Bjerknes’ model of an extratropical cyclone was produced in 1918 after investigating the structure of a large number of cyclones in north-western Europe.
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The construction of the cyclone model was based on the findings of J. Bjerknes and H.Solberg that the basic structures of all the middle-latitude cyclones were similar.
The Norwegian meteorologists found that a cyclone is made up of two air masses with contrasting physical properties (temperature density and moisture content). One of the air masses is of polar origin and consists of colder and drier air. The other opposing air mass is subtropical in origin and consists of warm and moist air.
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The cyclone model helps in understanding and predicting the sequence of weather of the temperate latitudes. Provides a clear picture of the distribution of clouds, and thus the region of possible precipitation associated with an extra tropical cyclone originating on the polar fronts.
Another characteristic feature is that there is lack of continuity in the distribution of temperature, wind and weather. A temperate cyclone takes about 2 to 4 days to pass over a region. During this short period of time abrupt changes in weather conditions are the rule rather than exception.