Geoscience Reference
In-Depth Information
Air Masses and Fronts
The second letter is uppercase and designates the latitude
position of the source region:
Have you ever experienced hot and humid weather for a few
days and then, boom , a strong storm blasted through the area,
and the temperature subsequently dropped sharply? You might
have even talked with your friends about how the weather
changed so fast. What you probably experienced was the pas-
sage of a cold front through your region and the air masses
associated with it. The first air mass was warm with high water
vapor content that could have made it uncomfortable to be out-
side. After the storm occurred, you noticed that a new air mass
dominated the area, one that was relatively cool (maybe even
cold) and dry, with sunny skies. Although you may not have
known why these weather fluctuations occurred, you certainly
noticed the change.
A = Arctic
P = Polar—from between 50° and 60° N or S latitude
T = Tropical—from between 20° and 35° N or S latitude
Using these designations, five principal types of air masses can
be identified that most often affect North America (Figure  8.1).
The most common cold air mass is the continental Polar (cP),
which forms over northern Canada and consists of stable, dry air.
You have probably experienced this type of an air mass when a
cold snap occurred in the middle of January or February. Such
an air mass can be further cooled when it is infused with conti-
nental Arctic (cA) air, which is extremely cold. This kind of air
mass usually originates over Siberia and drifts over the North Pole
on its way south into the United States. An air mass of this kind
makes you run from one building to another because it is so cold.
In northern Minnesota, for example, cA air has produced a record
low temperature of - 43°C ( - 46°F). Although cP air masses are
most intense in winter, they sometimes affect the weather in the
summer. Such an influence is particularly noticeable when it has
been hot and muggy for a few days and then the temperature cools
to perhaps 24°C (75°F) with a very comfortable northwest breeze.
Another dry, stable air mass that influences North America
is the continental Tropical (cT). This air mass forms over places
like Mexico and Arizona and thus is hot and arid. Temperatures
in these air masses can reach 45°C ( 115°F), with low relative
humidity of 40%. If you have ever been to the southwestern
deserts of the United States in the summer, at places such as
Phoenix, Arizona, or Las Vegas, Nevada, you probably expe-
rienced such an air mass that just . . . takes your breath away
because it is so hot.
In contrast to the continental air masses, two maritime air
masses periodically influence parts of the continent. One of these
air masses is the maritime Polar (mP), which consists of cool and
moist bodies of air that form over the northern Pacific Ocean and
northwest Atlantic Ocean. Given the influence of westerly winds
in the midlatitudes, this kind of air has a particularly strong influ-
ence in the Pacific Northwest. If you happen to live there, or have
visited places such as Seattle, Washington, or Portland, Oregon in
the winter, you certainly experienced the cool, damp air associat-
ed with an mP air mass. This air often has temperatures of about
12°C ( 53°F) and relative humidity of 70% to 80%. The other
maritime air mass is the maritime Tropical (mT), which forms
over places like the Gulf of Mexico and is distinctive because it is
warm and moist. You have most likely experienced this air in the
summer when it was hot and sticky outside, with temperatures
of about 33°C ( 95°F) and 60% relative humidity. This was an
mT air mass and was the kind of air that likely kept you in an air-
conditioned building much of the day.
Air Masses
Before we can properly investigate midlatitude circulatory pro-
cesses and how they influence weather, an air mass must first
be defined. This definition is essential because the formation of
midlatitude cyclones depends on the interaction of very large
bodies of air, covering huge geographical regions thousands of
square kilometers in size, that have distinctly contrasting physi-
cal properties.
An air mass is any large body of the lower atmosphere
that has fairly uniform conditions of temperature and mois-
ture. Although the terminology and definition sound like sci-
entific jargon, they actually represent conditions that you have
experienced and probably have noticed. The source region of
an air mass is any large body of land or water from which the
air derives these characteristics. Imagine, for example, that
a maritime air mass moves from the northern Pacific Ocean
to northern Canada, where it stagnates for a couple of weeks
in January. What will happen to that air mass? How will it
change character? From previous discussions, you should re-
alize that the temperature of the air mass will drop while it
is over Canada because the landmass is cold during winter.
In addition, the air mass would lose a lot of its moisture be-
cause cold air holds less water vapor than warm air. If the air
mass then moved south into the United States, perhaps due
to changes in the pressure gradient or the overall circulatory
system, it would be cold and dry by the time it reached where
you live.
Air masses are categorized by a combination of letter
designations. The first letter is lowercase and designates the
moisture source of the air mass:
c = continental (dry)
m = maritime (moist)
Fronts
We have just discussed the character of air masses, the geo-
graphical regions where they originate, and how they move
into North America. Now let's examine how they interact
Air mass A large body of air in the lower atmosphere that has
distinct temperature and humidity characteristics.
 
 
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