Air mass: an extremely large body of air (thousands of square kilometers) whose properties of temperature and moisture are fairly similar in any horizontal direction at any given altitude. The properties of an air mass is determined by the type of surface over which it develops, source region. The source region should be generally flat and of uniform composition, with light surface winds such as ice- and snow-covered arctic plains in winter and subtropical oceans and desert regions in summer. To have a uniform air mass in temperature and moisture, air mass must stay in its source region for several days or weeks.

Air masses are grouped into four general categories according to their source region: i) maritime polar (mP), ii) continental polar (cP), iii) maritime tropical (mT), and iv) continental tropical (cT). An additive fifth air mass, arctic (A), is distinguished from cP by its bitter cold. Maritime air masses show different characteristics in temperature over Atlantic than over Pacific.

Table 11.1, page #289 (Ahrens)

The source region of cT air mass is the subtropical deserts of Mexico and southwestern US primarily in summer, while mT air mass develops over tropical and subtropical oceans. The source region of mP air mass is over cold ocean waters of North Pacific and North Atlantic, while cP air mass develops over the northern interior of North America. Arctic air mass, on the other hand, forms over ice- and snow-covered regions of Siberia, the Arctic Basin, Greenland, and northern North America.

Figure 11.2, page #289 (Ahrens)

Air masses do not remain in their source region indefinitely but move from place to place changing their temperature, humidity, and/or stability. The modification in air mass occurs primarily by i) exchanging heat or moisture, or both, with the surface over which the air mass travels, ii) radiative cooling, and iii) processes associated with large-scale vertical motion.

In winter, when cP air travels southward from Canada into the US, its temperature modifies quickly if the air reaches over snow-free ground which is heated by solar radiation, while the modification of cP air is slow when it travels over snow-covered ground where incoming solar radiation is reflected rather than absorbed and heating the ground.

Figure 11.3, page #290 (Ahrens)

Figure 11.4, page #292 (Ahrens)

When cP air travels over warm Great Lakes or Atlantic water, in addition to temperature, its humidity also modifies leading low-level cloudiness and fog. If the onshore winds are present over Great Lakes, a localized snow-storm forms on downwind side of the lake, known as lake-effect snow. Such storms are common in late autumn and early winter before the lakes become frozen.

Figure 1, page #291 (Ahrens)

Siberian express is an extremely cold Arctic air mass situated over Siberia that crosses over the North Pole and moves into Canada, and plunges into the Great Plains, and southward.

Figure 3, page #294 (Ahrens)

When cool and humid mP air travels inland off the Pacific Ocean, the is forced up the windward slopes of coastal mountain ranges and undergo adiabatic cooling, leading development of clouds and precipitation. As the mP air descends the leeward slopes, clouds dissipate as a results of adiabatic warming. A similar process is repeated when mP air passes up and over the Rockies and it becomes considerably milder and drier that its original air.

Figure 11.6, page #293 (Ahrens)

Figure 11.7, page #293 (Ahrens)

Figure 11.8, page #295 (Ahrens)

The mT and cT air masses do not modify as readily as polar air. When mT air travels northward over a colder ground of the eastern US in winter, it is stabilized leading to suppression the convective activity and is pushed back soutward. When mT air travels over a warmer ground of the eastern US in summer, it becomes warmer and continues its journey northward to Canada. Similar features are true for the cT air.

Figure 11.9, page #295 (Ahrens)

Front: a narrow zone of transition between air masses of contrasting density, that is, air masses of different temperatures or different water vapor concentrations or both. Front is also characterized by an abrupt change in wind direction and speed across its surface, know as wind shift, which is associated with convergence, leading to upward motion, clouds, and perhaps precipitation.

The formation, strengthening (stronger density gradient), or regeneration of a front is called frontogenesis, while the weakening (weaker density gradient) or dissipation of a front is called frontolysis. Precipitation associated with the front tends to increase or diminish in intensity as the front strengthens or weakens.

Front slopes back from the Earth's surface toward colder (denser) air. It intersects the isobars at the surface in a trough region. Each Isobar kinks as it crosses the front.

Frontal fog: a ground-level cloud induced by mixing. It occurs when precipitation falls from relatively warm air into the relatively cool air near the surface and evaporates as increasing water vapor concentration to saturation. Frontal fog is observed just ahead of a warm front or just behind a cold front.

There are four types of fronts: stationary, cold, warm, and occluded. Occluded fronts may be of either the cold-type or the less common warm-type.

Stationary front: a nearly stationary narrow zone of transition between contrasting air masses; winds blow parallel to the front but in opposite directions on the two sides of the front. A stationary front in the high plains separates cold dense cP air of Canada from the milder mP air of North Pacific.

Figure 11.14, page #299 (Ahrens)

Stationary front is often associated with a wide region of clouds and rain or snow on the cold side of the front. The clouds and precipitation result from overrunning, as warm humid air flows upward over the cooler air mass, more or less along the frontal surface, cools through adiabatic expansion which triggers condensation and precipitation.

Warm front: a narrow zone of transition between advancing relatively warm (less dense) air and retreating relatively cold (dense) air. A warm front in the eastern US separates advancing mT air of Gulf of Mexico from retreating cold mP air of North Atlantic.

Figure 11.20, page #304 (Ahrens)

Figure 11.21, page #304 (Ahrens)

As the stationary front, warm front is associated with a broad cloud and precipitation shield that may extent hundred of kilometers ahead of the surface front.

The type of frontal weather depends on the stability of the warmer air. When warm air is stable, a frontal inversion may exist in the upper frontal region, a steady light-to-moderate rainfall or frontal fog is observed in the presence of nimbostratus or stratus clouds, respectively. When the warm air is unstable, brief periods of heavy rainfall are observed in the presence of cumulonimbus clouds.

Cold front: a narrow zone of transition between advancing relatively cold (dense) air and retreating relatively warm (less dense) air. A cold front in New England separates advancing cool mP of Atlantic from retreating warmer cP air of Mid-Atlantic, known as back-door front.

Figure 11.17, page #301 (Ahrens)

Figure 11.15, page #300 (Ahrens)

Over North America, the temperature contrast across a cold front is typically greater than that across stationary or warm front. A cold frontal passage is associated with a sharp temperature drop in winter and a noticeable humidity drop in summer.

The slope of a cold front is steeper (1:50 to 1:100) than the slope of a warm front (1:150). A typical flow aloft across the front is from the cold to the warm side, opposite in stationary or warm front. Low-level air motion in cold front also differs from that in stationary or warm front.

Figure 11.13, page #298 (Ahrens)

Warm Front Features:

Table 11.3, page #306 (Ahrens)

Cold Front Features:

Table 11.2, page #303 (Ahrens)

Occluded front (occlusion): a narrow zone of transition formed when a cold front overtakes a warm front. When air behind the advancing cold front is colder than the air ahead of the warm front, it is called cold-type occlusion. When air behind the advancing cold front is not as cold as the air ahead of the warm front, it is called warm-type occlusion.

The cold-type occlusion is often observed in northern US and southern Canada, while the warm-type occlusion is less common, but often observed in the northerly portions of western coasts, such as in Pacific Northwest.

When an occluded begins to rotate counter-clockwise (in the Northern hemisphere), it sometimes called bent-back occlusion. The point of occlusion where occluded, warm, and cold front meets is called triple point where conditions are favorable for formation of a new cyclone.

Figure 11.22, page #307 (Ahrens)

Dry line: a boundary between warm, dry and warm, humid air in a southeast sector of a mature midlatitude cyclone. It is likely site of a severe thunrderstorm formation.

Figure 4, page #305 (Ahrens)