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 ...

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

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was List of wars and anthropogenic disasters by death toll, one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, ...

within the Bergen School of Meteorology. In this theory,

cyclone In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an an ...

s develop as they move up and along a frontal boundary, eventually occluding and reaching a

barotropic In fluid dynamics, a barotropic fluid is a fluid whose density is a function of pressure only. The barotropic fluid is a useful model of fluid behavior in a wide variety of scientific fields, from meteorology to astrophysics. The density of most ...

ally 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

cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...

s 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 of low pressure. It often forms behind an extratropical cyclone (to the west in the Northern ...

above the surface

warm front A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold f ...

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 in Northern Europe, the mainland territory of which comprises the western and northernmost portion of the Scandinavian Peninsula. The remote Arctic island 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 A squall line, or more accurately a quasi-linear convective system (QLCS), is a line of thunderstorms, often forming along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front (which often are accom ...

or cold front. Areas of clouds and

rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...

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 l ...

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 In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather (such as cloudy, windy, with possible ...

, 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 under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...

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 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 In meteorology, an occluded front is a type of weather front formed during cyclogenesis. The classical and usual view of an occluded front is that it initiates when a cold front overtakes a warm front near a cyclone, such that the warm air is separ ...

. 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 A cold-core low, also known as an upper level low or cold-core cyclone, is a cyclone aloft which has an associated cold pool of air residing at high altitude within the Earth's troposphere, without a frontal structure. It is a low pressure syste ...

. 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 Stratiform may refer to: * Any of the stratus family of clouds (fog, stratus clouds, altostratus clouds, cirrostratus clouds, nimbostratus clouds) and the precipitation coming from them. * Any occurrence of layered strata (see stratigraphic unit ...

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 comprises 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 throughou ...

. 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 fr:Cyclogénèse#Modèle norvégien

Development of the theory

Evolution

Conveyor belt

See also

References