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MET 1010
Introduction to Weather
Dr. Arturo Rodriguez
Miami Dade College
North Campus
Air masses, Fronts, and Middle-Latitude Cyclones
Chapter 8
Air masses, Fronts, and MiddleLatitude Cyclones
Air masses, Fronts, and Middle-Latitude Cyclones
Air Mass
Air Mass is an extremely large body of air whose properties
of temperature and moisture content (humidity), at any
given altitude, are fairly similar in any horizontal direction.
can cover hundreds of thousands of square miles.
there can be small variations
Source Regions are simply geographic areas where an air
mass originates. Should be:
uniform surface composition - flat
light surface winds
The longer the air mass stays over its source region, the
more likely it will acquire the properties of the surface
below.
Air masses, Fronts, and Middle-Latitude Cyclones
Five basic types of air masses determine
the USA's weather. They can bring
anything from scorching heat to bonechilling cold depending on the type of air
mass. The USA's most violent weather
usually occurs in spring when
continental polar air clashes with
maritime tropical air.
These air masses are:
Air masses, Fronts, and Middle-Latitude Cyclones

Continental Arctic (cA): Extremely cold temperatures
and very little moisture. These usually originate north of the
Arctic Circle, where days of 24 hour darkness allow the air
to cool to sometimes record-breaking low temperatures.
Such air masses often plunge south across Canada and
the USA during winter, but very rarely form during the
summer because the sun warms the Arctic.
 Continental polar (cP): Cold and dry, but not as cold as
Arctic air masses. These usually form farther to the south
and often dominate the weather picture across the USA
during winter. Continental polar masses do form during the
summer, but usually influence only the northern USA.
These air masses are the ones responsible for bringing
clear and pleasant weather during the summer to the
North.
Air masses, Fronts, and Middle-Latitude Cyclones

Maritime polar (mP): Cool and moist. They usually
bring cloudy, damp weather to the USA. Maritime polar
air masses form over the northern Atlantic and the
northern Pacific oceans. They most often influence the
Pacific Northwest and the Northeast. Maritime polar air
masses can form any time of the year and are usually
not as cold as continental polar air masses.
 Maritime tropical (mT): Warm temperatures with
copious moisture. Maritime tropical air masses are
most common across the eastern USA and originate
over the warm waters of the southern Atlantic Ocean
and the Gulf of Mexico. These air masses can form
year round, but they are most prevalent across the
USA during summer. Maritime tropical air masses are
responsible for the hot, humid days of summer across
the South and the East.
Air masses, Fronts, and Middle-Latitude Cyclones
Continental Tropical (cT): Hot and very dry. They
usually form over the Desert Southwest and
northern Mexico during summer. They can bring
record heat to the Plains and the Mississippi
Valley during summer, but they usually do not
make it to the East and the Southeast. As they
move eastward, moisture evaporates into the air,
making the air mass more like a maritime tropical
air mass. Continental tropical air masses very
rarely form during winter, but they usually keep the
Desert Southwest scorching above 100 degrees
Fahrenheit during summer.
Air masses, Fronts, and Middle-Latitude Cyclones
On some maps, you may see a letter "K"
or "W" attached to the two-letter initials
describing an air mass. The "K" means
that the air moving across a region is
colder than the land surface temperature
while "W" indicates that the air is warmer
than the land surface temperature. The
USA's most violent weather usually
occurs during spring when continental
polar air clashes with maritime tropical
air.
Table 8-1, p.203
Fig. 8-2, p.203
Air masses, Fronts, and Middle-Latitude Cyclones
Fronts
Front is the transition zone between air masses
with distinctly different properties.
The differences in density are most often caused
by temperature differences. Separate air masses
with different humidities as well. We identify
fronts by the movement of this transition zone
and the properties that move over a
geographical location. What weather changes do
you expect when TV weather person says a cold
front is moving through the area?
Air masses, Fronts, and Middle-Latitude Cyclones
How do you identify a front on a surface
weather map or by your own weather
observations? Look for:
Sharp temperature changes over a relatively
short distance
Change in moisture content
Rapid shifts in wind direction
Pressure changes
Clouds and precipitation patterns
Air masses, Fronts, and Middle-Latitude Cyclones
Cold fronts
The term "cold front" is one of meteorology's
most misused terms. Many people say "cold front"
when they are really talking about the mass of cold
air that moves in behind the front. In weather, all
fronts are boundaries between masses of air with
different densities, usually caused by temperature
differences.
A cold front is a warm-cold air boundary with the
colder air replacing the warmer.
Fig. 8-13, p.214
Air masses, Fronts, and Middle-Latitude Cyclones
While a winter cold front can bring frigid air,
summer cold fronts often can more accurately
be called "dry" fronts. As anyone who's ever
suffered through a few days of hot, humid air
anywhere east of the Rockies can tell you, cold
fronts are welcome visitors because they often
bring air that might be only a few degrees
cooler, but much less humid.
The weather map symbol for a cold front is a
blue line with triangles pointing the direction
the cold air is moving.
Air masses, Fronts, and Middle-Latitude Cyclones
As a cold front moves into an area, the
heavier, cool air pushes under the lighter,
warm air it's replacing. The warm air cools as it
rises. If the rising air is humid enough, water
vapor in it will condense into clouds and
maybe precipitation.
In the summer, an arriving cold front can
trigger thunderstorms, sometimes severe
thunderstorms with large hail, dangerous
winds and even tornadoes.
Air masses, Fronts, and Middle-Latitude Cyclones
As a cold front arrives in a particular place,
the barometric pressure will fall and then
rise. Winds ahead of a cold front tend to be
from a southerly direction while those
behind the front - in the cooler air - tend to
be northerly. In fact, weather stations use
the shift from a southerly to a northerly wind
direction as the indication that a cold front
has passed the station.
Table 8-2, p.215
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
Warm Fronts
The term "warm front" sounds like something you'd
like to have coming your way on a cold winter's day.
Think again. A warm front is the boundary between
warm and cool, or cold, air when the warm air is
replacing the cold air. That sounds like what you
want. However, warm fronts often bring days of
inclement weather.
Warm fronts often form to the east of low pressure
centers, where southerly winds push warm air
northward.
As the warm air advances northward it rides over the
cold air ahead of it, which is heavier. As the warm
air rises the water vapor in it condenses into clouds
that can produce rain, snow, sleet or freezing rain,
often all four.
Fig. 8-16, p.216
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
The warm front symbol on a weather map
marks the warm-cold boundary at the earth's
surface. The circles on the red line point in the
direction the warm air is moving. As you move
into the cold air the warm-cold boundary is
overhead. The boundary, along with clouds
and precipitation, can stretch hundreds of
miles over the cold air. This is why a slowmoving warm front can mean hours, if not
days, of cloudy, wet weather before the warm
air finally arrives.
Table 8-3, p.217
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
Since warm air is lighter and less dense
than cold air, the cold air ahead of a warm
front at the surface must retreat before
warm air can move in. Sometimes, cold air
is very stubborn and hard to move, which
slows the warm front down and can lead to
several days of wet weather. This happens
often during winter along the East Coast as
cold air banks up against the Appalachian
Mountains. It is commonly referred to as
cold air damming.
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
 Stationary fronts prolong bad weather
A cold front is the boundary between cool
and warm air when the cool air is replacing
the warm air. A warm front is the boundary
when the warm air is winning the battle.
When the pushing is a standoff, the
boundary is known as a stationary front.
Stationary fronts often bring several days of
cloudy, wet weather that can last a week or
more.
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
Since neither the warm air nor the cold air are
advancing, the stationary front weather map
symbols combine both the cold front and the
warm front symbols.
Maps show stationary fronts with alternating
triangles pointing away from the cold air and
half circles pointing away from the warm air.
Color maps alternate the cold front blue and
warm front red.
A weather map's frontal position shows where
the boundary touches the Earth. The boundary
can be thousands of feet above the ground a
couple of hundred miles away from the surface
front.
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
If there's enough humidity in the air,
clouds and precipitation will form as
warm air overruns cool air along a
stationary front.
Sometimes, stationary fronts can stay
stationary or nearly so for days. When
this happens, the sky can stay gray with
rain or snow. Stationary fronts are also
good places for new low pressure areas
to begin and grow into storms.
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
Occluded fronts can signal weakening of storm
Often, in the later stages of a storm's life cycle, a frontal
occlusion occurs.
This happens when the air in the warm sector of the storm
is lifted off the ground.
This can happen in two ways:
 A cold occlusion, which occurs when the air behind the
front is colder than the air ahead of the front. In this
situation, the coldest air undercuts the cool air ahead of the
front and the occluded front acts very similar to a cold front.
 A warm occlusion, which occurs when the air behind the
front is warmer than the air ahead of the front. In this
situation, the cool air is lighter than the coldest air ahead of
the front. As a result, the cool air rises up and over the
coldest air at the surface and the occluded front acts very
similar to a warm front.
Fig. 8-17, p.218
Table 8-4, p.219
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
In both types of occlusions, the occluded front
has well defined vertical boundaries between
the coldest air, the cool air, and the warm air.
Many weather textbooks state that occluded
fronts occur when the cold front catches up
with and overtakes the warm front, but many
scientists disagree. They say that frontal
occlusions occur when storms redevelop
farther back into the cold air. In most cases,
storms begin to weaken after a frontal
occlusion occurs.
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
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Extratropical cyclone
Forms outside the tropics.
Center of storm is colder than the
surrounding air.
Has fronts.
Strongest winds in the upper atmosphere
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
Extratropical storms are major weather makers
From fall through the winter and well into
spring, extratropical storms dominate the
weather across much of the United States
and other parts of the globe outside the
tropics. "Extratropical" means the storms
originate outside the tropics. These storms
move generally west to east across the
oceans and continents
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
The extratropical storm's center is an
area of low atmospheric pressure with
winds going counterclockwise in the
Northern Hemisphere, clockwise south of
the equator. The winds pulls cold air
toward the equator from the polar
regions and bring warm air toward the
poles. The clash of warm and cold air
leads to the widespread precipitation the
storms bring.
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
How tropical and extratropical storms
differ
All storms are areas of low atmospheric
pressure with winds that spiral in toward the
center. Most of the Earth's storms are
"extratropical" because they form outside of
the tropics and have the characteristics
listed below.
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
Tropical storms, as the name says, form
over tropical oceans. Many grow into
hurricanes -- or typhoons if they are west of
the International Date Line in the northern
Pacific Ocean. Tropical storms, hurricanes
and typhoons often move out of the tropics
to hit places in the middle latitudes
Air masses, Fronts, and Middle-Latitude Cyclones Circulation



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Tropical storms:
Form over a tropical ocean.
Center of storm is warmer than the
surrounding air.
Have no fronts.
Strongest winds are near the Earth's surface
Air masses, Fronts, and Middle-Latitude Cyclones Circulation


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Extratropical storms:
Form outside the tropics.
Center of storm is colder than the
surrounding air.
Have fronts.
Strongest winds in the upper atmosphere.
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
Coastal storms can confound forecasters
Intense areas of low pressure, develop off the
East Coast during late fall, winter and early spring.
They usually make life hard for forecasters. The
storms are called "northeasters" because they
usually bring strong northeast winds over the East
as they move north along the Atlantic Coast.
Northeasters often bring heavy rain, heavy snow
and severe coastal flooding to the East.
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
As a storm rapidly intensifies, winds blow
warm air inland from over the relatively warm
Atlantic Ocean water.
At the same time, cold air moves south over
the East Coast. The combination of warm and
cold air can produce snow, sleet, freezing rain
and ordinary rain.
The exact track of the storm's center
determines the dividing line between rain and
snow. If the storm moves over the coast or
inland just east of the Appalachian Mountains,
it will usually push enough warm air inland to
bring rain to the coastal plain, with snow
confined to the mountains and points west of
the mountains.
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
If the storm moves further east over the
Atlantic Ocean, heavy snow can fall
along the coastal plain.
Historically, northeasters with an
easterly track have brought the East
Coast its heaviest snowfalls. If these
storms are unusually intense and
develop quickly they are known as bomb
cyclones
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
Alberta Clippers reinforce cold air
During major cold outbreaks over the USA in the
middle of winter, you might hear forecasters
talking about an "Alberta clippers."
An Alberta clipper is an area of low pressure that
generally forms over Alberta, Canada, east of
the Rocky Mountains. They develop east of the
Rockies because air flowing eastward over the
mountains creates favorable conditions.
Once an Alberta Clipper forms it usually moves
very rapidly to the southeast across the USA's
northern Plains and then to the east off the
mid-Atlantic Coast.
Air masses, Fronts, and Middle-Latitude Cyclones Circulation
Clippers usually cause only light precipitation
with very few producing major snowstorms.
However, if conditions are favorable, some
Alberta clippers can rapidly intensify off the
East Coast once the storm taps the relatively
warm moist air over the Atlantic Ocean.
The storms that rapidly intensify sometimes
spread heavy snow over New England and
southeastern Canada. Generally, the main
weather features associated with Alberta
clippers are some light snow and a
reinforcement of cold air over the USA.