Chapter 11, Air Masses, Fronts, and the Wave Cyclone Model 11-1
11 Air Masses, Fronts, and the
Wave Cyclone Model
11.1 Introduction
An air mass is a large body of air with generally uniform temperature and humidity.
A wave cyclone is a low-pressure circulation that forms and moves along a front.
This chapter will discuss air masses, fronts, and the wave cyclone model.
Chapter 11, Air Masses, Fronts, and the Wave Cyclone Model 11-2
11.2 Air Masses
The area from which an air mass originates is called a source region.
Air mass source regions range from extensive snow -covered polar areas to deserts to tropical oceans. The
United States is not a favorable source region because of the relatively frequent passage of weather
disturbances that dis rupt any opportunity for an air mass to stagnate and take on the properties of the
underlying region. The longer the air mass stays over its source region, the more likely it will acquire the
properties of the surface below.
11.2.1 Air Mass Classification
Air masses are classified according to the temperature and moisture properties of their source regions
(see Figure 11-1).
11.2.1.1 Temperature Properties
• Arctic (A)—An extremely deep, cold air mass that develops mostly in winter over arctic surfaces
of ice and snow.
• Polar (P)—A relatively shallow, cool to cold air mass that develops over high latitudes.
• Tropical (T)—A warm to hot air mass that develops over low latitudes.
11.2.1.2 Moisture Properties
• Continental (c)—A dry air mass that develops over land.
• Maritime (m)—A moist air mass that develops over water.
11.2.1.3 Five Air Masses
When this classification scheme is applied, the following five air masses (see Figure 11-1) may be
identified:
• Continental Arctic (cA)—Cold, dry.
• Continental Polar (cP)—Cold, dry.
• Continental Tropical (cT)—Hot, dry.
• Maritime Polar (mP)—Cool, moist.
• Maritime Tropical (mT)—Warm, moist.
Note: Maritime Arctic (mA) is not listed, since it seldom (if ever) forms.
Chapter 11, Air Masses, Fronts, and the Wave Cyclone Model 11-3
Figure 11-1. Air Mass Classification
11.2.2 Air Mass Modification
As these air masses move around the Earth, they can begin to acquire different attributes. For example, in
winter, an arctic air mass (very cold and dry air) can move over the ocean, picking up some warmth and
moisture from the warmer ocean and becoming a maritime polar (mP) air mass—one that is still fairly cold
but contains moisture. If that same polar air mass moves south from Canada into the southern United States,
it will pick up some of the warmth of the ground, but due to lack of moisture, it remains very dry. This is
called a continental polar (cP) air mass.
The Gulf Coast states and the eastern third of the country commonly experience the tropical air mass in the
summer. Continental tropical (cT) air is dry air pumped north off of the Mexican Plateau. If it becomes
stagnant over the Midwest, a drought may result. Maritime tropical (mT) air is air from the tropics that has
moved north over cooler water.
A warm, moist air mass moving over a cold surface (see Figure 11-2) produces stable air associated with
stratiform clouds, fog, and drizzle.
Figure 11-2. Air Mass Modification—Warm, Moist Air Mass Moving Over a Cold Surface
Chapter 11, Air Masses, Fronts, and the Wave Cyclone Model 11-4
11.2.2.1 Lake Effect
Lake effect is the effect of any lake in modifying the weather near its shore and for some distance
downwind. In the United States, the term is applied specifically to the region around the Great Lakes, and
sometimes the Great Salt Lake in Utah. A lake effect can sometimes generate spectacular snowfall amounts
to the lee side of the Great Lakes. This phenomenon is termed “lake effect snow.”
In autumn and winter, cumuliform clouds and showers often develop in bands over, and to the lee of, large,
ice-free lakes (see Figure 11-3). As initially cold, dry, stable polar air over land flows over the relatively
warm water, the air is heated and moistened, and stability decreases. Shallow cumuliform clouds develop
with low tops. The strength of the convection increases with increasing temperature differences between
warm water and cold air, increasing wind speeds, and decreasing relative humidity within the cold, dry air.
Figure 11-3. Lake Effect
11.3 Fronts
Air masses can control the weather for a relatively long time period ranging from days to months. Most
weather occurs along the periphery of these air masses at boundaries called fronts. A front is a boundary or
transition zone between two air masses. Fronts are classified by which type of air mass (cold or wa rm) is
replacing the other (see Figure 11-4).
Figure 11-4. Fronts
Chapter 11, Air Masses, Fronts, and the Wave Cyclone Model 11-5
Fronts are usually detectable at the surface in a number of ways: significant temperature gradients, or
differences, exist along fronts (especially on the cold air side); winds usually converge, or come together,
at fronts; and pressure typically decreases as a front approaches and increases after it passes.
Fronts do not exist only at the surface of the Earth; they have a vertical structure in which the front slopes
over the colder (denser) air mass.
11.3.1 Warm Front
A warm front occurs when a warm mass of air advances and replaces a body of colder air. Warm fronts
move slowly, typically 10 to 25 mph. The slope of the advancing front slides over the top of the cooler air
and gradually pushes it out of the area. Warm fronts typically have a gentle slope, so the warm air rising
along the frontal surface is gradual (see Figure 11-5). This favors the development of widespread layered
or stratiform cloudiness and precipitation along, and ahead of, the front if the warm rising air is stable.
Generally, prior to the passage of a warm front, cirriform or stratiform clouds, along with fog, can be
expected to form along the frontal boundary. In the summer months, cumulonimbus clouds (thunderstorms)
are likely to develop.
Light to moderate precipitation is probable, usually in the form of rain, sleet, snow, or drizzle, accentuated
by poor visibility. The wind blows from the south -southeast, and the outside temperature is cool or cold
with an increasing dewpoint. Finally, as the warm front approaches, the barometric pressure continues to
fall until the front passes completely.
During the passage of a warm front, stratiform clouds are visible, and drizzle may be falling. The visibility
is generally poor but improves with variable winds. The temperature rises steadily from the inflow of
relatively warmer air. For the most part, the dewpoint remains steady and the pressure levels off. After the
passage of a warm front, stratocumulus clouds predominate , and rain showers are possible. The visibility
eventually improves, but hazy conditions may exist for a short period after passage. T he wind blows from
the south-southwest. With warming temperatures, the dewpoint rises and then levels off. There is generally
a slight rise in barometric pressure, followed by a decrease of barometric pressure.
Figure 11-5. Warm Front
Chapter 11, Air Masses, Fronts, and the Wave Cyclone Model 11-6
11.3.2 Cold Front
A cold front occurs when a mass of cold, dense, and stable air advances and replaces a body of warmer air.
It is so dense, it stays close to the ground and acts like a snowplow, sliding under the warmer air and forcing
the less dense air aloft. Cold fronts have a steep slope, and the warm air is forced upward abruptly
(see Figure 11-6). This often leads to a narrow band of showers and thunderstorms along, or just ahead of,
the front if the warm rising air is unstable. Cold fronts move more rapidly than warm fronts, progressing at
a rate of 25 to 30 mph. However, extreme cold fronts have been recorded moving at speeds of up to 60 mph.
The rapidly ascending air causes the temperature to decrease suddenly, forcing the creation of clouds. The
type of clouds that form depends on the stability of the warmer air mass. A cold front in the Northern
Hemisphere is normally oriented in a northeast to southwest manner and can be several hundred miles long,
encompassing a large area of land.
Prior to the passage of a typical cold front, cirriform or towering cumulus clouds are present, and
cumulonimbus clouds may develop. Rain showers may also develop due to the rapid development of clouds.
A high dewpoint and falling barometric pressure are indicative of an imminent cold front passage.
As the cold front passes, towering cumulus or cumulonimbus clouds continue to dominate the sky.
Depending on the intensity of the cold front, heavy rain showers form and may be accompanied by
lightning, thunder, and/or hail. More severe cold fronts can als o produce tornadoes. During cold front
passage, the visibility is poor with winds variable and gusty, and the temperature and dewpoint drop rapidly.
A quickly falling barometric pressure bottoms out during frontal passage, and then begins a gradual
increase. After frontal passage, the towering cumulus and cumulonimbus clouds begin to dissipate to
cumulus clouds with a corresponding decrease in the precipitation. Good visibility eventually prevails with
the winds from the west -northwest. Temperatures remain cooler and the barometric pressure continues
to rise.
Fast-moving cold fronts are pushed by intense pressure systems far behind the actual front. The friction
between the ground and the cold front retards the movement of the front and creates a steeper frontal
surface. This results in a very narrow band of we ather, concentrated along the leading edge of the front. If
the warm air being overtaken by the cold front is relatively stable, overcast skies and rain may occur for
some distance behind the front. If the warm air is unstable, scattered thunderstorms and rain showers may
form. A continuous line of thunderstorms, or squall line, may form along or ahead of the front. Squall lines
present a serious hazard to pilots as squall -type thunderstorms are intense and move quickly. Behind a
fast-moving cold front, the skies usually clear rapidly, and the front leaves behind gusty, turbulent winds
and colder temperatures.