AIR MASSES & FRONTS - VIDEO

Large body of air with similar temperature and moisture characteristics in any horizontal direction

cover many 1000's of square kilometers
Part of weather forecasting is determining air mass characteristics, how they may be modified, and their movement

AIR MASSES

AIR MASS SOURCE REGIONS - DEFINED

Air masses originate in source regions
source regions:
- flat, uniform composition
- light winds
So, where are the good source regions?

snow covered Artic plains, tropic-subtropic oceans, forests, mountains, large bodies of water

Air masses tend to clash and interact in the middle latitudes.

Air Mass Characteristics

There are two main characteristics that define air masses -

temperature and moisture content.
Generally, the temperature and moisture content of air masses are abbreviated as two letters.
The first letter is a lower case letter and is used to symbolize the overall moisture in the air.
The second letter used to symbolize a type of air mass is capitalized. The capital letter symbolizes the temperature or thermal properties of the air.

AIR MASS CLASSIFICATION - VIDEO

4 general air mass classifications categorized according to the source regions. Source regions should be:

uniform surface composition - flat
light surface winds

polar latitudes P - typically located poleward of 60 degrees north and south
tropical latitudes T - typically located within about 25 degrees of the equator
continental c - located over large land masses--dry
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

four general categories according to source region (see table)
extremely cold cP air is sometimes denoted at cA
extremely hot, humid mT air is sometime denoted by mE

Arctic air is often symbolized with an A. This type of air mass is characterized by extremely cold temperatures. Equatorial air is symbolized with an E. The air is hot because it originates in the equatorial regions.

It is important to remember that the classification system for an air mass is based on source region and not the destination of the air mass. As the air mass moves over Earth, the characteristics can slowly change and the air mass itself will eventually change.

Air Masses on the move:

if an air mass is colder than the surface over which it is moving, "k" is added
if an air mass is warmer than the surface over which it is moving, "w" is added

Example - a cP air mass moving over the great lakes in December becomes cPk

Source Region	Polar	Tropical
Land - continental (c)	cP (cold, dry, stable)	cT (hot, dry, stable air aloft; unstable surface air
Water - maritime (m)	mP (cool, moist, unstable)	mT (warm, moist; usually unstable)

Five Air Masses affect the U.S. during the course of a typical year:

continental
polar
continental arctic
continental tropical
mmaritime polar
maritime tropical

Continental air masses are characterized by dry air near the surface while maritime air masses are moist.
Polar air masses are characterized by cold air near the surface while tropical air masses are warm or hot. Arctic air masses are extremely cold.

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.

Continental polar (cP) or continental arctic (cA) air masses are cold, dry, and stable.

These air masses originate over northern Canada and Alaska as a result of radiational cooling. They move southward, east of Rockies into the Plains, then eastward. Continental polar or continental arctic air masses are marked by surface high pressure, cold temperatures, and low dew points.

Maritime polar (mP) air masses are cool, moist, and unstable.

Some maritime polar air masses originate as continental polar air masses over Asia and move westward over the Pacific, collecting warmth and moisture from the ocean. Some mP air masses originate from the North Atlantic and move southwestward toward the Northeast States. The latter air mass generally is colder and drier than the mP off of the Pacific.

Maritime tropical (mT) air masses are warm, moist, and usually unstable.

Some maritime tropical air masses originate in the subtropical Pacific Ocean, where it is warm and air must travel a long distance over water. These rarely extend north or east of southern California. Some maritime tropical air masses originate over the Gulf of Mexico and Caribbean Sea. They can be associated with fog and low clouds as they moves northward. In the spring and summer, this air mass accounts for the thunderstorms in the Great Plains and elsewhere.

Continental tropical (cT) air masses are hot, dry, unstable at low levels and generally stable aloft (upper-level ridge)

Continental tropical air masses originate in northern Mexico. They are characterized by clear skies and negligible rainfall. If one moves into the Great Plains and stagnates, a severe drought can result.

Air masses can be modified significantly as they pass over regions with different characteristics. When air masses are modified, they are renamed according to their new characteristics.
Topography can play a crucial role in the modification of air masses. For example, the Rocky Mountains cause flow from the west to be lifted over the mountains. The originally mP air loses its moisture as it precipitates, leaving dry air to move eastward. Hence, mP air becomes cP air after it is forced over the Rockies.

Most large-scale weather events occur at the boundary of two or more air masses.

AIR MASSES - SOURCE REGIONS

Source region is the location from which an air mass originates.
Examples of source regions include the icy continental portions of Canada in the winter. As a giant parcel of air moves over this source region, the air will acquire the characteristics of the underlying surface. In this case, the air will be dry (continental) and mild to cold (polar).

The source region in weather usually refers to the location where the different types of air masses formed.
In general, a source region covers several million square miles. The conditions on the surface of Earth are nearly homogeneous. As an example, the ocean is a common source region for the formation of air masses. Air will remain over the ocean for a period of time sufficient enough to alter the properties of that air parcel.
As an air mass stays over a region, it will acquire the properties of that land or ocean region. For instance, in the example above, the air over the ocean would become moist provided it remain in the same general location for an extended period. Uniformity within the air mass is reached in anywhere from several days to several weeks.

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.

FRONTS - click

Fronts are the boundaries between two air masses. Fronts are classified as to which type of air mass (cold or warm) is replacing the other.

For example, a cold front demarcates the leading edge of a cold air mass displacing a warmer air mass.

A warm front is the leading edge of a warmer air mass replacing a colder air mass. If the front is essentially not moving (i.e. the air masses are not moving) it is called a stationary front.

Symbols used to identify fronts on a weather map

Fronts don't just exist at the surface of the earth, they have a vertical structure or slope as well. Warm fronts typically have a gentle slope so the air rising along the frontal surface is gradual. This usually favors the development of widespread layered or stratiform cloudiness and precipitation along and to the north of the front. The slope of cold fronts are more steep and air is forced upward more abruptly. This usually leads to a narrow band of showers and thunderstorms along or just ahead of the front, especially if the rising air is unstable.

Cold fronts typically move faster than warm fronts, so in time they "catch up" to warm fronts. As the two fronts merge, an occluded front forms.
In the occluded front, the cold air undercuts the cooler air mass associated with the warm front, further lifting the already rising warm air.

Fronts are usually detectable at the surface in a number of ways. Winds usually "converge" or come together at the fronts. Also, temperature differences can be quite noticeable from one side of the front to another. Finally, the pressure on either side of a front can vary significantly.

Here is an example of a location that experiences typical warm frontal passage followed by a cold frontal passage: Clouds lower and thicken as the warm front approaches with several hours of light to moderate rain. Temperatures are in the 50s with winds from the east.

As the warm front passes, the rain ends, skies become partly cloudy and temperatures warm into the mid 70s. Winds become gusty from the south. A few hours later, a line of thunderstorms sweeps across the area just ahead of the cold front. After the rain ends and the front passes, winds shift to the northwest and temperatures fall into the 40s and skies clear.

Cold Front the front edge of a moving mass of cold air that pushes beneath a warmer air mass like a wedge.

Warm Front the front edge of advancing warm air mass that replaces cooler air with warmer air.

Occluded Front a front that forms when a cold air mass overtakes a warm air mass and lifts the warm air mass off the ground and over another air mass.