Air Masses and Fronts

Air Masses An air mass is a large body of air with generally uniform temperature and humidity. The area from which an air mass originates is called a "source region."

Origin of Air Masses Air mass source regions range from extensive snow covered polar areas to deserts to tropical oceans. The United States is not a favorable source region because of the relatively frequent passage of weather disturbances. Disturbances disrupt any opportunity for an air mass to stagnate and take on the properties of the underlying region.

The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below. The four principal air mass classifications that influence the continental United States according to their source region are: Polar latitudes - Located poleward of 60° north and south. Continental - Located over large land masses between 25°N/S and 60°N/S. Maritime - Located over the oceans between 25°N/S and 60°N/S Tropical latitudes - Located within about 25° of the equator.

As these air masses move around the earth they can begin to 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).

The Gulf Coast states and the eastern third of the country commonly experience the tropical air mass in the summer. Continental tropical (cT) air is dry air pumped north, off of the Mexican Plateau. If it becomes stagnant over the Midwest, a drought may result. Maritime tropical (mT) air is air from the tropics which has moved north over cooler water. Air mass abbreviation is on the ESRT:

Cyclones and Anticyclones Cyclones and anticyclones are air masses, in the Northern Hemisphere, which differ in air pressure at their center and in the direction in which their winds spiral.

Fronts 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.

Cold Fronts A cold front demarcates the leading edge of a cold air mass displacing a warmer air mass.

Warm Fronts A warm front is the leading edge of a warmer air mass replacing a colder air mass.

Stationary Front If the front is essentially not moving (i.e. the air masses are not moving) it is called a stationary front.

Occluded Fronts In an occluded front, two cold fronts trap a warm front and force it upwards

Here are the symbols used on weather maps to indicate the four types of fronts:

Which kind(s) of fronts?

Structure of Warm Fronts 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.

Structure of Cold Fronts 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 move faster than warm fronts! 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.

How do we detect fronts? Fronts are usually detectable at the surface in a number of ways: Winds usually "converge" or come together at the fronts. Temperature differences can be quite noticeable from one side of the front to another. The pressure on either side of a front can vary significantly.

Lab 15: Weather Patterns

What is the general appearance of the isotherms on this map? How does the temperature change from north to south on this map? Near which cities is the temperature gradient the greatest? Calculate the temperature gradient between Galveston and Kansas City SHOW ALL WORK AND LABEL PROPERLY. Calculate the temperature gradient between Cincinnati and Chicago. SHOW ALL WORK AND LABEL PROPERLY.

What is the general appearance of isobars on this map? The high pressure center is near which city?  What is the highest air pressure on the map?  What is the highest value for an isobar on Map B?  What is the highest value for an isobar on map B?  As you travel from Salt Lake City to Los Angeles, what change in atmospheric pressure would you observe?  Calculate the pressure gradient between Little Rock and Galveston. (SHOW ALL WORK AND LABEL PROPERLY)

What region of the country has the greatest change in wind direction?  Describe the general surface wind pattern around the low pressure area.  Are surface winds around a low pressure area convergent or divergent?  Describe the general surface wind pattern around the high pressure.  Are surface winds around a high pressure area convergent or divergent?

Where is precipitation occuring relative to the continental polar and maritime tropical air masses on this map?  Describe, step by step, how clouds form over a low pressure center.

With respect to the cold front, where does the precipitation occur? With respect to the warm front, where does precipitation occur?  two characteristics that are used to describe an air mass?  Compare the characteristics of an air mass to its source region. Compare the following conditions on either side of the cold front: temperature air pressure wind direction

Answer Keys

Special New York Weather: The Lake Effect Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer lake water. This warms the lower layer of air which picks up water vapor from the lake, rises up through the colder air above, freezes and is deposited on the leeward (downwind) shores.

Lake Effect Snow

The striped areas on the map below show regions along the Great Lakes that often receive large amounts of snowfall due to lake-effect storms. These storms generally develop when cold air moves to the east over warmer lake water cold air moves to the west over warmer land regions warm air moves to the east over colder lake water warm air moves to the west over colder land regions