figure 19. surface winds and friction
The theoretical explanation of the development of cyclones that succeeded the polar fro n t
theory was first introduced in the 1940’s. This theory, termed the baroclinic theory,
identified instabilities in the upper level westerlies as the key to cyclone development.
The baroclinic theory is better able than the polar front theory to predict when and
where mid-latitude cyclones will develop. With the advent of satellite observations in
the 1960’s, the basic insights of the baroclinic theory were confirmed although, as will
be explained in section 5, satellites have also identified large weather-making systems
that are not fully explained by baroclinic processes.
While the baroclinic theory is quite complex and cannot be fully described here, we can
point to the key factors that result in extra-tropical cyclone development and the manner
in which they interact. Through satellite images and surface and upper air charts (all
now routinely available on the Internet), these factors can be identified and tracked so
that simple, but often accurate, forecasts of cyclone development can be made.
With the advent of routine upper air observations, it was found that cyclonic distur-
bances tend to occur just ahead (east) of the base of the trough (figure 3) and that
these upper air waves—which are quasi-horizontal—amplify along with the developing
Historical Background
Serial ascents of balloon-bornemeteorographs in the late 1920’s and early
1930’s were able to provide clues regarding the upper-air conditions associated
with cyclonic disturbances. These showed the vertical extent of the frontal
zones - rather than abrupt discontinuities between air masses - and some indi-
cation of upper level wave structure. After the Second World War, a
radiosondenetwork that spanned the globe was set up which allowed for
daily analysis of upper air patterns. This allowed, for the first time, routine
observation of the strength and extent of the polar jet stream.