Air Masses and Fronts Lesson

An air mass is a huge body of air that has similar temperature, pressure, and humidity throughout.

A front is the border between two air masses that collide and humidity is a measure of the amount of moisture in the air.

Cold Front, Warm Front, Occluded Front, Stationary Front

Humidity is a measure of the amount of water vapor in the air.

Relative humidity is the amount of water vapor in the air compared to the amount of water vapor the air could potentially hold.

Air masses form when air stays over a region long enough to take on the temperature and humidity characteristics of that region. It takes a week or more for an air mass to form.

The density of air masses varies with the temperature and humidity of the air. Warm air masses are less dense than cold air masses, and humid air masses are less dense than dry air masses. When air masses with different densities meet, the two masses do not mix. As with oil and water, a distinct boundary forms between the air masses. In the experiment, the oil represented a warm air mass and the colored water a cold air mass. As with the oil and water, warm, less dense air moves over cold, denser air.

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.

When we classify air masses we use two characteristics; temperature and humidity.

Fronts

There are two types of fronts: warm and cold.

When air masses clash a front will form.

Cold fronts happen when a cold air mass pushes a warm air mass up.

Coldfront

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.

Bjerknes

Warm Front

Warm fronts happen when a warm air mass catches up with a cold air mass and the warm air mass stretches up over the cold air mass. Stratus clouds are formed. Warm fronts can produce steady, light rain or snow can occur.

Vilhelm Bjerknes (1862-1951), a Norwegian physicist and meteorologist, coined the term front to describe the boundary between warm and cold air masses. The leading edge of a warm air mass advancing into a region occupied by a cold air mass is called a warm front. A cold front occurs when a cold air mass advances into a region occupied by a warm air mass. If the boundary between the cold and warm air masses doesn't move, it is called a stationary front. The boundary where a cold air mass meets a cool air mass under a warm air mass is called an occluded front. At a front, the weather is usually unsettled and stormy, and precipitation is common.

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. Front Symbols

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