Download File

Chapter 6 Weather Patterns
Lesson 3 Air masses and their
Movements
What is an air mass?
An air mass is a huge body of air in the lower
atmosphere that has similar temperature,
humidity and air pressure at any given height.
Types of Air Masses
Four major types of air masses influence the
weather that occurs in North America.
1. Maritime tropical
2. Continental tropical
3. Maritime polar
4. Continental polar
Air masses and North America
The masses that affect North America affect the
weather in the continent.
A tropical air mass is warm and travels from the
tropic with low pressure
A polar is cold and is found in 50 degrees
latitude in North and South in North America
A Maritime form over oceans and a continental
form over land.
How Air Masses Move
When air masses move and interact with other
air masses weather can change.
Air masses are moved by prevailing winds and
jet streams.
Air mass movement in North America
The air masses that affect North America are
moved by the prevailing westerlies.
This is the major wind belt over North America
that moves from West To East.
Air Mass Movement in North America
Jet streams over North America also help to
move these air masses over the continent.
Jet streams are high speed wind bands that
blow high above the atmosphere.
Jet streams over North America also blow
masses from West to East.
Fronts
The boundary between air masses are called
fronts.
When air masses are pushed over the continent
they can encounter other air masses. When this
occurs the collide but sometimes don’t easily
mix.
Main Types of Fronts
Colliding air masses can form four types of
fronts:
1. Cold fronts
2. Warm fronts
3. Stationary fronts
4. Occluded fronts
Cold Fronts
A cold front is created when a cold air mass and
a warm air mass collide.
Cold air sinks and warm air rises pushing the
cold air to flow under the warm air.
As the warm air rises it begins to cool and
expand causing clouds and even rain or snow to
fall.
Cold fronts arrive fast, change weather and can
cause storms.
Warm Fronts
Warm fronts occur when a fast moving warm air
mass overtakes a slow moving cold air mass.
Warm fronts move slow and tend to cause rainy
or cloudy conditions for days. After a warm front
passes the weather tends to be warm and
humid.
Occluded Fronts
An occluded front is more complex. It’s when a
warm air mass is caught between two cooler air
masses. The two cool air masses push the warm
air up and then the cooler air meets and may
mix.
As the warm air rises it condenses, turns into
clouds and may cause precipitation.
Occluded fronts turn weather cloudy, rainy or
snowy.
Stationary Fronts
A stationary front occurs when a warm and cold
air mass meet but cannot over take each other.
The air masses cannot push each other out of
the way and therefore do not move.
Where the cold and warm air mass meet the
water vapor in the warm air condenses into
clouds, rain, snow or fog.
If the stationary front stalls for days, it may
cause cloudy days and precipitation.
Cyclones and Anticyclones
Sometimes when air masses collided the
boundary of the masses, the fronts, can become
distorted because of land features, such as
mountains, or strong winds such as jet streams.
This distortion can cause the wind to twirl , the
swirling air can cause low pressure centers to
form and create cyclones or anticyclones.
Cyclones
A cyclone is a swirling center of low air pressure.
Winds in a cyclone spin inward toward the
center.
In the Northern Hemisphere the Earth’s rotation
deflects winds to the right, so cyclones winds
spin counterclockwise when seen from above.
Cyclones and decreasing air pressure are
associated with clouds, winds and precipitation.
Anticyclones
Anticyclones are high pressure centers of dry air.
Winds in anticyclones move outward from the
center. The descending air in an anticyclone
generally causes dry, clear weather.