CHAPTER 9. Air Masses and Fronts
Chapter Overview:
This chapter details the various types of air masses, their characteristics, and their source regions.
Modification of air masses associated with advection is also described. The characteristics and
effects of cold, warm, stationary, and occluded fronts are also given.
Chapter at a Glance:
Air masses contain uniform temperature and humidity characteristics that may affect areas over
vast distances. Fronts are boundaries between unlike air masses and as such are spatially limited.
Fronts are also inherently linked to mid-latitude cyclones.
• Formation of Air Masses - Surface energy and moisture exchanges cause initial
temperature, pressure, and moisture characteristics in overlying air. Such exchanges,
however, are limited in spatial scope leading to variations in these parameters from place to
place.
A. Source Regions - Source regions are the sites of air mass genesis. When air is
allowed to stagnate over particularly large surface regions, typically those which
are topographically uniform, the overlying air gains surface characteristics. Thus,
an air mass has similar moisture and temperature characteristics reminiscent of its
source region. As the mid-latitudes are quite volatile, most source regions exist in
either high or low latitudes. Air masses are characterized based on their moisture
and temperature characteristics. By convention, moisture considerations are
expressed first and in lower case with air designated as either maritime or
continental. Temperature characteristics follow and are expressed in upper case.
From warmest to coldest, air may be deemed tropical, polar, or arctic. Once
formed, air masses migrate within the general circulation. Upon movement, the air
masses displace residual air over locations thus changing temperature and humidity
characteristics. Further, the air masses themselves moderate from surface
influences.
B. Continental Polar (cP) and Continental Arctic (cA) Air Masses - Wintertime
source regions for continental polar air include northern Canada and Asia. Because
of this, cP air takes on cold, dry characteristics and is inherently stable. During
summer, cP air is warmer and more humid but still cool and dry as compared to
other air masses. Continental arctic air represents extremely cold and dry
conditions as it contains very low water vapor amounts. The boundary between cA
and cP air is the arctic front. <ME9.1>
1. Modification of cP Air Masses - Migrations of cP air induce colder, drier
conditions over affected areas. However, as cP air migrates toward lower
latitudes, it warms from beneath. As it warms, moisture capacity increases
while stability decreases. <ME9.2>
C. Maritime Polar (mP) Air Masses - Maritime polar air masses form over upper
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latitude oceanic regions and are, therefore, cool and moist. Along the west coast of
the United States, mP air affects regions during winter and may be present before
mid-latitude cyclones advect over the continent. Along the east coast, mP air
typically affects regions after cyclone passage as the mP air wraps around the area
of low pressure. Such an occurrence is referred to as a nor’easter <WEB> for the
dominant northeasterly winds. <ME9.3>
D. Continental Tropical (cT) Air Masses - Continental tropical air is mainly a
summertime phenomena exclusive to the desert southwest of the US and northern
Mexico. This air is characteristically hot and very dry owing to its source region.
Because of the extreme thermal qualities, cT air is very unstable, yet clear
conditions predominate due to the very low moisture content. Thunderstorms may
occur when moisture advection takes place or when the air is forced orographically.
<ME9.4>
E. Maritime Tropical (mT) Air Masses - Maritime air masses form over low
latitude oceans and as such are very warm and humid. Because of the temperature
and moisture present in mT air, it is inherently unstable and large thunderstorms are
very common. The Gulf of Mexico provides moisture too much of the eastern US
through advection of mT air. This air may undergo further decreases in static
stability during summer as the air passes over the very warm continent. In such
instances, the high humidity and high heat may be a concern. Further, advection of
mT air also promotes the so-called Arizona monsoon.
• Fronts - Fronts separate unlike air masses. Fronts, therefore, bring about changes in
temperature and moisture conditions as one air mass is replaced by another. There are four
general types of fronts in association with a mid-latitude cyclone with the name reflective
of the advancing air mass.
A. Cold Fronts - When cold air displaces warm air, a cold front results. <CD8.3>
Cold fronts are indicative of heavy precipitation events, rainfall or snow, combined
with rapid temperature drops. Such extreme precipitation events stem from rapid
vertical lifting associated with the steep cold front boundary profile. Because cold
air is dense, it spills across the surface producing the steeply inclined leading edge.
Warm, moist air ahead of the front is forced aloft with great vertical displacement.
<CD8.3> This accounts for large vertical cumulonimbus clouds capable of heavy
precipitation. Such sharp transitions between the colder, drier air behind the front
and the warmer, moister air ahead of the front, can be easily detected on satellite
images and radar composites.
B. Warm Fronts - Warm fronts are created when warm air displaces colder air.
Even though the warmer air advances, it nevertheless is displaced aloft. <CD8.3>
This overrunning process places large amounts of warm, moist air over cooler,
drier air through extensive spatial areas. Shallow horizontal stratus clouds
dominate and bring light precipitation to affected regions. Stable regions above the
warmer air aloft help propagate vertically limited clouds and light precipitation.
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Further, frontal fogs may occur as falling raindrops evaporate in the colder air near
the surface. In a similar situation, but with more extreme temperatures near the
surface, sleet and freezing rain may result.
C. Stationary Fronts - When two unlike air masses remain side by side, with
neither encroaching upon the other, a stationary front exists between them.
Although termed stationary, the fronts migrate slowly. As with other fronts,
warmer air is displaced over colder air. The subjective decision of whether a front
is stationary or not arises from the analysis of many weather charts over time.
Problems may stem from weather stations being spatially separated in addition to
the fact that fronts are zones of transition rather than sharp boundaries. <Web>
<ME9.5>
D. Occluded Fronts - When two fronts meet, the warm air mass between them is
displaced aloft. Such a situation results in an occluded front. This typically occurs
when a cold front closes on, and meets a warm front as it circulates about the low
pressure center of a mid-latitude cyclone. Upon occlusion, cold air will occupy the
surface completely around the low, while warmer air is displaced aloft. A cold-type
occlusion occurs throughout eastern portions of continents where a cold front
associated with cP air meets a warm front with mP air ahead. Such a situation
resembles a cold front with much vertical forcing. A warm-type occlusion is
typical of the western edges of continents where the cold front is associated with
mP air. This air invades an area in which colder cP air is entrenched. The resulting
vertical profile resembles a warm front in that displacement of air aloft occurs
along a shallow slope. <Web>
E. Drylines - Because humidity is an important determinant of air density, air
masses with similar temperatures but strong humidity gradients will act as fronts.
Boundaries between dry and moister air are called drylines. They frequently occur
throughout the Great Plains and are an important contributor to storm development.
Chapter Boxes:
9-1 Special Interest: Maritime Air Masses Invade Eastern North America - Many classic
storms which occurred throughout the US during the winters of 1994, 1995, and 1996 are
compared and contrasted. Isobaric charts and satellite images augment the examples.
<Web> <ME9.1>
9-2 Special Interest: The Pineapple Express - United States west coast precipitation is often
linked to storms which traverse the Pacific Ocean. Since the storms mainly involve mP air,
temperatures are warmer than those that occur in similar situations throughout the central
and eastern US. Also, precipitation is almost always in the form of rainfall except in high
elevations. When average storm tracks are displaced equatorward from normal, higher
than average precipitation occurs throughout southern California. This Pineapple express
accounts for higher overall precipitation but because of higher temperatures, less snowfall
occurs in high elevations.
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CD Rom Unit 8 - Cyclones and Anticyclones:
1. Introduction - The Unit introduction describes cyclones and anticyclones as
being important contributors of weather conditions, and ultimately climate, for
mid-latitude locations.
2. Fundamentals - A map of a surface low centered on the US reiterates concepts
that dictate wind direction and speed (PGF, CF and F). A similar animation
appears in relation to a high pressure center. Profile view diagrams are also
provided demonstrating the concepts of convergence and divergence.
3. Fronts and Cyclones - A two-dimensional map of a cold front is depicted to
detail that fronts are simply boundaries between unlike air masses. An animated
three-dimensional map of a cold front shows how air moves toward a cyclone
center while rising. A similar diagram details air rise and movement in relation to a
warm front.
4. Cyclone/Anticyclone Formation - Profile diagrams of a surface low highlight
surface convergence and divergence aloft. An interactive animation of westerly
flow in the upper atmosphere over a low pressure area allows visualization of
streamline spreading (diffluence). A similar graphic details confluent regions
associated with a high. A map depicts ridges and troughs with the convergent
regions situated west of the trough axis and the convergent region situated east of
the trough axis. The position of the surface high and low is superimposed,
respectively.
5. Cyclones: 3D Flow - A composite view of a mid-latitude cyclone in
three-dimensions is depicted. Interactive animations detail aspects of the dry,
warm, and cold conveyor belts allowing full visualization of the systems, from a
profile and birds-eye view, both separately and in unison.
Related Web Sites:
Nor’easter: http://users1.vastnet.net/mitchd/weatherold.htm
Occluded and Stationary Fronts: www.nws.noaa.gov
Current Front Locations: http://weather.unisys.com/surface/sfc_front.html
Media Enrichment:
ME9.1 - Satellite movie of the Jan. 6-8, 1996 blizzard.
ME9.2 - The perfect storm - a nor’easter that merged with the remnants of Hurricane Grace in
1991.
ME9.3 - Water vapor image from a Pacific Ocean storm.
ME9.4 - A water vapor image over northern Africa.
ME9.5 - Eastern US water vapor image.
Key Terms:
air masses
warm front
fronts
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continental arctic air masses
source regions
occluded front
drylines
stationary front
cold front
maritime tropical
northeasters
overrunning
air masses
continental tropical air masses
maritime polar air masses
continental polar air masses
Review Questions:
1. What are the requirements for an area to serve as a source region?
It must be large and relatively flat with somewhat homogeneous surface temperatures and
moisture characteristics. Air must be allowed to stagnate over these locations as well,
which is why source regions are not usually found in the mid-latitudes.
2. Where are the primary air mass source regions in North America located?
The Gulf of Mexico, the southwestern and northwestern North Atlantic, the southeastern
and northeastern North Pacific, the desert southwest of the US and northern Mexico, and
northern Canada are all source regions for North America.
3. Describe the characteristics of cA, cP, cT, mT, and mP air masses.
All air mass are based upon temperature and humidity characteristics. Air mass humidity
characteristics are defined as either maritime or continental. Maritime air masses have
fairly high humidity values while continental air masses have low humidity values.
Temperature characteristics are defined as being tropical, polar, or arctic as air
temperatures decrease, respectively. Types of air masses are simply combinations of
humidity and temperature characteristics. Types, therefore, include: Continental Polar
(cP), Continental Arctic (cA), Maritime Polar (mP), Continental Tropical (cT), and
Maritime Tropical (mT).
4. Of the five types of air masses, which are the hottest, driest, coldest, and dampest?
Continental tropical is usually the hottest and driest air mass, while mT air masses are
usually the dampest. The coldest, by far, are cA air masses.
5. What is the primary difference between arctic and polar air masses?
Continental Arctic air masses are much colder and drier than polar (either cP or mP) air
masses.
6. Which of the air mass types are likely to be stable or unstable?
Stability is a relative term in many cases. However, the most stable air masses are those
that are cold and/or dry. Overall, the most stable air mass is cA. Continental polar is also
very stable. Maritime polar air masses are less stable than the other two but more stable
than mT and cT. Of the latter, mT is inherently more unstable than cT due to a higher
moisture content.
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7. Describe the changes that occur when a continental air mass migrates out of its source region.
It typically moves equatorward and therefore passes over regions which are warmer with
higher humidities. Therefore, the air mass will undergo modification which will cause it to
warm while moisture is added. The stability of the air mass will decrease with these
modifications.
8. Describe the structure of cold, warm, stationary and occluded fronts.
Fronts are named for the temperature characteristics of the advancing air mass. Cold fronts
indicate that cold air is advancing upon, and displacing, warmer air. Warm fronts are
indicative of warm air advancing upon, and displacing, colder air. Stationary fronts
describe neutral situations in which air masses exist next to one another but neither is being
displaced. Occluded fronts occur when two fronts meet.
No matter which front occurs, warm air is always displaced aloft due to its lower density as
compared to colder and (usually) drier air. Each front is also indicative of various types of
weather. The most violet weather stems from cold fronts as steep frontal boundaries
displace warmer air through rapid vertical lifting. Warm fronts typically produce light
precipitation over extensive spatial areas as a shallow sloping frontal boundary displaces
warmer air aloft. Stationary fronts describe wide zones of transition with warmer air
displaced aloft. Occluded fronts may be described as being either a warm-type, which
closely resembles a warm front situation, or a cold-type, which parallels a cold front
situation.
9. What is overrunning?
Overrunning refers to warm air lifting over colder air in relation to warm front movement.
Overrunning typically causes overcast conditions and light precipitation over a widespread
area.
10. Why do cold fronts have steeper slopes than warm fronts?
This relates to how cold air moves across the surface. Because it is very dense, it tends to
spill across the surface which produces a steeply inclined leading edge.
11. How does the alternative models of the occlusion process differ from the traditional model?
The traditional view of occlusion holds that a cold front sweeping around a low pressure
core catches up to the slower moving warm front. The warm air that occupies the space
between the two fronts, the warm sector, is displaced aloft. The occlusion occurs as the
difference in temperature from one side of the occluded front to the other is minimized as
both regions are occupied by relatively cool air. While such a condition may apply to some
mid-latitude occlusions, others form differently. Occlusions may occur when the low
pressure core, near the cold front warm front junction changes shape and stretches
backwards from its original position. This is revealed in an elongated isobaric pattern,
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which normally would be circular. In other cases, the cold front moves eastward relative to
the warm front so that the junction between them also progresses eastward, thus creating an
occlusion. A final way involves an upper level frontal zone overtaking and joining with a
surface front.
12. What is the difference between warm-type and cold-type occlusions?
A warm-type occlusion usually occurs along the western edges of continents where the
cold front is associated with mP air. This air invades an area in which colder cP air is
entrenched. The resulting vertical profile resembles a warm front in that displacement of
air aloft occurs along a shallow slope. Cold-type occlusions resemble a cold front as cP air
moves into a region of mP air. This situation produces a steep vertical displacement of the
mP air which resembles a cold front. This usually occurs along eastern continent locations.
13. What are drylines and why are they important?
Drylines are simply density fronts. These boundaries separate dry and moist air masses.
They are important primarily in the creation of volatile weather as moist air is lifted in
advance of denser, dry air.
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