CY C L O N I C DI S T U R B A N C E S
A N D BA R O C L I N I C IN S TA B I L I T Y
ection 3
In this section, the wave motion that is characteristic of the weather in the mid-latitudes
is investigated in more detail. A pattern of regular storms in the mid-latitudes has been
known for many years (see historical note on page 25). However, the first modern par-
adigm for describing the development of mid-latitude disturbances did not appear until
the time of World War I. At that time, Vilhelm Bjerknes— a noted hydrodynamicist, his
son Jacob, and other Norwegian scientists set up a research facility in Bergen, Norway.
Because of the war, all sources of weather data were cut off. To prepare local forecasts,
the group—later known as the Bergen School, set up a dense observational network
across Norway. The data collected from this network was used to develop what has
come to be known as the polar front theory. This theory postulated the existence of the
now-familiar warm and cold fronts, as well as the three-dimensional motions associated
with them. Although many of the concepts associated with the polar front theory had
already existed or been hinted at, the scientists of the Bergen School created a com-
plete and coherent three-dimensional picture of the life cycle of extra-tropical cyclones.
The data upon which this theory was based was primarily a network of surface obser-
vations, supplemented by limited upper air data. The polar front theory predates many
observing systems in use today including the global upper air observation network,
radar, and satellites. However, the basic insights contained in this paradigm still form
part of the current understanding of extratropical cyclone development and are a use-
ful place to begin to understand what we see on the satellite images.
figure 13. panel 1, a-d four-stage pressure and front fields
panel 2, a-d four-stage wind and temperature field
The evolution of the wave as described by the polar front theory is shown in figure 13.
The symbols for fronts are shown in the under weather symbols. The wave
passes through several distinct stages with characteristic surface weather phenomena
associated with each stage.
S
Polar Front Theory
1a. 1b. 1c. 1d.
2a. 2b. 2c. 2d.
cold
warm
cold
warm
warm
LL LL L
L LL
