Air Masses And Where They Come From

Sometimes the weather stays the same for days: humid and hazy, or crisp and dry. Then, in a few hours, everything changes. That change usually marks the arrival of a new air mass. Understanding air masses is the first step toward understanding fronts, the boundaries where much of our active weather happens.

What Is An Air Mass In Weather?

An Air Mass is a huge body of air, often more than 1,500 km (1,000 miles) across and several kilometers deep, with fairly uniform temperature and humidity at any given height. Air masses form when air sits over one kind of surface long enough to take on its character, usually several days to a couple of weeks.

Source Regions

The place where an air mass forms is its Source Region. Good source regions share three traits:

• A large, uniform surface, such as an ocean, a snow-covered plain or a desert.
• Light winds, so air lingers long enough to adjust.
• Often a stationary high pressure system, whose slow, sinking air lets the air mass settle in.

The midlatitudes, including most of the United States, make poor source regions: the surface is varied and storms keep moving air in and out. Instead, the US is a battleground where air masses from elsewhere meet.

How Are Air Masses Classified?

Air masses are named with a two-letter code.

First letter, moisture (where it formed):

• c (continental): formed over land, so it is dry.
• m (maritime): formed over water, so it is moist.

Second letter, temperature (latitude of origin):

• A (arctic): extremely cold
• P (polar): cold
• T (tropical): warm or hot
• E (equatorial): very warm and humid, rarely used in North American analysis

What Are The 5 Air Masses In North America?

Continental Polar (cP)

Continental Polar air forms over central and northern Canada, especially in winter. It is cold, dry and stable. When cP air sweeps south behind a cold front, it brings clear blue skies, low dew points and chilly nights. In summer, cP air is milder and brings the refreshing, low-humidity days that follow a cold front.

Continental Arctic (cA)

Continental Arctic air forms over the Arctic Ocean ice, northern Canada, Alaska and Siberia during the long polar night, when the snow surface radiates heat to space with no sunlight to offset it. It is the coldest air mass on Earth, with surface temperatures of -30 to -50 C. Arctic outbreaks can plunge the Plains below 0 F and send freezes deep into Texas and Florida.

Maritime Tropical (mT)

Maritime Tropical air comes from the Gulf of Mexico, the Caribbean and the subtropical Atlantic. It is warm and very humid, with dew points often in the upper 60s and 70s F. It brings muggy summers east of the Rockies and supplies the moisture for thunderstorms, severe weather and heavy rain. A separate mT source in the subtropical Pacific affects California and the Southwest, notably during the summer monsoon and with atmospheric rivers in winter.

Maritime Polar (mP)

Maritime Polar air forms over the cold waters of the North Pacific and North Atlantic. It is cool and moist. mP air flowing onto the Pacific Northwest brings the region's famous gray skies and drizzle, and heavy snow when it is lifted over the Cascades and Sierra Nevada. Along the East Coast, mP air from the North Atlantic feeds the cold, raw northeast winds of nor'easters.

Continental Tropical (cT)

Continental Tropical air forms over the deserts of northern Mexico and the US Southwest, mainly in summer. It is hot and very dry, with afternoon temperatures that can exceed 110 F and dew points in the 20s or 30s F. When cT air spreads over the southern Plains, it can bring extreme heat and drought. Its sharp boundary with mT air is the dryline, covered later in this unit.

How Do Air Masses Change?

When an air mass leaves its source region, the surface underneath starts changing it, a process called Air Mass Modification.

• Cold air over warm water is heated from below and gains moisture. It becomes unstable, producing cumulus clouds and showers. Lake-effect snow is the classic example: cA or cP air crossing the relatively warm Great Lakes picks up heat and moisture and dumps it downwind.
• Warm air over a cold surface is cooled from below and becomes stable. That can produce widespread fog and low stratus, like the advection fog that forms when mT air glides north over cold ground or cold coastal waters.
• Air crossing mountains loses moisture on the windward side and arrives drier on the lee side.

Meteorologists sometimes add a third letter to describe stability: k for air colder than the surface beneath it (unstable) and w for air warmer than the surface (stable). So cPk is cold polar air heating from below over warmer ground.