During World War I, Vilhelm Bjerknes, a Norwegian physicist with expertise in radio science and fluid mechanics, was asked to form a Geophysical Institute in Bergen, Norway. Cut-off from weather data due to the war, he arranged for a dense network of 60 surface weather stations to be installed.  Some of his students were C.-G. Rossby, H. Solberg, T. Bergeron, V. W. Ekman, H. U. Sverdrup, and his son Jacob Bjerknes.

Jacob Bjerknes used the weather station data to identify and classify cold, warm, and occluded fronts.  He published his results in 1919, at age 22.  The term “front” supposedly came by analogy to the battlefronts during the war.  He and Solberg later explained the life cycle of cyclones. Their description is known as the Norwegian cyclone model.

The Bergen School of Meteorology still exists today, although it no longer has the notoriety of the 1900s.  This photo is of Bergen, Norway.  The school is spread throughout the city, so this picture depicts the current university too.

The remainder of this module and the next module on midlatitude cyclones examines the concepts in the Norwegian cyclone model.  This conceptual model was developed as weather balloon data became available, and researchers could analyze the vertical structure of the atmosphere.  Prior to weather balloon data, only surface observations were available.  Thus, the Norwegian cyclone model fits both of these data sets into our understanding of the atmosphere.

The Norwegian cyclone model has a particular life cycle that storm systems undergo.  This life cycle has these stages:

1. Initial conditions
2. Beginning stage
3. Intensification
4. Mature stage
5. Dissipation

Initial conditions are for a stationary front to exist.  Cold air is on one side and warm air is on the other side.  Winds are generally parallel to the front as is shown in these illustrations.  The first illustration is a weather map view as if looking at it from above.  The second illustration is a three-dimensional view.  Both views represent the same conditions, just with a different perspective.

The beginning stage is a wave or kink that develops on the stationary front.  This kink is associated with a weak surface low pressure.  Winds start to move cyclonically around this low pressure with a cold and warm front forming.  Precipitation will begin to develop with the heaviest near the low center.  These illustrations depict this beginning stage.  The green area indicates precipitation in the map view (left one) with the heaviest along the front (dark green).  The black lines in the map view are isobars.

Intensification sees the low pressure deepening along with well-defined cold and warm fronts.  Winds are northerly behind the cold front and southerly behind the warm front.  The area of precipitation has expanded with the heaviest precipitation still associated with the low center.  These illustrations depict this stage in the life cycle.

The mature stage has the cold front starting to lift the warm air aloft, which is the beginning of an occluded front.  The low-pressure center stays with the formation of the occlusion.  Precipitation wraps around this low center.   These illustrations depict the mature stage.

Dissipation occurs as the cold front continues to advance on the warm front.  The occluded front lengthens which eventually removes the supply of warm, moist air.  The low pressure weakens and gradually dissipates.  These illustrations depict the final dissipation stage of the storm system.

Video: Mod 6.5.6 Norwegian cyclone model life cycle ( 3:34 min.)

This video describes the life cycle of a storm system using the Norwegian cyclone model.

Three other boundaries are seen on weather surface analysis maps in the United States.  These boundaries are a dry line, squall line, and a trough.  A dry line marks the boundary between a moist air mass and a dry air mass. It typically lies north-south across the central and southern high Plains states (NM, CO, TX, KS, and OK) during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west).

The dry line typically advances eastward during the afternoon and retreats westward at night.

A dry line passage results in a sharp drop in humidity, a rise in temperatures, clearing skies, and a wind shift from south or southeasterly to west or southwesterly.  These changes occur in reverse order when the dry line retreats westward.

Which is heavier, dry air or moist air?   We know colder air is denser (or weighs more) than warm air; thus, colder air always slides under warmer air when the two collide.  To determine how dry and moist air interact, we need to know which one weighs more.  This question can be answered by looking at the molecular weights of air and water.  From Module 1, we know that air is largely comprised of 78% nitrogen and 21% oxygen.  Taking the molecular weights of N₂ (28) and O₂ (32) along with their percentages (0.78 and 0.21) gives a molecular weight of dry air as 29.  Water (H₂O) has a molecular weight of 18 (2 + 16).  Water vapor is much lighter than air.  Therefore, dry air with little or no moisture will be heavier than moist air.

Since drier air weighs more than moist air, as the dryline moves east it lifts moist air. Therefore, severe and sometimes tornadic thunderstorms can develop along a dry line or in the moist air just to the east of it.  A dry line is represented by a brown line with unfilled half circles on the side that the dry air is moving towards.  These illustrations depict the dry line symbol and how it interacts with moist air.

The second boundary seen on surface maps is a squall line.  A line of thunderstorms that moves ahead of an approaching cold front is called a squall line.  A squall line only proceeds a cold front when certain atmospheric conditions are present.  A secondary line of weaker thunderstorms or showers would then be associated with the actual cold front passage.  Squall lines will be discussed in more detail in Module 9.

Current surface front analyses for the United States can be seen on the National Oceanic and Atmospheric Administration (NAOA) website.