- In stage 1a. and 1b., a stationary polar front exists in a region of locally lower
pressure (pressure trough) between two air masses. Cool polar air is to the north
and warmer tropical air to the south. This is a local expression of the stable
condition shown in figure 11 (page 23) re g a rding the general circulation. - A kink or open wave forms in stage b with low pressure at the center of the wave.
The inverted V-shape in stage b now contains the familiar cold and warm fronts.
The cold front moves faster and eventually catches up to warm front. - The top of the inverted V becomes closed in stage c. This is the occlusion stage
of a mature system, the storm is now intense with a distinct comma shaped cloud
pattern associated with it. - As the occlusion progresses in stage d, the main area of warm, moist air becomes
isolated from its source. The storm will spin about itself and slowly dissipate. This
isolated area of warm air (warm eddy) in stage d is an example of the poleward
transfer of heat that acts to restore the Earth system to balance.
Historical Note
Advances in the field of meteorology have paralleled general technological advances.
The invention of the telegraph in 1845 allowed, for the first time, the rapid communi-
cation of weather data and the ability to create timely weather maps. The day-to-day
weather motions revealed by these charts provided the ability to provide short-term
forecasts. The first regular storm warnings were issued in the Netherlands in 1860. As
the network of surface observations increased, and theoretical understanding
improved, the first general theory of wave development, the polar front theory, was
introduced in the early 20th century (1917–1922).
The shortcoming of weather analysis up to the early 1920’s was the dearth of observa-
tions of upper air conditions. However, advances in radio technology and associated
improvements in storage battery technology made possible the invention of the radio
meteorograph (radiosonde). Inexpensive radiosondes were the key to the develop-
ment, during the period from 1920–1950, of a global network of regular upper air
observations. The data from this network stimulated theoretical investigations of the
physics of the atmosphere culminating, just after the Second World War, in the work of
Jule Charney and Arnt Eliassen. These scientists, working independently, adapted the
general equations of hydrodynamics to provide the possibility of a mathematically man-
ageable description of three-dimensional atmospheric motion.
The problem with theoretical investigations of atmospheric motion was the inability to
carry out the immense number of calculations involved in solving the equations of
motion. The advent of the general purpose (programmable) computer in the early
1950’s finally surmounted this problem and allowed rapid and significant advances in
meteorology. In fact, the first peacetime use of a multipurpose electronic digital com-
puting machine (the Electronic Numerical Integrator and Computer or ENIAC) was to
predict weather. In the following years, advances in semi-conductor technology has
made computers more powerful and able to solve more complex forecast problems.