Download 17-1 Air Masses and Fronts (pages 578–585) Weather Patterns 1

17-1
Air Masses and Fronts (pages 578–585)
Weather Patterns
1. Identify the major types of air masses in
that affect the weather in North America, and
describe how they move.
2. Name the main types of fronts.
3. Explain the type of weather that is
associated with cyclones and anticyclones.
Types of Air Masses (pages 579–580)
1. Identify the major types of air masses in
that affect the weather in North America….
Key Concept: Four major types of air masses
influence the weather in North America:
maritime tropical, continental tropical, maritime
polar, and continental polar.
An air mass is a huge body of air. The air
at any given height in an air mass all has
about the same temperature, humidity,
and air pressure.
A tropical air mass forms in the tropics. A
tropical air mass has warm air. A polar
air mass forms near the poles. A polar air
mass has cold air.
A maritime air mass forms over an ocean.
A maritime air mass has humid air. A
continental air mass forms over land. A
continental air mass has dry air.
A maritime tropical air mass has warm,
humid air. A maritime polar air mass has
cool, humid air.
A continental tropical air mass has hot, dry
air. A continental polar air mass has cold,
dry air.
Air mass
humid
cool
dry
cold
How Air Masses Move (page 581)
1. Identify the major types of air masses
in that affect the weather in North
America, and describe how they move.
Key Concept: In the continental United
States, air masses are commonly moved by
the prevailing westerlies and jet streams.
Air masses move. Global winds usually
move air masses from one part of Earth to
another.
The major wind belts over the United
States are the prevailing westerlies. These
winds usually push air masses from west
to east across the United States.
Jet streams also push air masses across the
United States from west to east.
As air masses move, they bump into each
other. A front is a boundary where two air
masses meet. Storms and other types of
weather develop along fronts.
front
Types of Fronts (pages 582–583)
2. Name the main types of fronts.
Key Concept: Colliding air masses can
form four types of fronts: cold fronts,
warm fronts, stationary fronts, and
occluded fronts.
A cold front forms when a cold air mass
slides under a warm air mass.
Thunderstorms can occur at a cold front.
A warm front forms when a warm air
mass moves over a cold air mass. Light
rain or snow may fall.
Sometimes cold and warm air masses meet, but
neither air mass moves over or under the other.
The two air masses face each other in a
“standoff.” This is called a stationary front. A
stationary front can bring many days of clouds
and precipitation.
A warm air mass sometimes is caught between
two cooler air masses. This forms an occluded
front. Occluded means to be cut off. In an
occluded front, the warm air mass is cut off from
the ground. The weather may turn cloudy, and
rain or snow may fall.
Cyclones and Anticyclones (pages 584–585)
3. Explain the type of weather that is
associated with cyclones and
anticyclones.
Key Concept: Cyclones and decreasing air
pressure are associated with clouds, wind, and
precipitation.
A cyclone is a swirling center of low air
pressure. The winds in a cyclone spiral
around a center. In a cyclone, winds spiral
inward towards the low-pressure center.
The Coriolis
effect makes
winds spin
counterclock
wise in a
Northern
Hemisphere
cyclone.
Air spins upward in a cyclone. The result is
clouds, wind, and precipitation.
Cyclones play a large part in the weather
of the United States.
Key Concept: The descending air in an
anticyclone generally causes dry, clear
weather.
An anticyclone is a high-pressure center of dry
air. An anticyclone is the opposite of a cyclone.
In an anticyclone, winds spiral outward away
from the high-pressure center.
The Coriolis effect makes winds spin
clockwise in a Northern Hemisphere
anticyclone.
The air spins down in an anticyclone. The
result is dry, clear weather.