Air Masses & Fronts Reading
What is an air mass?
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 large
(hundreds of miles) areas. 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. There are 4 general air mass classifications categorized according to the source region: polar, tropical, continental and marine.
1.) Polar latitudes (P) - located pole-ward of 60 degrees north and south
2.) Tropical latitudes (T) - located within about 25 degrees of the equator
3.) Continental (c) - located over large land masses, dry
4.) Marine (m) - located over the oceans
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
What type of air masses affect the United States?
There are many types of air masses that can affect the U.S., since it is such a large country. Below are a few examples:
cP - wintertime bitter cold can extent to Southern U.S. 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 U.S. 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 U.S. 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 U.S. http://www.weatherwizkids.com/weather-forecasting.htm

What is a front?
A front is a boundary between two different air masses, resulting in stormy weather. A front usually is a line of separation between warm and cold air masses.
How do you identify a front on a surface weather map or by your own weather observations?
Look for: Sharp temperature changes over a relatively short distance, change in moisture content, rapid shifts in wind direction, pressure changes, clouds and precipitation patterns.
What is a cold front?
A cold front is a boundary between two air masses, one cold and the other warm, moving so that the colder air replaces the warmer air. A cold front is represented as a blue line with the teeth pointing toward the direction on movement.
What is a warm front?
A warm front is a boundary between two air masses, one cool and the other warm, moving so that the warmer air replaces the cooler air. A warm front is represented as a red line with half circles pointing toward the direction on movement.
What is a stationary front?
A stationary front is a boundary between two air masses that more or less doesn’t move, but some stationary fronts can wobble back and forth for several hundred miles a day. A stationary front is represented as an alternating warm and cold front symbol.
What is a occluded front?
An occluded front is a combination of two fronts that form when a cold front catches up and overtakes a warm front. An occluded front is represented as a purple line with teeth and half circles. http://www.weatherwizkids.com/weather-forecasting.htm