Air Masses and Fronts

Air Masses 

Air masses can differ from each other in temperature, moisture, and pressure. These characteristics are generally the result of conditions in the area over which the air mass originates. Global winds such as the jet streams flow between air masses and direct their movement across Earth...Click for Air Masses Explained

Although air masses maintain their individual identities, their movement causes them to interact with each other, and it is at the boundaries between air masses that weather fronts form. The movement of a front over an area brings characteristic changes in temperature, pressure, winds, cloud cover, and precipitation. Air masses are categorized by the aforementioned qualities as Artice (very cold), Polar (cold) or Tropical (warm) and Continental (dry) or Marine (moist)....Click for Air Mass Classifications

Fronts

Cold Front

A cold front forms when a mass of relatively cold air advances toward a mass of warmer air. Because cold air is denser than warm air, it wedges underneath the mass of warm air, forcing the warm air to rise and cool.

 The cold front moves quickly and has a steep edge that creates the rapid uplift needed to form cumulus clouds. A strong cold front may produce cumulonimbus clouds and thunderstorms, tornadoes, or snow squalls. Cold fronts also bring abrupt temperature changes. As a cold front advances, temperatures can drop more than 8°C (15°F) within a couple of hours... 

Warm Front

When a mass of warmer air advances to replace colder air, the boundary is called a warm front. This type of front moves more slowly than a cold front. Warm fronts also have gentle slopes and are associated with less severe weather. As the warm air advances, it gently slides over and replaces the cold air. Typically, the leading edge of the warm front first produces high cirrus clouds. Later, the gradual lifting of the rest of the front produces layered clouds such as cirrostratus, altostratus, and nimbostratus clouds. Because of the gentle slope and slow movement, warm fronts can produce steady precipitation that lasts for days...

On a weather map, fronts are shown as colored lines with markings. Cold fronts are drawn as blue lines with triangles pointing in the direction of movement. Warm fronts are drawn as red lines with scallops that face the direction of movement. Additionally, two other types of fronts may be shown: stationary and occluded. Stationary fronts — where there is little or no movement of air masses — are illustrated with alternating warm- and cold-front symbols. Occluded fronts — where three different air masses meet and keep warm air trapped away from the surface — are shown as purple lines with alternating triangles and scallops.

Frictional drag on the lower levels of the advancing cold air causes the leading edge to buckle, resulting in a relatively steep slope on the cold front. As a result, the fast moving cold air forces the warm air rapidly aloft, and if conditions are right, thunderstorms – even severe thunderstorms – can result. Precipitation occurs in a narrow band just ahead of and just behind the cold front.

At the warm front, the cold air is retreating and the warmer lighter air rides up over it so that the slope of the frontal surface is reversed. Because the surface friction is stronger, the slope is shallower which, in turn, slows the speed of the warm front. Along the gentle slope of the warm front, the overrunning warm air tends to produce stratus-type clouds. Precipitation is less intense along this front and occurs for hundreds of kilometers in advance of the front itself.

Occluded Front

Cold fronts travel much faster than warm fronts. As a result, the cold front overtakes the warm front, forcing the warm air completely aloft, allowing the cold and cool air masses to mix. The warm air aloft gradually cools and stops rising. In the absence of any new influx of warm air, the pressure stops falling and the storm dies. Initially, the boundary between the cold and cool air is called an occluded front, but as the air masses become wellmixed, the occluded front dissipates and a stationary front may form again on the southern boundary of the cold air mass.