Meteorology: Understanding the Atmosphere            Ackerman and Knox



An air mass is an extremely large body of air whose properties of temperature and moisture content (humidity), at any given altitude, are fairly similar in any horizontal direction. Air masses can cover hundreds of thousands of square miles.

Air mass source regions are geographic areas where an air mass originates. Source regions should be:

  1. uniform surface composition - flat
  2. light surface winds

The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below.

Classification: 4 general air mass classifications categorized according to the source region.

  1. polar latitudes P - typically located poleward of 60 degrees north and south
  2. tropical latitudes T - typically located within about 25 degrees of the equator
  3. continental c - located over large land masses--dry
  4. marine m - located over the oceans----moist

We can then make combinations of the above to describe various types of air masses.

cP continental polar cold, dry, stable
cT continental tropical hot, dry, stable air aloft--unstable surface air
mP maritime polar cool, moist, and unstable
mT maritime tropical warm, moist, usually unstable

Air masses in the U.S. include

cP -- wintertime bitter cold can extent to Southern US and even Florida causing crop damage. Require long, clear nights, which means strong radiational cooling of air near the surface. A stable air mass. Little moisture added so air is dry

mP -- Winter cP air moves over a region such as the NE Pacific, picking up some warmth and moisture from the warmer ocean. In the case of the Pacific NW mountains force the air to rise (orographic lifting) causing rain.

mT -- wintertime source for the SW US is the subtropical East Pacific Ocean. mT air that influences weather east of the Rocky Mountains comes from the Gulf of Mexico, but only influences winter weather in the SE states. Occasionally, slow moving weather systems in SW flow aloft can draw up moisture at mid and low levels producing precipitation.

cT -- Continental tropical air usually only influences the US in summertime as warm, dry air is pumped up off of the Mexican Plateau. It is usually fairly stable and dry, and if it becomes stagnant over the midwest, results in a drought. Deaths associated with the 1995 heat wave in the midwest were the result of cT and mT air which stagnated over the central and eastern part of the US this last summer.

Air masses can control the weather for a relatively long time period: from a period of days, to months. Most weather occurs along the periphery of these air masses at boundaries called fronts.