Die Entstehung einer Höhenzyklone über Nordamerika

The existence of a high-level cyclone always depends on masses of increased potential vorticity in the upper troposphere. The origin of those masses is studied in a single case of cut-off. It is found that the upper part of them comes from the polar stratosphere, while the lower part consists of tropospheric air. Both from the vorticity equation and from the single wind observations, the existence of a non-conservative force results, parallel and south to the jet-stream. This force can be regarded as the immediate cause of the cut-off. An explanation of the force is not attempted.