Introduction

We’ve already learned that areas of low and high pressure can be identified based on the isobars on a weather map. We’ve also learned that some regions on Earth typically have low pressure and while others typically have high pressure. In this chapter we’ll learn that some regions of the atmosphere have similar air properties and are named by those properties as a collective mass of air or “air mass”. High pressure systems, especially, are very common air masses.

Air Masses

An air mass is an extensive body of air featuring generally similar temperature and moisture characteristics. They can extend thousands of square kilometers. Air mass are identified based on their temperature and humidity characteristics as well as their geographical region of origin. For example, if an air mass is dry and warm and originated from the tropical region over a continent, it would be called a continental Tropical (cT) air mass. If an air mass is humid and cold and originated over the ocean in the high latitudes, it would be called a maritime Polar (mP) air mass. The air mass name will always begin with a lower-case letter signifying either continental (c) or maritime (m) and a second capital letter signifying Equatorial (E), Tropical (T), Polar (P), Arctic (A), or Antarctic (AA).

Maritime air masses are humid air masses originating from oceans or large bodies of water. Continental air masses are dry air masses originating from land. Equatorial air masses are warm moist air masses originating from the equatorial region. Tropical air masses are warm air masses originating from the lower latitudes. Polar air masses are cold air masses originating from the upper latitudes. Arctic air masses are composed of extremely cold air that originated from the poles. Arctic air masses are even colder and drier than Polar air masses, and Antarctic air masses are even more extreme than Arctic air masses.

The common types of air masses are maritime Tropical (mT), maritime Polar (mP), continental Polar (cP), continental Tropical (cT), and continental Arctic (cA).

• Maritime Tropical (mT) air masses are warm, humid air masses originating from the oceans in the tropics.
• Maritime Polar (mP) air masses are cold, humid air masses originating from the oceans in the polar latitudes.
• Continental Polar (cP) air masses are cold, dry air masses originating from land regions in the polar latitudes.
• Continental Tropical (cT) air masses are hot, dry air masses originating from land in the tropics.
• Continental Arctic (cA) air masses are cold, dry air masses originating from the North Pole.
• Continental Antarctic (cAA) air masses are extremely cold and dry air masses originating from land at the South Pole.

See the below image to visualize where these different types of air masses typically originate.

Creation of Air Masses

Air masses develop when air is present over a surface for an extended period of time. This typically occurs in a high pressure system with light wind. The areas where air masses develop are called source regions. Air masses over warm surfaces usually develop faster than those over colder surfaces because there is weaker turbulence in the stable air over the cold surface. When air masses shift from their source regions, they change over time due to the surfaces and terrain over which the air masses flow.

Movement of Air Masses

Air masses do not remain over their source regions permanently. Slight changes in weather patterns may shift the air mass to a new location. As air masses move, two things can occur. First, as the air shifts over the different surface characteristics, the air mass begins changing. This process is called air mass modification. For example, an mP airmass that moves from the Pacific ocean over the mountains in the western continental US will typically dry as it crosses the mountains, rains out its moisture, and warms over the land surface until it becomes a cT airmass. The second thing that happens when air masses move is that they may collide with other air masses. When a collision occurs, the two air masses develop a boundary called a front.

Surface Fronts

Surface fronts are the boundaries or transition zones between air masses at the Earth’s surface. Changes in temperature, humidity, wind, pressure, visibility, as well as particular cloud and precipitation patterns are often observed at fronts. There are four main types of fronts.

1. Cold fronts
2. Warm fronts
3. Occluded fronts
4. Stationary fronts

Fronts are named based on the characteristics of the air mass that is replacing the prior air mass. For example, if a cold air mass is moving toward a warm air mass, the boundary between them will be called a cold front because the cold air is effectively replacing the warm air from the perspective of a stationary point on Earth’s surface.

Fronts are usually associated with low pressure systems. Frontal boundaries on a map are labeled in the location where the temperature gradient of the front meets the Earth’s surface.

Cold Fronts

Cold fronts are a transition zone in which the advancing cold air mass replaces a retreating warmer air mass. On weather maps, cold fronts are drawn as blue lines with blue triangles pointing toward the warmer air mass, in the direction of frontal movement, as seen in the above image.

Because warm air is less dense than colder air, the cold air stays on the bottom and warm air is forced to rise above the advancing cold air. This forced lifting results in typical cloud patterns ahead of a cold front, which include cirrus and cirrostratus clouds. The number of cumulus clouds in the warm air mass increases as the frontal boundary approaches. Because the warm air mass is forced to rise, atmospheric instability occurs along the cold front and results in towering cumulus and cumulonimbus clouds, which may produce heavy rain and thunderstorms along the frontal boundary. During the passage of a cold front, the wind direction generally shifts from south or southwest (in the warm sector) to west or northwest (in the cold sector) in the northern hemisphere. After a cold front’s passage, fair weather returns with the appearance of cumulus and stratocumulus clouds.

Warm Fronts

Warm fronts are a transition zone in which the advancing warm air mass replaces a retreating colder air mass. On weather maps, warm fronts are drawn as red lines with red semicircles pointing toward the colder air mass in the direction of the frontal movement.

Advancing warm air is forced to rise above the retreating cold dense air. Again, because of this forced lifting, typical cloud patterns are common ahead of a warm front. These include upper-level clouds such as cirrus and cirrostratus clouds before clouds thicken and lower-level clouds like altostratus, nimbostratus, and fog near the frontal boundary. Because the air mass is rising along the warm front, clouds form and steady precipitation may occur. During the passage of a warm front, wind direction shifts from east or southeast (in the cold sector) to southwest (in the warm sector) in the northern hemisphere. After a warm front’s passage, cloud cover and precipitation decreases with only scattered cumulus clouds remaining.

Occluded Fronts

Occluded fronts are a frontal boundary that forms when a cold front catches up to a warm front. Cold fronts move faster than warm fronts, so cold fronts can sometimes catch up to warm fronts, but not the other way around.

There are two types of occlusions: cold occlusions and warm occlusions. Cold occlusions (shown above) occur when the advancing air mass is colder than the retreating air mass. Warm occlusions occur when the advancing air mass is warmer than the retreating air mass. On weather maps, occluded fronts are drawn as purple lines with purple triangles and purple semi circles. You’ll notice that this symbol is a combination of the cold front and warm front symbols, which isn’t surprising because an occluded front is effectively a combination of the two.

Occluded fronts are the final stage of a frontal boundary because warm air above cool air is a stable scenario.

Stationary Fronts

Stationary fronts are a type of frontal system that are almost stationary with the winds flowing nearly parallel and from the opposite paths in each side separated by the front. On weather maps, stationary fronts are drawn as alternating blue and red lines with blue triangles pointing toward the warmer air mass and red semicircles pointing toward the colder air mass. This is the only scenario where the direction of the symbols does not indicate a direction of movement.

Other Frontal-like Features

Not all weather features have frontal characteristics, such as a trough and a dryline. While troughs feature a change in cloud and precipitation pattern, it lacks the sharp change in temperature and moisture as observed in fronts. Drylines are a boundary between warm, moist air and warm, dry air. Because both air masses are warm, a dryline cannot be classified as either a warm front or a cold front. Drylines commonly occur during spring and summer in southwestern United States, particularly in Texas. Warm, humid air from the Gulf of Mexico meets with the warm, dry air from the desert plateau. During the afternoon, convective clouds and even thunderstorms can develop in drylines as moist air rises over the denser dry air.