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Weather Patterns
• Air Mass: A large
body of air that has
properties similar to
the part of Earth’s
surface over which it
develops.
• Air masses cover
thousands of square
miles.
• Six major air masses
affect weather in the
Unites States.
Weather Patterns
•Air pressure is measured using a barometer.
•Weather has high- and low-pressure systems.
•Winds blow from areas of high to areas of low pressure.
High Pressure
Low Pressure
• Winds blow away from a
center of high pressure.
• Winds blow into a lowpressure area in the
northern hemisphere and
swirl in a counterclockwise direction.
• These winds spiral
clockwise in the northern
hemisphere.
• High-pressure areas are
associated with fair
weather and are called
anticyclones.
• Low pressure systems are
called cyclones and are
associated with stormy
weather.
Weather Patterns
• Low pressure systems at Earth’s surface are regions of
rising air.
• Clouds form when air is lifted and cools.
• Areas of low pressure usually have cloudy weather.
• Sinking motion in high-pressure air masses makes it
difficult for air to rise and clouds to form.
• That’s why high pressure usually means good weather.
Weather Patterns: Fronts
• A boundary between two air masses of different density,
moisture, or temperature is called a front.
• Cloudiness, precipitation, and storms sometimes occur at
frontal boundaries.
• Four types of fronts include cold, warm, occluded, and
stationary.
Cold Front
A cold front forms when a
mass of cold air meets &
pushes warm air up
rapidly.. Violent storms are
associated with a cold
front. Fair, cool weather
usually follows.
Warm Front
A warm front forms
when a mass of warm
air overtakes a cold air
mass and moves over it.
Rain and showers
usually accompany a
warm front. Hot, humid
weather usually follows.
Occluded Front
When a cold front overtakes
a warm front, an occluded
front forms.
As the warm air is pushed
upward, the cold air meets
cool air. An occluded front
may also occur when cool air
overtakes a cold front and
warm air is pushed upward.
An occluded front produces
less extreme weather than a
cold or warm front.
Stationary Front
When a warm air mass
meets a cold air mass
and no movement
occurs, a stationary
front forms. Rain may
fall in an area for many
days when a stationary
front is in place.
Weather Patterns: Severe Weather
• Thunderstorms can stall over
a region, causing flash
flooding.
• Flash floods can occur with
little warning.
• Strong Winds >89mph are
classified as severe.
• Hail can destroy property and
farm crops.
• Lightning
• Tornadoes
Weather Patterns: Lightning
• In a storm cloud, warm air is lifted rapidly as cooler air
sinks. This movement of air causes different parts of the
cloud to become oppositely charged.
• When current flows between regions of opposite electrical
charge, lightning flashes.
•occurs within clouds, b/w clouds, or from cloud to ground.
•Thunder results from rapid heating of air around a bolt of
lightning. Temperature can reach 30,000°C!
•The extreme heat causes air around the lightning to expand
rapidly, then quickly cools and contracts.
•rapid movement of molecules form sound waves heard as
thunder.
• Thunder-snow video, click here
• Hail Video – click here
Weather Patterns: Tornadoes
• Tornado: A violently rotating column of air in contact
with the ground.
• Wind Shear: Wind at different heights blows in different
directions and at different speeds. This can create a rotating
column parallel to the ground.
• Updraft tilts the rotating column upward into the
thunderstorm creating a funnel cloud.
• If the funnel comes into contact with the ground, it is called
a tornado.
• Although tornadoes rarely exceed 200m in diameter and
usually last only a few minutes, they often are extremely
destructive.
Moore Tornado 2013
How a tornado forms!
2011 EF5 Joplin Tornado
Weather Patterns: Hurricanes
• Steered by surface winds, they can travel west, gaining
strength from the heat and moisture of warm ocean water.
• Damage: High winds, tornadoes, heavy rains, lightning,
high waves, and flooding can destroy crops, demolish
buildings, kill people and animals.
• When over water, the warm, moist air rises and provides
energy for the storm.
• When it reaches land, its supply of energy disappears and
the storm loses power.
Weather Patterns: Blizzards
The National Weather
Service classifies a winter
storm as a blizzard if:
1.The winds are 56 km/h.
2.The temperature is low.
3.The visibility is less than
400 m in falling or
blowing snow.
4.These conditions persist
for three hours or more.
Severe Weather Safety
• When severe weather threatens, the National Weather
Service issues a watch or warning.
• Watches are issued when conditions are favorable for
severe thunderstorms, tornadoes, floods, blizzards, and
hurricanes.
• METEROLOGISTS ARE ASKING YOU TO
WATCH AND SEE WHAT WEATHER MAY FORM.
• When a warning is issues, severe weather conditions
already exist.
• THEY ARE WARNING YOU TO GET AWAY
FROM THE DANGEROUS WEATHER!!!
• You should take immediate action.