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AIR MASS CLASSIFICATION

METEOROLOGIST JEFF HABY

The primary air mass classifications are given below:

mT= Maritime Tropical
mP= Maritime Polar
cP = Continental Polar
cT = Continental Tropical
A = Arctic
H = Highland


Maritime Tropical

The maritime tropical air mass is most often felt in the Southeast US. In the winter this air mass is shoved toward the equator but in summer it can cover much of the US east of the Rockies. This air mass results from the warm waters of the Gulf of Mexico and Gulf Stream. The warm waters in this region evaporate an enormous volume of water. Cold water currents tend to stabilize the atmosphere and produce little evaporation while warm waters destabilize the atmosphere and add moisture. The warm waters warm the low levels of the atmosphere. Temperatures in this air mass warm to highs in the 80's and 90's in the summer and the 70's and 80's in winter. High dewpoints characterize mT air. At all times of the year dewpoints are greater than 50 F. The majority of US thunderstorm activity develops within the mT airmass, most being by way of scattered thermodynamic thunderstorms and thunderstorms out ahead of fronts.
As the maritime tropical airmass moves over land it begins to "pick up" characteristics of a continental climate. This is particularly true when the mT airmass moves toward the North. The mT airmass modifies due to lower sun angles, drier land below, and cooler land below.


Maritime Polar

The source region for mP air is over cold ocean currents or high latitude ocean waters. This air does not have the moisture content as mT air. Since mP air is always near saturation, orographic lifting of the air mass can produce widespread rain or snow. This air mass is notorious for producing fog, drizzle, cloudy weather and long lasting light to moderate rain. The temperature of mP air ranges from just above freezing to below 70 F. mP air is modified as it moves over elevated terrain. On the windward side of mountain ranges, mP air can produce an abundance of rain and snow. Once on the lee side of mountains, the mP airmass modifies into a continental airmass. These air masses produce cold fronts but the air is not as cold as polar or arctic fronts. They are often termed "Pacific fronts" or "back-door cold fronts".


Continental Polar

This airmass has low dewpoints, cold temperatures and a high degree of stability. The denseness of cP air creates surface high pressure and a trough aloft, especially when cP air moves into lower latitudes. Precipitation in association with cP air is usually light due to the dryness and low moisture capacity of the air. Precipitation is most common on the "edges" of cP air, especially where it intersects and displaces mT air. Precipitation within a cP air mass is elevated and dynamically induced. These dynamical uplift mechanisms include jet streaks, isentropic lifting and positive differential vorticity advection. Cold surface temperatures and a dry boundary layer inhibit thermodynamic convection.
cP air modifies rapidly as it moves to the South. The dewpoints remain low but the temperature of this airmass increases when moving South due to the following: warmer soil temperature, a shallower airmass, higher sun angles and a lack of surface snow cover. cP air will modify less rapidly if soil temperature are abnormally low to the south (especially if surface snow cover exists). On some occasions the subtropical jet will "overrun" the shallow cP air. If this occurs, the cP air will modify less rapidly due to a much reduced solar heating. Once cP air modifies significantly it no longer makes sense to label it Polar air. After modification, cP air becomes modified cP air or modified mid-latitude continental air.


Continental Tropical

The source region for cT air is the desert Southwest, the high plains and Mexico. The air has low dewpoints and warm to hot afternoon temperatures but with mild nighttime temperature. Due to the buoyancy and elevation of cT air across North America, this air will advect into the mid levels of the atmosphere once it moves out of its source region. This creates a cap of mild dry air. If this air advects over PBL mT air, the severe thunderstorm threat increases significantly. The boundary of cT is most noticeable with the creation of a dryline. A dryline separates mT air from cT air. Depending on the strength of the dryline, convergence along the dryline and the dynamics above the dryline, severe thunderstorms can form near a dryline boundary.


Arctic

The source region for A air is northern Canada. It has the same characteristics as Polar air except it is colder with even lower dewpoints. This air often forms when a high pressure area becomes nearly stationary over Eastern Alaska and the Yukon. Due to a near lack of winter solar radiation, abundant surface snow/ice cover and the continuous emission of radiation from the Earth's surface the air will progressively become colder and colder. Temperatures can reach -30 F to -60 F. If the jet stream becomes meridional during the same time frame Arctic air builds, very cold air will spread into Southern Canada and the US. Once Arctic air moves into the Southern US it modifies to Polar air and then eventually to modified Polar Air behind the cold front boundary.


Highland

This air mass occurs in regions with large elevation changes over short distance. It is not a source region for one particular type of air mass. Since highland climates are in an elevated terrain, they can promote dryness in the interior of the highland climate. When air masses enter a highland climate they modify due to these elevation changes. mP and mT air is dried (on lee-ward side) due to orographic descent. cP air has difficulty entering a highland climate due to the high density of the cP air. Cold dense air has difficulty moving over elevated terrain.


Some important points to keep in mind concerning air masses:

*Surface low pressure and fronts are most often found within the transition zone of air masses
*Fronts occur on the edges of polar air masses
*Cold/ dry air masses are stable due to having a higher density and higher average molecular weight (dry air is more dense than moist air)
*Warm moist air masses are unstable due to a lower density due to thermal expansion and a lower molecular weight.
*Air masses are 3-D. Polar and Arctic air masses become shallower moving away from the source region.
*mT air can isentropicallly lift over mP or cP air creating elevated precipitation. This is especially true north of warm fronts
*Low pressure forces air mass movement. With a strong low pressure, abnormally warm air will be to the SE of the low with abnormally cool conditions to the west of the low.
*The mid-latitudes are unique in that they can experience several different air mass types over the course of a year. Tropical and Polar areas tend to have more uniform weather throughout the year, although the tropics can experience a wet and dry season while the polar elevation temperature depends heavily on the sun angle from season to season.
*Air masses and air mass modification are determined by latitude, altitude, ocean currents, sunshine hours, sunshine angle, vegetation, soil temperatures, snow cover, prevailing wind, etc.


QUESTIONS AND ANSWERS

1. Next to each state, name the top 1 or 2 air masses that dominate the state during a year.

Florida -- maritime tropical is dominant by far
Colorado-- East side - continental polar, modified continental tropical, West side- Highland
Oregon-- Maritime Polar
New Mexico-- Continental Tropical
Ohio-- Many different air masses through the course of a year


2. Why does maritime tropical air tend to be vertically deeper than polar or arctic air?

Answer: Warm air is less dense than cold air. Moist air is less dense than dry air. Warm and moist air has a much greater thickness than cold and dry air and is thus vertically deeper.


3. From Chart #1, how will the cold front over Northern California modify as it moves into Nevada?

Answer: The front will lose its moisture on the windward facing mountain slopes in California. The front will bring in cooler and moist air to coastal regions. As it moves into the Nevada, the front will not have as much of an influence in Nevada as compared to California. The front will not have its moisture in Nevada but will bring in cooler temperatures.


4. Which air mass on average covers the smallest area over the United States?

Answer: Continental Tropical


5. Why does polar air modify most rapidly when it moves over the Gulf of Mexico?

Answer: The Gulf evaporates moisture into the drier air. The ocean surface conducts heat into the polar air and warms it significantly. The Gulf turns the air from dry and cold to more moist and warmer.


6. Why are thunderstorms uncommon with a mP or Arctic air mass? Why are they more common in a maritime tropical air mass?

Answer: Not enough moisture and instability in mP or Arctic air; mT air is more buoyant and has a high moisture concentration.


7. In what way is the continental tropical airmass important for severe weather?

Answer: When cT air is advected into the mid-levels of the atmosphere it creates a capping inversion which traps heat and moisture in the PBL. Once this building heat breaks the cap, explosive convection can occur. The dry mid-level air also promotes a large evaporational cooling potential that results in microbursts. The dry air also promotes convective instability.