Weather Situation September,14th 2012

Full Earth September, 14th 2012, 11.00 UTC Sourece: EUMETSAT

General Synopsis of North Atlantic and Europe: A Cut-Off Low over Mediterranian has detached from Jetstream further north. Mediterranian Cut-Off Low looks really beautiful! Jetstream at the North is decernible as a band of cloud.

Usually the pathways of low pressure weather systems are strongly controlled by current of the Jetstream. But sometimes the Jetstream  slows down or even breaks, so that weather systems can detach (Cut-Off). After that they move independently until they decay or are captured by a trough of a new established Jetstream.

Low pressure systems on Northern Hemisphere are spinning anticlockwise due to Coriolis force. Inside the system air is rising and cooling. If air-moisture is sufficient condensing water vapor starts to form clouds, made of tiny water droplets (or tiny ice crystals in higher altitudes). It might rain or snow. Bad Weather!

For comparison the model calculation chart for windspeed and Jetstream in the higher troposphere:

Windspeed and Jetstream in the higher Troposphere. Source:

The mathematic circulation model shows almost perfect all features of reality!

Jens Christian Heuer



Nice Animation of a Jetstream Source: NASA – GISS

Jetstreams are driven by atmospherical differences in temperature and thus in air pressure. There are horizontal gradients in air-temperature of atmosphere, for example between polar regions and mid latitudes, but there are also vertical gradients leading to convection and to cloud formation if air moisture is sufficient.

A Jetstream forms Rossby waves, troughs and ridges traveling streamwise and also eddies, the weather-making high and low pressure systems. These eddies arise from turbulences in jetstream like the vortexes of a raging river do.

Jens Christian Heuer

A nice animation: