Download Weather Fronts - Hobbs High School

Air Masses and Fronts
Air Masses
• An air mass is an
immense body of air
(1600 km/1000 mi or
more across and
several km/2 mi thick)
that is characterized
by similar
temperatures and
amounts of moisture
at any given altitude.
Movement of Air Masses
• Because of its size, it may take several
days for an air mass to move over an
area, so that area experiences fairly
constant weather during its passage.
• Remember last week’s very cool weather?
We were in a large cold air mass.
How much warmer was the air
mass when it reached Tampico,
Mexico, than when it formed?
• When an air mass
moves out of the
region over which it is
formed, it carries its
temperature and
moisture conditions
with it.
• As it moves, its
characteristics
change and so does
the weather it
causes.
Classifying Air Masses
The area over which an
air mass gets its
characteristic properties
of temperature and
moisture is called its
source region.
• Polar (P) air masses
form at high latitudes
• Tropical (T) air masses
form at low latitudes
Moisture Classification
In addition to their overall temperature, air
masses are classified according to the
surface over which they form and describe
the air mass’ moisture characteristics:
• Continental (c) air masses form over land
and are likely to be dry
• Maritime (m) air masses form over water
and are humid or moist.
Four Basic Types of Air Masses
• Using this
classification,
there are
4 basic
types of
air
masses:
•
(embedded movie)
Continental Polar (cP)
SOURCES: Canada, Alaska, northern US
• Are uniformly cold and dry in winter and
cool and dry in summer
• Along with maritime tropical air
masses, responsible for much of the
weather in North America.
• Can bring heavy snow to the leeward
shores of the Great Lakes (downwind from
the Lakes)
Maritime Polar (mP)
SOURCES: North Pacific, North Atlantic
• Cool, humid, unstable maritime air
• Often starts as cP air over Siberia but
becomes more humid and warmer in its long
journey over the Pacific Ocean
• mP air masses in the Atlantic affect weather
in the northeastern states when New England
is north or northwest of a passing low
pressure center, leading to winter storms
called nor’easters
Continental Tropical (cT)
SOURCES: Mexico, southwestern US
• Hot, dry air masses that usually do not
influence weather outside of their source
region (have the least influence on US
weather)
• In the summer, can cause extremely HOT
and dry conditions in the Great Plains region
• In the fall, when moving north toward the
Great Lakes region, causes warm and mild
conditions called “Indian summer”
Maritime Tropical (mT)
SOURCE: Gulf of Mexico, Pacific off
Mexico
• Warm to hot, very moist, usually unstable
• Along with cP air masses, play a dominant
role in North American weather
• Responsible for most of the precipitation
that falls in the eastern two-thirds of the
US (east of the Rocky Mountains)
• In summer, can bring high temperatures
and oppressive humidity
Weather Fronts
Air Mass Review
We learned that air masses are
• Large (1600 km by several km deep)
• Maintain the temperature and moisture
characteristics of their source regions
• Labeled by their source region
So, what happens when air masses meet?
What is a front?
• A boundary that separates two air
masses
• Very narrow—between 15 and 200 km wide
• Warmer air overlies cooler, denser air
• Both air masses can move at the same
speed and direction, but most often, one
moves faster than the other, advancing into
the slower mass with some mixing of air
Types of fronts
• Classified by the temperature of the
ADVANCING air mass
• Four kinds:
Warm
Cold
Stationary
Occluded
• Cold, dense air moves
into an area occupied by
warm air
• Twice as steep as warm
front, moves faster
• If the warm air is unstable,
violent weather occurs
with cumulonimbus clouds,
heavy downpours and
gusty winds
Cold Front
• Behind the front,
cumulus or
stratocumulus
clouds form where
the cold, stable air
mass dominates.
Warm Front
• Warm air moves into an area
formerly covered by cooler air
• Gradual, low slope
• Cloud sequence: cirrus,
cirrostratus, altostratus, finally
stratus and nimbostratus about
300 km ahead of front
• Moves slowly
• Light-to-moderate
precipitation over an
extended period
• Winds shift from EAST to
SOUTHWEST
Stationary Front
• Sometimes there is a
standoff, and neither
air mass moves
• The resulting surface
front doesn’t move
• Can have gentle to
moderate
precipitation
Occluded Front
• An active cold front
overtakes a warm
front, forcing the
warm air upward so
that it is above the
surface
Mid-Latitude Cyclones
• Apply your new understanding of fronts to weather
patterns in the contiguous US
• The main weather
producers are mid-latitude
cyclones or low pressure
systems that travel from west
to east and cause stormy
weather
• These storms usually have a
cold front and a warm front
associated with them
The mid-latitude cyclone starts with a
frontal boundary, with wind moving
in opposite directions.
Over time, a ‘kink’ or ‘wave’
develops, and warm air moves
poleward and cold air moves
equatorward. This develops into a
counterclockwise rotation with
falling pressure at the ‘kink.’
As the cold air overtakes the warm
air, an occlusion begins,
strengthening the storm. Pressure
at the center of the storm continues to
drop and wind speeds increase.
During the winter, this phase of the storm can bring heavy
snowfalls and blizzard-like conditions.
During the summer, this phase of the storm can bring
severe thunderstorms and tornadoes.
Airflow Aloft Maintains Cyclones
• Cyclones often exist for a week or longer. Remember
that lows are areas of surface convergence, so for
them to stay as storms this long, there must be
divergence aloft that maintains the surface storm.
• Most of the time air aloft is what fuels a mid-latitude
cyclone.
Lastly, where does the diverging air aloft go?
Where does the inflowing surface air come from?
Notice that cyclones (lows) and anticyclones (highs)
occur together.
Like cyclones, anticyclones depend on air aloft to
maintain their circulation.
Reviewing Concepts
Answer these questions on a sheet of paper:
1. What happens when two air masses
meet?
2. Describe a warm front (clouds, precip.,
timing, symbol on weather map)
3. What are the characteristics of a cold
front?
4. What are the stages in the formation of
an occluded front?
5. What is a mid-latitude cyclone?
6. What sustains a mid-latitude cyclone?