Middle Latitude Cyclones

A middle latitude cyclone (MLC) is a large disturbance that lasts for several days, and is the most significant weather-maker of the middle latitudes. This storm is also known as an extratropical cyclone or a wave cyclone. The term “cyclone” denotes a central low pressure with closed isobars. Middle latitude cyclones, hurricanes, tornadoes, and dust devils are cyclones of various causes, sizes, and strengths. In particular, a middle latitude cyclone is the largest of all cyclones and is a complicated admixture of the polar front jet stream, air masses, and fronts. The MLC is a seminal concept in understanding the middle latitude weather of our planet and was first proposed by Norwegian scientists shortly after World War I.

The origin of an MLC, called cyclogenesis, is directly related to energy imbalances between equator and poles. As such, the strongest MLCs are usually reserved for the winter season when the imbalance is greatest. Near the polar front jet stream, cold, dense air is to the polar side and warm, less dense air is to the tropical side. These air masses do not mix unless they are energized by a disturbance in the polar front jet stream. As such a disturbance passes overhead an air aloft diverges (pulls apart horizontally). This creates a surface low pressure zone into which air converges and rises to make cloudiness and precipitation. However, this is not all: the convergence of air into the surface low makes the polar and tropical air masses push on each other thus creating active fronts. The strength of an MLC cyclone is largely dependent on the upper air flow. If the flow aloft strengthens, divergence is favorable for evacuating air from the surface cyclone and strengthening the storm. If the upper divergence decreases, the surface circulation is damped. The paths of MLCs are readily forecasted because they are steered in the direction of the air in the polar front jet stream.

A MLC experiences several stages as it crosses the United States. At first, tropical and polar air masses encounter each other with neither being the aggressor. An upper disturbance in the polar front jet stream causes a center of low pressure (the cyclone), a cold front, a warm front, and an incipient wave. With the passage of a few hours the MLC forms a large wave and there is a circulation established (a counterclockwise spiral toward the center in the Northern Hemisphere). Within the wave is, typically, a maritime tropical air mass while pushing the cold front along is a continental polar or Arctic air mass. It is at this mature wave stage that the cyclone is strongest and the most severe weather is expected. With the passage of more time, an occluded front starts to form. The occluded front occurs because cold fronts travel faster than warm fronts. As cold air overtakes the maritime tropical air within the wave, the warm air is forced aloft ending the air mass contrast along that part of the front. Occlusion progresses southward from the cyclone’s center in the Northern Hemisphere. As the cyclone occludes its strength abates and all that remains is residual cloudiness and light precipitation. The cyclone has “died” but, before this, much polar and tropical air has mixed and the boundary between the polar and tropical air is established in another position.