An occluded front represents a complex atmospheric boundary where multiple air masses converge, leading to intricate weather patterns. This meteorological phenomenon involves a dynamic interaction where a cold air mass overtakes a warm air mass, lifting it away from the Earth’s surface. Understanding the specific types of air masses involved is important for comprehending the varied weather conditions that can result from these fronts.
Understanding Air Masses
An air mass is a vast body of air that exhibits relatively uniform temperature and humidity. These properties are acquired from the region over which the air mass originates, known as its source region. If an air mass remains over a particular area for an extended period, it adopts the temperature and moisture qualities of that surface.
Air masses are broadly classified based on their temperature and moisture content. Temperature classifications include polar (cold), tropical (warm), and arctic (extremely cold). Moisture classifications distinguish between continental air masses, which are typically dry because they form over land, and maritime air masses, which are moist due to their oceanic origin. Combining these characteristics allows for descriptions such as continental polar (cP), denoting a cold, dry air mass, or maritime tropical (mT), indicating a warm, moist air mass.
Formation of Occluded Fronts
An occluded front forms when a faster-moving cold front effectively catches up to and overtakes a slower-moving warm front. This process typically occurs in the mature stages of a mid-latitude cyclone, marking a significant phase in the storm’s evolution. As the cold front advances, its denser, colder air wedges underneath the less dense, warm air mass of the warm front.
This interaction forces the warm air mass to lift completely off the ground, becoming separated from the surface. The point where the cold front merges with the warm front at the surface creates the occluded front. This lifting leads to complex atmospheric layering.
Air Masses in Cold Occlusions
A cold occlusion occurs when the air mass behind the advancing cold front is colder than the cool air mass located ahead of the warm front it is overtaking. Three distinct air masses are involved in a vertical arrangement. The foremost air mass is a cool air mass, which was originally ahead of the warm front.
The second air mass is the warm air mass, which is lifted aloft as the cold front approaches. The third and coldest air mass is found behind the cold front, and it is this very cold air that undercuts both the cool air ahead and the lifted warm air. This type of occlusion is commonly observed in continental interiors.
Air Masses in Warm Occlusions
In contrast, a warm occlusion forms when the air mass behind the cold front is less cold than the cool air mass situated ahead of the warm front. Here, the temperature gradient between the air masses behind the cold front and ahead of the warm front is reversed compared to a cold occlusion. The three air masses present are the colder air mass ahead of the original warm front, the warm air mass being lifted, and the less-cold air mass that constitutes the advancing cold front.
The less-cold air mass behind the cold front rides up and over the colder air mass that was initially ahead of the warm front. The warm air mass is still lifted aloft. This type of occluded front is more frequently encountered in maritime climates.