Chapter 10Chapter 10

Mid-latitude CyclonesMid-latitude Cyclones

Bergen school - Vilhelm Bjerknes:The Polar Front Theory was postulated in the early

part of the twentieth century to describe the formation,development, and dissipation of mid-latitude cyclones.

Mid-latitude cyclones are large systems that travel great distances and often bring precipitation

and sometimes severe weather to wide areas.Lasting a week or more and covering largeportions of a continent, they are familiar as

the systems that bring abrupt changesin wind, temperature, and sky conditions.

The three-cell model divides the circulation of each hemisphere into

three distinct cells:the heat-driven Hadley cell that circulates air between the Tropics and subtropics,a Ferrel cell in the middle latitudes, and a polar cell.

From Chapter 8

Cyclogenesis is the formation of a mid-latitude cyclone.

Initially, the polar front separates the cold easterlies and the warmer westerlies.

As cyclogenesis begins, a “kink” develops along the boundary.The cold air north of the front

begins to push southward behind the cold front, and air behind the warm front advances northward,

creating a counterclockwise rotation around a weak low-pressure system.

With further intensification, the low pressure deepens even

further and distinct warm and cold fronts emerge from the

original polar front. Convergence associated with the low pressure

can lead to uplift and cloud formation, while linear bands of

deeper cloud cover develop along the frontal boundaries.

Occlusion represents the endof the cyclone’s life cycle

and takes place as the centerof the low pressure pulls backfrom the warm and cold fronts.

The figure depicts the typicalstructure of a mature cyclone

and the processes causing uplift.Shaded areas represent the

presence of cloud cover.The numbers representan approximation of theprecipitation probability.