Air Mass

A large body of air that has a similar temperature and water content or humidity throughout is called an air mass. Air masses have similar temperature and moisture/dryness. When different air masses meet it causes the weather to change.

Fronts

When two different air masses collide a front forms. Typically this is the boundary between cold and warm air masses.

Weather

Weather fronts appear as different colored lines that extend outward from the pressure center. They mark the boundary where two opposite air masses meet.

Warm fronts are indicated by curved red lines with red semicircles.

• Light rain and humid temperatures

Cold fronts are curved blue lines with blue triangles.

• likely to produce thunderstorms

• violent storms generally form

Stationary fronts have alternating sections of red curves with semicircles and blue curves with triangles.

• Neither air mass that meets is stronger, so it doesn't move

• it can stay over one area for a long time

Occluded fronts are curved purple lines with both semicircles and triangles. Weather fronts are found only on surface weather maps.

An air mass is a large body of air with generally uniform temperature and humidity. The area over which an air mass originates is what provides its characteristics. The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below.

There are two broad overarching divisions of air masses based upon moisture content.

Continental air masses, designated by the lowercase letter "c", originate over continents or land and are therefore dry air masses.

Maritime air masses, designated by the letter "m", originate over the oceans and are therefore moist air masses.

Each of the two divisions are then divided based upon the temperature of the surface over which they originate.

• Arctic air masses, designated by the letter "A", originate over the Arctic or Antarctic regions and therefore are very cold.

• Polar air masses, designated by the letter "P", originate over the higher latitudes of both land and sea and are therefore not as cold as Arctic air masses.

• Tropical air masses, designated by the letter "T", originate over the lower latitudes of both land and sea and therefore are warm/hot.

Putting both designations together, we have, for example, a "continental arctic" air mass designated by "cA", which originates over the poles and is therefore very cold and dry.

• Continental polar (cP) is not as cold as the Arctic air mass but is also very dry.

• Maritime polar (mP) is also cold but moist due to its origin over the oceans.

• The desert region air masses (hot and dry) are designated by "cT" for "continental tropical".

• Hot and moist maritime Tropical (mT) air masses develop due to the fact that this air mass develops near the equator over the ocean.

As these air masses move around the Earth, they can acquire additional attributes.

For example, in winter, an arctic air mass (very cold and dry air) can move over the ocean, picking up some warmth and moisture from the warmer ocean and becoming a maritime polar air mass (mP) – one that is still fairly cold but contains moisture. If that same polar air mass moves south from Canada into the southern U.S., it will pick up some of the warmth of the ground, but due to lack of moisture, it remains very dry. This is called a continental polar air mass (cP).