The older of the models of

extratropical cyclone Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of p ...

development is known as the Norwegian cyclone model, developed during and shortly after

World War I World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...

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.Shaye Johnson
The Norwegian Cyclone Model.
Retrieved on 2006-10-11. 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 A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface Trough (meteorology), trough of Low-pressure area, low pressure. It often forms behind an extratropica ...

above the surface

warm front Warm, WARM, or Warmth may refer to: * A somewhat high temperature; heat * Kindness Music Albums * ''Warm'' (Herb Alpert album), 1969 * ''Warm'' (Jeff Tweedy album), 2018 * ''Warm'' (Johnny Mathis album), 1958, and the title song * ''Warm'' ( ...

Development of the theory

Polar front theory is attributed to Jacob Bjerknes, derived from a coastal network of observation sites in

Norway Norway, officially the Kingdom of Norway, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. The remote Arctic island of Jan Mayen and the archipelago of Svalbard also form part of the Kingdom of ...

during

. 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

rain Rain is a form of precipitation where water drop (liquid), droplets that have condensation, condensed from Water vapor#In Earth's atmosphere, atmospheric water vapor fall under gravity. Rain is a major component of the water cycle and is res ...

fall appeared to be focused along these convergence zones. The concept of frontal zones led to the concept of

air mass In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to ...

es. The nature of the three-dimensional structure of the cyclone would wait for the development of the upper air network during the 1940s.University of Oklahoma
The Norwegian Cyclone Model.
Retrieved on 2007-05-17.

Evolution

A wave along a frontal boundary, in the form of a broad area of low pressure, develops as an upper level disturbance moves towards that portion of the boundary.

Precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

will begin to form ahead of the surface low, within the cold sector of the cyclone poleward of the

. As the low deepens, both the

cold Cold is the presence of low temperature, especially in the atmosphere. In common usage, cold is often a subjectivity, subjective perception. A lower bound to temperature is absolute zero, defined as 0.00K on the Kelvin scale, an absolute t ...

and warm fronts around the low become better defined. As the low matures, it couples with the upper level disturbance moving into the cyclone's cold sector. The cold front catches up to the westward portion of the warm front, forming an occluded front. Eventually, the cyclone stacks with the upper level disturbance, becoming isolated within the cold sector, and begins to weaken as it becomes far removed from the original temperature discontinuity along the cold and warm fronts. At this point, it becomes a cold-core low. The frontal boundary becomes weaker and surrounds the equatorward portion of the cyclone, waiting for the next upper level disturbance to form a new low pressure area.JetStream
Norwegian cyclone model.
Retrieved on 2007-05-17.

Conveyor belt

A conveyor belt, also referred to as the warm conveyor belt, describes the flow of a stream of warm moist air originating within the warm sector of an extratropical cyclone ahead of the cold front which slopes up above and north of the surface warm front. The idea of the conveyor belt originated in 1969.University of Oklahoma
Reexamining the Cold Conveyor Belt.
Retrieved on 2007-05-17. The left edge of the conveyor belt is sharp due to the higher density air moving in from the west forcing a sharp slope to the cold front. An area of stratiform precipitation develops north of the warm front along the conveyor belt. Active precipitation north of the warm front implies potential for greater development of the cyclone. A portion of this conveyor belt turns to the right (left in the Southern Hemisphere), aligning with the upper level westerly flow. However, the western portion of this belt wraps around the northwest (southwest in the Southern Hemisphere) side of the cyclone, which can contain moderate to heavy precipitation. If the air mass is cold enough, the precipitation falls in the form of heavy

snow Snow consists of individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water througho ...

. Theory from the 1980s talked about the presence of a cold conveyor belt which originates north of the warm front and flows along a clockwise path (in the northern hemisphere) into the main belt of the westerlies aloft, but there has been conflicting evidence as to whether or not it actually exists.

References

{{reflist Synoptic meteorology and weather Extratropical cyclones fr:Cyclogénèse#Modèle norvégien

Development of the theory

Evolution

Conveyor belt

See also

References