Download The purpose of this module is to introduce air masses. You will be

The purpose of this module is to introduce air masses. You will be able to describe from
where they originate and some basic properties about each air mass affecting the United
States. Not all air masses are the same. When two different types of air masses collide
with each other, different types of weather related events might occur.
Just as any country or state has boundaries, so do air masses. The boundaries of an air
mass are called a front. This module examines fronts, with detailed explanations about
cold fronts and warm fronts.
WHAT IS AN AIR MASS?
Air Masses = uniform bodies of air
An air mass is a large body of air that has similar temperature and moisture properties
throughout. Air masses are extremely large. Judging from the picture below, an air mass
can easily encompass 10 or even more entire states! An air mass takes on the temperature
and humidity content (moisture level) of the area in which it forms. Thus, an air mass
over an ocean area would probably be moister that an air mass formed over a desert.
Likewise, an air mass formed over Canada would probably be a cold air mass, whereas
an air mass formed over Mexico would likely be a warm (and dry) air mass.
Let’s examine some typical air masses.
Maritime Tropical Air Masses = warm temperatures and rich in moisture
The maritime tropical air masses originate over the warm waters of the tropics and Gulf
of Mexico. Here the heat and moisture of waters below are transferred to overlying air
above. The northward movement of tropical air masses transports warm moist air into
the United States, increasing the potential for precipitation.
Continental Polar Air Masses = cold temperatures and little moisture
In contrast, continental polar air masses (cP), originate over the northern plains of
Canada. These air masses are typically transported in a south or southeast direction. Air
masses formed over Canada are obviously cold. Those who live in northern portions of
the United States expect cold weather during the winter months. These conditions usually
result from the invasion of cold arctic air masses that originate from the snow-covered
regions of northern Canada. However since there are no large oceans, or seas in Canada,
these air masses are also considered to be dry air masses, too. When a cold dry air mass
meats a warm humid air mass the result is a lot of rain, snow, or even tornadoes!
Once an air mass moves out of its source region, it is modified as it encounters surface
conditions different than those found in the source region. For example, as a polar air
mass moves southward, it encounters warmer landmasses and consequently, is heated by
the ground below. Air masses typically clash in the middle latitudes (the United States),
producing some very interesting weather.
Since the terrain is generally flat in the central United States with no large mountain
ranges, arctic air masses entering the United States and can easily slide all the way to
Texas and Florida!
WHAT IS A FRONT?
Fronts = the boundaries between air masses
A front is defined as the transition zone between two air masses! As an example in the
diagram below there is a front that exists between the advancing cold air mass and the
retreating warm air mass.
We will examine three basic types of fronts:
Stationary Front
A front that is not moving.
Cold Front
Leading edge of colder air that is replacing warmer air.
Warm Front
Leading edge of warmer air that is replacing cooler air.
HOW ARE FRONT AND AIR MASSES DIAGRAMMED?
But, before we begin the study of fronts, you have to understand the “language” of
weather maps. Pay close attention and you will be able to understand the weather report
on the nightly news and explain it to you parents.

The leading edge (the moving portion) of a cold front is always indicated by a
blue line with forward pointing blue triangles.
Blue line with blue triangles = cold front (ahead of the cold air mass)

The leading edge (the moving portion) of a warm front is always indicated by a
red line with forward pointing red semi-circles.
Red line with red semicircles = warm front (ahead of the warm air mass)

So, in the picture below, the warm air mass is moving up, and the cold air mass is
moving down and to the left – trying to fill in the area behind the warm air mass
WHAT IS A STATIONARY FRONT?
Stationary Front = a front that is not moving
When a warm or cold front stops moving, it becomes a stationary front. A stationary front
is represented by alternating blue and red lines with blue triangles pointing towards the
warmer air and red semicircles pointing towards the colder air.
WHAT IS A COLD FRONT?
Cold Front = transition zone when cold air moves into warm air
A cold front is defined as the “transition zone” where a cold air mass is moving into the
area of a warmer air mass. Cold fronts generally move from northwest to southeast. The
air behind a cold front is noticeably colder and drier than the air ahead of it. When a cold
front passes through, temperatures can drop more than 15 degrees within the first hour!
The map below shows a typical weather map of a cold front. Notice that the cold air is
moving to the east (to the left). Notice that the temperatures behind the cold front are
much cooler then the warm air ahead of it. In the next few hours, the warm air ahead of
the cold front will be replace by that cold air!
There is typically a noticeable temperature change from one side of a cold front to the
other. In the map of surface temperatures below, the station east of the front reported a
temperature of 55 degrees Fahrenheit while a short distance behind the front, the
temperature decreased to 38 degrees. An abrupt temperature change over a short distance
is a good indicator that a front is located somewhere in between.
When colder air is replacing warmer air, then the front is called a cold front. Common
characteristics associated with cold fronts have been listed in the table below.
Weather Change
Temperature
Pressure
Precipitation
From warm humid air to colder less
humid air
The air pressure steadily increases
Short Period of Showers while the
front is passing which could last about
2 to 3 hours
On the other hand, if warmer air is replacing cold air, then the front should be analyzed as
a warm front.
WHAT IS A WARM FRONT?
Warm Front = transition zone from cold air to warm air
A warm front is defined as the transition zone where a warm air mass is replacing a cold
air mass. Warm fronts generally move from southwest to northeast and the air behind a
warm front is warmer and moister than the air ahead of it. When a warm front passes
through, the air becomes noticeably warmer and more humid than it was before.
Symbolically, a red solid line represents a warm front with red semicircles pointing
towards the colder air and in the direction of movement. . Common characteristics
associated with warm fronts have been listed in the table below.
Weather Change
Temperature
Pressure
Precipitation
From cool temps to warmer temps
The air pressure steadily drops
Long Period of Showers while the
front is passing which could last up to
24 hours
HOW DOES PRECIPITATION FROM?
Precipitation Along a Cold Front
(precipitation when a cold air mass overtakes a warmer air mass)
The animation below depicts the development of precipitation ahead of and along a cold
front. The surging blue mass represents colder air behind the cold front (solid blue line)
while the yellow shading indicates the warm moist air mass ahead of the front which is
being moved out of the way.
Animation by: Hall
As the front advances, the colder air lifts the warmer air ahead of it (red arrows). As the
warm air rises, it condenses to produce clouds and precipitation.
.
If condensation of water vapor persists, precipitation may develop, typically in a narrow
band just ahead of the cold front.
Due to the steep slope of a cold front, vigorous rising motion is often produced, leading
to the development of showers and occasionally severe thunderstorms.
Precipitation Along a Warm Front
(precipitation when a warm air mass overtakes a colder air mass)
The animation depicts the development of precipitation ahead of and along a warm front.
The region shaded in blue represents a colder air mass while the yellow shading indicates
the warm moist air behind the warm front (solid red line).
Animation by: Hall
Since warm air is less dense then cold air, the leading edge of the warm front has a
tendency to move up and over the colder air mass ahead of it. As the warm air rides up
and over the cold air mass it produces clouds and rain. The clouds and rain actually
occur many hours before warm air actually arrives to the area. Because the lifting of the
warm air is very gradual and steady, generally widespread and light intensity
precipitation develops ahead of a warm front.
A Closer Examination of the Animation:
Initially, a warm air mass (in yellow) nudges against a colder air mass (in blue) ahead of
it, (separated from each other by the warm front). The lighter warm moist air behind the
front is lifted upward and "overrides" the colder air.
A layer of thin clouds is occasionally observed more than a 400 miles in advance of a
surface warm front. As the front gets closer, the clouds thicken and eventually light
precipitation begins to fall.
Adapted from the works of WW2010. Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign. Credits and Acknowledgments