds
• The movement of warm air and
cold air can cause parts of clouds
to become oppositely charged
• When current flows between these
regions, lightning flashes
• As the extreme heat cools sound
waves are given off and causes
thunder
• Wind at different
heights blows in
different directions and
speeds called wind
shears
• Wind shears create a
rotating column parallel
to the ground
• Updrafts can tilt the
rotating column upward
creating an area of low
pressure in the form of
a funnel cloud
• If the funnel cloud
comes in contact with
the Earth’s surface it is
a tornado.
Tornadoes
Hurricane
• Large, swirling lowpressure system that
forms over the Atlantic
ocean
• Winds of at least 119 km/h
• Similar storms are called
typhoons in the Pacific
Ocean and cyclones in the
Indian Ocean
• The strongest hurricanes
affecting North America
usually form east of Africa
and travel west
• When hurricanes strike
land, high winds,
tornadoes, heavy rains
and waves can cause
damage
Blizzards
• Winter storm
• Winds are 56 km/h
• Temperature is -7 0 C
or below
• Visibility is < 400 m
• Conditions consist for
three hours or more
Weather
• 4 major air masses affect weather in the United States
• An air mass has the same characteristics as the area over
which it formed
• Air masses, pressure systems, and fronts cause weather
to change
• Fair weather is associated with high pressure areas
• Cloudy weather and severe weather are associated with
low pressure areas