Air Masses and Fronts
Air Masses
The weather of the United States east of the Rocky Mountains is dominated by large masses of air that travel south from
the wide expanses of land in Canada, and north from the warm waters of the Gulf of Mexico. Not surprisingly, the term
air mass means a large body of air (sometimes as large as one million square miles!) that can be characterized by their
temperature and humidity. Such large bodies of air form over large portions of the surface of the Earth, and we use the
term source region to describe the areas of the Earth over which large air masses form.
The diagram below shows the major source regions and air masses for the United States:
In particular, the weather in the midwest and eastern US are predominantly influenced by two of these air masses: the
cold, dry air mass coming from Canada, and the warm, moist air mass that forms over the Gulf. The cold, dry Candian air
mass is an example of a cP air mass, standing for continental (dry) polar (cold). The warm, moist air that forms over the
Gulf and generates almost all of the precipitation over the Eastern 2/3 of the United States is an example of an mT air
mass, meaning maritime (moist) Tropical (warm).
Air Mass
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:
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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.
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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.
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.
Fronts
Fronts are the boundaries between two air masses. Fronts are classified as to which type of air mass (cold or warm) is
replacing the other. For example, a cold front is the leading edge of a cold air mass displacing a warmer air mass. A warm
front is the leading edge of a warmer air mass replacing a colder air mass. If the front is essentially not moving (i.e. the
air masses are not moving) it is called a stationary front. Cold fronts typically move faster than warm fronts, so in time
they "catch up" to warm fronts. In weather, all fronts are boundaries between masses of air with different densities,
usually caused by temperature differences.
Cold Fronts
A cold front is where a cold air mass is pushing into a warmer air
mass. Cold fronts can produce dramatic changes in the weather.
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.
They move fast, up to twice as fast as a warm front. Cold air is dense so it is able to quickly plow a warm air mass ahead
of it. 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.
If enough moisture exists in the atmosphere at this time, then clouds and possibly thunderstorms could develop.
Cold fronts are usually accompanied with areas of low pressure. Winds blow counter-clockwise around an area of low
pressure in the Northern Hemisphere. With this in mind, the wind direction before a cold front passes would usually be
out of the south or southwest with warm air temperatures. After the cold front passes, the winds would shift to out of
the west or northwest and the air temperature would steadily drop.
As a cold front arrives in a particular place, the barometric pressure will fall and then rise, and commonly, when the cold
front is passing, winds become gusty; there is a sudden drop in temperature, and heavy rain, sometimes with hail,
thunder, and lightning. Lifted warm air ahead of the front produces cumulus or cumulonimbus clouds and
thunderstorms. Atmospheric pressure changes from falling to rising at the front. After a cold front moves through your
area you may notice that the temperature is cooler, the rain has stopped, and the cumulus clouds are replaced by
stratus and stratocumulus clouds or clear skies.
On weather maps, cold fronts are shown as blue lines with triangles pointing in the direction of the motion of the cold
air mass:
Warm Front
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.
A warm front is where a warm air mass is pushing into a colder air mass. Warm fronts move more slowly than cold fronts
because it is more difficult for the warm air to move against the cold, dense air. 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.
You will often see high clouds like cirrus, cirrostratus, and middle clouds like altostratus ahead of a warm front. These
clouds form in the warm air that is high above the cool air. As the front passes over an area, the clouds become lower
and rain is likely. There can be thunderstorms around the warm front if the air is unstable.
Precipitation increases over an area as the front approaches. Winds ahead of the approaching warm front will be out of
the south or southeast in the Northern Hemisphere. After the warm front has passed, the winds will shift to out of the
south or southwest and the air temperatures will have warmed and leveled off. Behind the warm front is a warm sector,
where the air mass is usually relatively warm and moist. Severe storms could possibly be located in this warm sector that
is ahead of the approaching cold front if the air mass is unstable.
We can understand the basic weather patterns associated with a warm front by considering the following crosssectional view:
Occluded Fronts
Occluded fronts form when a cold front catches up to a warm front because cold fronts move along much faster than
warm fronts do. The cold front over takes the warm front and cold air moves into cooler air ahead of the warm front.
Occluded fronts are represented on weather maps as alternating warm and cold front symbols in purple pointing in the
direction the front is moving.
Occluded fronts usually form around areas of low atmospheric pressure. Occluded fronts are not particularly common
in the southeast US. The low pressure centers may begin in the south, but will typically move to the northeast and form
an occlusion farther north. The precipitation patterns for an occluded front typically follow the same patterns as a cold
front. Temperature will vary after the occluded front passes depending on the different air masses that are interacting
with the occlusion.
There is often precipitation along an occluded front from cumulonimbus or nimbostratus clouds. Wind changes direction
as the front passes and the temperature changes too. The temperature may warm or cool. After the front passes, the
sky is usually clearer and the air is drier.
On a weather map, like the one on the left, an occluded front looks like a purple line with half triangles and half
semicircles along it pointing in the direction that the front is moving. It ends at a low pressure area shown with a large ‘L’
on the map, and at the other end connects to cold and warm fronts.
Figure E is an example of an occluded front. The
colder, drier air is moving in to replace the cooler air
above the warm front. The cooler air will rise up over
the colder, drier air and the front will move in the
direction the purple symbols are pointing.
Figure E
Image from University of Illinois
Stationary Fronts
A stationary front is the boundary between two different air masses with different densities when neither is advancing.
The air masses are not moving strongly enough to overtake each other, so the front remains in one place. Winds along a
stationary front are usually parallel to the front blowing in opposite directions on either side which is one reason why
the front does not move. Stationary fronts essentially have no surface movement, or very slow movement of less than 5
mph at the surface. However, warm air from the warm air mass can still be moving over the top of the colder air farther
up in the atmosphere.
When one of the air masses moves into the other air mass, the stationary front will begin to move. When this transition
occurs, the front will then either be classified as a warm front or a cold front, depending on which air mass is pushing
ahead.
Along some stationary fronts, the weather is clear to partly cloudy. If both of the air masses are relatively dry, then no
precipitation will fall. However, in some stationary fronts where there is a lot of water vapor in the warmer air mass,
significant rain or freezing rain can occur. Some of the worst ice storms and flooding in the Southeast occur in these
conditions because rain falls continuously over an area for several days as the front stays in one place.
Because a stationary front marks the boundary between two air masses, there are often differences in air temperature
and wind on opposite sides of it. The weather is often cloudy along a stationary front and rain or snow often falls,
especially if the front is in an area of low atmospheric pressure.
On a weather map, a stationary front is shown as alternating red semicircles and blue triangles like in the map at the
left. The blue triangles point in one direction and the red semicircles point in the opposite direction