Chapter 9 Air Masses and Fronts

The areas where air masses form are called source regions The areas where air masses form are called source regions. Based on moisture content, air masses can be considered either continental (dry) or maritime (moist). According to their temperature, they are either tropical (warm), polar (cold), or arctic (extremely cold). Meteorologists use a two-letter shorthand scheme for categorizing air masses. A small letter c or m indicates the moisture conditions, followed by a capital letter T, P, or A to represent temperature.

winter cP air masses are extremely dry. Continental polar (cP) air masses form over large, high-latitude land masses. In addition to having very low temperatures, winter cP air masses are extremely dry. Continental arctic (cA) air is colder than continental polar and separated by a transition zone similar to the polar front called the arctic front.

Maritime polar (mP) air masses are similar to continental polar air masses but are more moderate in both temperature and dryness. Maritime polar air forms over the North Pacific as cP air moves out from the interior of Asia. Maritime polar air also affects much of the East Coast with the circulation of air around mid-latitude cyclones after they pass over a region. The resultant winds are the famous northeasters or nor’easters (above) that can bring cold winds and heavy snowfall.

Continental tropical (cT) air forms during the summer over hot, low-latitude areas. These air masses are extremely hot and dry, and often cloud-free. Maritime tropical (mT) air masses develop over warm tropical waters. They are warm (though not as hot as cT), moist, and unstable near the surface, which are ideal conditions for the development of clouds and precipitation.

A cold front occurs when a wedge of cold air advances toward the warm air ahead of it. A warm front represents the boundary of a warm air mass moving toward a cold one. A stationary front differs in that neither air mass has recently undergone substantial movement. Occluded fronts appear at the surface as the boundary between two polar air masses, with a colder polar air mass usually advancing on a slightly warmer air mass.

In a typical mid-latitude cyclone, cold and warm fronts separated by a wedge of warm air meet at the center of low pressure. Cold air dominates the larger segment on the north side of the system.

The warm air (in red) is blowing toward the northeast. The cold air Cold fronts typically move more rapidly and in a slightly different direction from the warm air ahead of them. This causes convergence ahead of the front and the uplift of the warm air that can lead to cumuliform cloud development and precipitation. In this example, the cold air (in blue) advances from west to east (notice that the wind speed depicted by the thin arrows increases with height). The warm air (in red) is blowing toward the northeast. The cold air wedges beneath the warm air and lifts it upward.

Warm fronts have gentler sloping surfaces and do not have the convex-upward profile of cold fronts. Surface friction decreases with distance from the ground, as indicated by the longer wind vectors away from the surface (a). This causes the surface of the front to become less steep through time (b).

Warm fronts separate advancing masses of warm air from the colder air ahead. As is the case with cold fronts, the differing densities of the two air masses discourage mixing, so the warm air flows upward along the boundary. This process is called overrunning, which leads to extensive cloud cover along the gently sloping surface of cold air.

Nonmoving boundaries are called stationary fronts. Although they do not move as rapidly as cold or warm fronts, they are identical to them in terms of the relationship between their air masses. As always, the frontal surface is inclined, sloping over the cold air.

The most complex type of front is an occluded front or an occlusion, which refers to closure such as the cutting off of a warm air mass from the surface by the meeting of two fronts. When the cold front meets the warm front ahead of it, that segment becomes occluded, as shown above. The warm air does not disappear, but gets lifted upward, away from the surface. The occluded front becomes longer as more of the cold front converges with the warm front.

Eventually, the cold front completely overtakes the warm front, as shown above, and the entire system is occluded. In this occlusion, the air behind the original cold front was colder than that ahead of the warm front. This is an example of a cold-type occlusion.

but both have been pulled back beyond the dashed line. Occlusions sometimes occur when the circular core of low pressure near the junction of the cold and warm fronts changes shape and stretches backward, away from its original position. In (a), the cold and warm fronts are joined at the dashed line. At some later time (b), the cold and warm fronts have the same orientation with respect to each other as they did in (a), but both have been pulled back beyond the dashed line. The circular isobar pattern of (a) becomes elongated to form a trough over the occluded region.

The dryline above (the dashed line) separates low humidity The boundaries separating humid air from dry air are called drylines and are favored locations for thunderstorm development. The dryline above (the dashed line) separates low humidity to the west while to the east humidity is higher as indicated by the dew point temperatures.

The next chapter examines mid-latitude cyclones.