Transcript Ch 16 2 Weather

16.2
Weather Patterns
Air Mass

A large body of air that contains properties
similar to the part of the Earth’s surface
where it developed.

Air masses developed over




Land = Dry
Water = Moist
Density, moisture and temperature are properties
of air masses.
A change in weather is due to the movement of air
masses.
Atmospheric Pressure

Changes over the Earth’s surface.




Winds blow from high pressure to low pressure.
High pressure areas experience fair weather
Low pressure areas experience cloudy weather.
Falling temperature means precipitation is
expected.
Fronts


A boundary between two air masses is called
a front.
Types of fronts include cold, warm, occluded,
and stationary.
Cold Front



A cold front occurs when colder air advances
toward warm air.
Cold Fronts are shown on maps using blue
lines and blue triangles.
The triangles point to the direction the cold
front is moving.
Warm Front



Warm fronts occur when a warmer air mass
moves toward an area of colder air.
Warm fronts are shown on weather maps
with the red lines and red semicircles.
The semicircles point in the direction the front
is moving.
Occluded Front



An occluded front involves three air masses
of different temperatures.
An example of how an occluded front forms is
when a cold air mass moves toward cool air
with warm air in the middle.
Occluded fronts are shown using purple lines
with triangles and semicircles.
Stationary Front



A stationary front occurs when a boundary
between air masses stops advancing.
A stationary front boundary is shown by
alternating blue triangles (cold air) and red
semicircles (warm air).
The blue triangles point to the colder air mass
and the red semicircles point to the warmer
air mass.
Thunderstorms



Severe thunderstorms occur when warm air
masses collide with colder air masses.
The warm less dense air is pushed up over the
cooler air mass.
The greater the temperature difference between
the air masses, the more severe the storm can
be.
Lightning


When electric current flows
between regions of opposite
electrical charge, lightning
flashes.
These areas of opposite
electrical charge can occur
within a cloud, between clouds,
or between a cloud and the
ground.
Thunder and Lightning




Thunder results from the rapid heating of air
around a bolt of lightning.
Lightning can reach temperatures of 30,000
degrees Celsius.
The extreme heat causes air to expand
rapidly then cools quickly and contracts.
This rapid movement of air molecules forms
sound waves and thunder is heard.
Tornados




A tornado is a violently rotating
column of air in contact with
the ground.
Winds at different heights blow
in different directions and
different speeds.
This difference, called wind
shear, creates a rotating
column parallel to the ground.
This rotating column is called a
funnel cloud.
Hurricanes



A hurricane is a large,
swirling, area of low
pressure that forms over
the warm Atlantic Ocean.
A hurricane turns heat
energy from the warm
ocean in to wind.
A storm must have wind
speeds higher than 119
kilometers per hour (75
mph) to be called a
hurricane.
Blizzards



Severe storms in winter are called blizzards.
A blizzard occurs if winds are 56 km/h, (35 mph)
the temperature is low, and visibility is less than
400 meters or a quarter of mile.
These conditions must exist for more than three
hours to be classified as a blizzard.
Severe Weather




When severe weather occurs STAY INSIDE.
The National Weather Service issues
watches and warnings.
Watches are issued when conditions are
favorable for severe weather.
Warnings are issued when severe weather
conditions already exist.