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Brief notes on the Origin of Extra tropical Cyclones

The fickleness and variability of weather characteristics caused by atmospheric disturbances that move from west to east in the belt of prevailing westerlies gave an incentive to the weather scientists to make investigations and researches.

NASA - Hurricane Season 2012: Tropical Cyclone Giovanna (Southern ...

Image Source: nasa.gov/images/content/622988main_20120214_Giovanna-MODIS_full.jpg

Fitzroy was pioneer in the field who in 1863 explained that most of the extra tropical cyclones originated because of the interaction of air masses having different physical properties.

His investigations revealed that the subtropical re­gions produced warm and humid air currents, and the cold and dry air streams had their source regions in the polar and Arctic regions.

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It must be appreciated that the concept of the involvement of air masses in the formation of temperate depressions was altogether new, and in a way laid the foundation of further investigations into the causes of their origin and structure.

Those were the days when weather observing stations were few and scattered. There was dearth of a systematic accumulation of observational data.

With the publication of ‘Weather’ by Abercromby in 1887, a new era of scien­tific and systematic investigations began. He prepared a diagram of a model extra tropical cyclone in which weather characteristics observed in its different sectors were depicted.

But the then weather scientists laid greater emphasis on the descriptive analysis of weather disturbances, and the physical processes responsible for their origin were completely ignored.

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In 1911, Shaw and Lempfert took the lead and started the unfinished task of finding out the exact causes of the origin of temperate cyclones along with the lines shown by Abercromby. These investigators accorded a more scientific background to the theories propounded by Fitzroy.

It was toward the end of World War I that the Norwegian meteorologists, V.Bjerknes, his son J.Bjerknes and associates, collected systematic synoptic observations to study the structure of a number of cyclones over Europe.

Their efforts brought about major advances in the understanding of extratropical cyclones. Their investigations and researches resulted in what is popularly known as the polar front theory of cyclones, or simply as wave theory.

It is also called the Bergen theory of the origin of cyclone. Tor Bergeron, an eminent weather scientist of Sweden, by the application of this theory to the analysis of weather maps or synoptic charts, made it all the more popular.

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Even though this theory could provide an adequate explanation of cyclone development, recently more theories for cyclone development such as Baroclinic wave theory have been formulated.

As more and more data from the middle and upper troposphere from satellite photos were made available, some modifications of the polar front theory were necessary. However, even today the Bjerknes model is an accepted working tool in the interpretation of mid-latitude cyclones.

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