Transcript Weather Changes As Air Masses Move

Section 17.1 Notes
Weather changes as air masses move
Air Masses Are Bodies of Air
• Air Mass – A large volume of air in which
temperature and humidity are nearly the same in
different locations at the same altitude.
• An air mass forms when the air over a large
region of Earth sits in one place for many days.
The air gradually takes on the characteristics of
the land or water below it.
• Earths surface can affect the air on Earth.
The Four Types of Air Masses
• Continental air masses form over land. Air
becomes dry as it loses moisture.
• Maritime air masses form over water. Air
becomes moist as it gains water vapor.
• Tropical air masses form near the equator. Air
becomes warm as it gains energy from the
warm land or water below.
• Polar air masses form far from the equator. Air
becomes cool as it loses energy from the cold
land or water below.
The Four Types of Air Masses
Movement of an Air Mass
• Air masses can travel away from the regions
where they form. They move with the global
pattern of winds.
• When an air mass moves to a new region, it
carries along its characteristic moisture and
temperature.
Cold Front
• Front – A boundary
between air masses.
• A cold front forms when
a cold air mass pushes a
warm air mass and
forces the warm air
mass to rise.
• Cold fronts often
produce cumulonimbus
clouds, which can cause
heavy storms.
Cold Front
Warm Front
• A warm front forms when a
warm air mass pushes a cold
air mass and the warm air
rises slowly over the cold air.
• As a warm front approaches,
you may first see high cirrus
clouds, then stratus clouds,
then lower stratus clouds.
• Warm fronts often brings
many hours of steady rain or
snow.
Warm Front
Stationary Front
• A stationary front
occurs when 2 air
masses push against
each other without
moving.
• A stationary front
becomes a warm or
cold front when one air
mass advances.
Stationary Front
Weather Fronts
High Pressure Systems
• Formed when air
moves all the way
around a highpressure center.
• Air sinks and spreads
out towards locations
with low pressure.
• Generally brings clear
skies and calm air or
gentle breezes.
Low Pressure Systems
• A large weather system
that surrounds a center
of low pressure.
• Air circles and moves
upward.
• The rising air produces
stormy weather.
Section 17.2 Notes
Low-pressure systems can become storms
Hurricanes form over warm ocean water
• Tropical storm – a low-pressure system that
starts near the equator and has winds that blow
at 65 km per hour.
• Hurricane – a tropical low-pressure system with
winds blowing at speeds of 120 km per hour.
Formation of Hurricanes
• Hurricanes most often strike between the
months of August and October.
• Ocean water has to be 80oF or higher for a
hurricane to form.
• Tropical storms and hurricanes normally move
west due to the trade winds.
• Hurricanes die out when they move over land.
Formation of Hurricanes
• The center (eye) of a hurricane is calm.
• The eye wall (surrounds the eye) has heavy rain
and tremendous winds.
Effects of Hurricanes
• Hurricanes can lift cars, uproot trees, and tear
the roofs off buildings due to strong winds.
• Heavy rain can also cause severe flooding.
• Storm surge – A rapid rise in water level in a
coastal area that occurs when a hurricane
pushes a high mass of ocean water, often
leading to flooding and widespread destruction.
Winter storms produce snow and ice
• Winter storms are formed when 2 air masses
collide (a cold, dry, dense air mass and a moist
air mass).
• Blizzards – Blinding snowstorms with winds at
least 56 km per hour and low temperatures.
• Lake-Effect Snowstorms – Cold air gains
moisture and warmth as it passes over a lake
and then moves over cold land where it cools
again and snows.
Winter storms produce snow and ice
• Ice Storms – When rain falls onto freezing-cold
ground and covers everything with heavy,
smooth ice.
Section 17.3 Notes
Vertical air motion can cause severe storms
Thunderstorms form from rising moist air
• Thunderstorm – A storm with lightning and
thunder.
• Can form at a cold front or within an air mass.
Formation of Thunderstorms: Step 1
• Rising humid air forms a cumulus cloud. The
water vapor releases energy when it condenses
into cloud droplets. This energy increases the
air motion. The cloud continues building up
into the tall cumulonimbus cloud of a
thunderstorm.
Formation of Thunderstorms: Step 2
• Ice particles form in the low temperatures near
the top of the cloud. As the ice particles grow
large, they begin to fall and pull cold air down
with them. This strong downdraft brings heavy
rain or hail – the most severe stage of a
thunderstorm.
Formation of Thunderstorms: Step 3
• The downdraft can spread out and block more
warm air from moving upward into the cloud.
The storm slows down and ends.
Effects of Thunderstorms
• Flash floods – heavy rainfall in the same area
that can cause rivers to overflow their banks.
• Winds – winds as high as 170 mi/hr that can
knock down entire forests.
• Hail – can wipe out entire fields, damage roofs,
and kill livestock.
• Lightning – damage power lines and equipment,
cause forest fires, and kill people.
Tornadoes form in severe thunderstorms
• Tornado – A violently rotating column of air
stretching from a cloud to the ground.
• More tornadoes occur in North America than
anywhere else in the world.
• Occur when warm air masses meet cold, dense
air and form thunderstorms normally in the
central plains of the United States.
Effects of Tornadoes
• Powerful winds can pick up or destroy
everything in the path of a tornado.
• Most tornadoes are small and last only a few
minutes.
• Larger tornadoes are less common but have
stronger winds and last longer.
• The path of a tornado is unpredictable.