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The Polar Front Theory of Cyclogenesis

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Instructor: Linda Fye
Learn about The Polar Front Theory of Cyclogenesis. Understand the different stages in midlatitude cyclone development, and the characteristics of cyclones around the planet.

The Polar Front Theory of Cyclogenesis

Every day we all experience different kinds of weather. Have you ever noticed that there's a pattern to those changes in weather? Depending on where you live, there could be different reasons for that, but if you live anywhere in the mid-latitudes, there is a specific weather phenomenon that is the primary cause of your weather changes. This phenomenon is the mid-latitude cyclone and the result of its formation and decline greatly affects mid-latitude weather. So, how does this happen?

The Polar Front Theory of Cyclogenesis was developed in the early 1900s to explain the formation of mid-latitude cyclones. But what is a mid-latitude cyclone and why is it important? A mid-latitude cyclone is a large-scale low pressure system that travels eastward around the planet between 30 and 70 degrees latitude. It is crucial to day-to-day weather changes and to bringing rain and storms to much of the planet. The polar front is the boundary between the cold winds from the poles and the warm winds from closer to the equator. The cold winds blow from the east between 60 and 90 degrees latitude and are called the polar easterlies. The warm winds blow from the west between 60 and 30 degrees latitude and are called the westerlies. These winds cause mid-latitude cyclones.

Cyclogenesis is when a mid-latitude cyclone goes from origin to maturity to dissipation in a series of stages. It occurs when the cold air blowing down from the poles meets warm air blowing up in the mid-latitudes. These winds of different temperatures meet at the polar front in the mid-latitudes. Cold polar air blows from the east, and warm tropical air blows from the west. Normally, the winds blow right past each other in a straight line, the polar front. This is known as the stationary front stage.

However, certain conditions cause the straight, smooth polar front to develop a buckle or wave shape, usually where there is a sharp temperature difference. These conditions include the air passing by mountains or other irregularities in the landscape, or the air passing over a temperature contrast between ocean and land or over ocean currents. This is known as the wave stage. The wave deepens and causes a low pressure area. The warm air from the tropics and cold air from the poles begins to flow counterclockwise around the low pressure, forming a cyclone. This is the open stage, or mature stage, of cyclogenesis. During the mature stage, a very distinct cold front forms at the front edge of the cold polar air. At the cold front, heavy precipitation and cloud formation occur. Also, a well formed warm front develops at the front edge of the warm mid-latitude air. Light precipitation and cloud development form at the warm front. During this time, there is a sector of warm air wedged between cold air on either side.

The mature stage will not last forever, though, mainly because the cold front and air behind it is moving faster than the warm air. Eventually, the cold front catches up to the warm front to begin the partially occluded stage. Occlusion is when the cold front meets the warm front, pinching off the sector of warm air. Because the cold air is denser than warm air, cold air sinks below the warm air, forcing it higher into the atmosphere. This continues until the warm air is pushed completely off of the surface of earth and a short period of heavy precipitation follows. This burst of activity is the occluded stage. Finally, storm activity slows down and weakens and this is the dissipated stage. Eventually, the cyclone gets completely cut off from the polar front and the life cycle of the cyclone ends.

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