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Oct 30, 2023

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Mid-Latitude Cyclones Air masses and Fronts This lecture discusses mid-latitude cyclones. 1

Mid-Latitude Cyclone A mid-latitude cyclone is a low pressure system that forms and moves in the mid-latitudes, which makes it extratropical. Cold fronts can bring extreme weather in the form of heavy, intense precipitation, hail, thunder and lighting, damaging winds, and even tornadoes. Warm fronts can bring sleet, freezing rain, and fog. Fronts are boundaries between different air masses.

Air Masses • Large-scale masses of air with similar air and dewpoint temperatures Air masses are large bodies of air that have similar air and dewpoint temperatures. They usually form over flat areas with similar surfaces and light winds. That allows the air mass to take on the characteristics of the surface. Air masses are usually about as large as a region or smaller, like the size of the southeastern United States.

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Air Mass Descriptors • Moisture • Moist: maritime (m) • Dry: continental (c) • Temperature (latitude) • Hot: tropical (T) • Cold: polar (P) • Cold: Arctic (A) Air masses with low dewpoint temperatures are continental, forming over land. Air temperatures that are warmer come from the tropics, so warmer air masses are tropical. As temperatures decrease, they are labeled polar or arctic if they are very cold. When we talk about air masses, we use a contraction that identifies the moisture content and air temperatures. Moisture is listed first, then air temperature. For example, an mT air mass is maritime tropical air mass, with high dewpoints and high temperatures. A cP air mass is cold and dry.

Fronts • Boundaries between different air masses Fronts are simply boundaries between two different air masses. A mid-latitude cyclone has attendant fronts that are whipped around the low pressure center. In the northern hemisphere, the circulation around a low pressure system is counterclockwise.

Symbols point in the direction the front is advancing This cold front is advancing to the east Westerly winds behind the cold front When the warm air is advancing, a warm front boundary is formed. When the cold air is advancing, a cold front is formed. If the cold front catches up to the warm front, an occluded front is formed. A mid-latitude cyclone usually starts with a stationary front, where neither air mass is advancing. The symbols for the fronts show the direction that the front is advancing. For example, in the mid-latitude diagram, the cold front is advancing to the east. The front is advancing to the east because of the westerly winds behind the cold front.

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Vertical Profile: Warm Front • Warm air advances • Overruns This is a cross-sectional (horizontal) view mT mP Precipitation Remember, a front is just a boundary between two different air masses. A warm front occurs when the warm air is advancing into a cooler air mass. Typically, the mT air mass is advancing into an mP air mass. Since the warm air mass isn’t dense enough to push the cooler air mass, the warm air overruns, or moves up and over the cooler wedge. This causes more horizontal lift, resulting in stratus clouds. Status clouds aren’t usually able to grow large cloud droplets, so warm fronts usually have light drizzly precipitation. However, the precipitation happens well ahead of the front line.

Vertical Profile: Cold Front • Cold air advances This is a cross-sectional (horizontal) view cP mT Precipitation A cold front occurs when the cold air is advancing into a warmer air mass. Typically, the cP air mass is advancing into an mT air mass. The cP air mass being colder and more dense can cause vertical movement. Tall cumulus clouds can form with enough moisture, and large droplets can grow in the cloud. Cold fronts are associated with heavy, intense precipitation, hail, lighting, and even more severe weather such as flash floods and tornadoes. The precipitation typically occurs just along the cold front boundary.

Occlusion Ahead: similar to ahead warm front Behind: similar to behind cold front Since cold air is more dense, it will also move faster around a mid-latitude cyclone. When a cold front catches up with a warm front, extreme lifting occurs and the storm occludes. Sometimes extreme weather can occur. However, the occlusion marks the end of the mid-latitude cyclone as the air masses start to mix.

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