<P> The older of the models of extratropical cyclone development is known as the Norwegian cyclone model, developed during and shortly after World War I within the Bergen School of Meteorology . In this theory, cyclones develop as they move up and along a frontal boundary, eventually occluding and reaching a barotropically cold environment . It was developed completely from surface - based weather observations, including descriptions of clouds found near frontal boundaries . Developed from this model was the concept of the warm conveyor belt, which transports warm and moist air just ahead of the cold front above the surface warm front . </P> <P> Polar front theory is attributed to Jacob Bjerknes, derived from a coastal network of observation sites in Norway during World War I . This theory proposed that the main inflow into a cyclone was concentrated along two lines of convergence, one ahead of the low and another trailing behind the low . The convergence line ahead of the low became known as either the steering line or the warm front . The trailing convergence zone was referred to as the squall line or cold front . Areas of clouds and rainfall appeared to be focused along these convergence zones . The concept of frontal zones led to the concept of air masses . The nature of the three - dimensional structure of the cyclone would wait for the development of the upper air network during the 1940s . </P>

The development of a mid-latitude cyclone according to the norwegian model begins along the​