In general, the development of weather systems over middle latitudes will be driven by the upper winds which circulate around the earth. The mathematics and dynamics of these processes is complicated but after much work development areas can be related to the pressure pattern at 30 000ft or so.
The study of the mathematics is very rewarding and brings together the conservation of momentum (and angular momentum), Newton’s Laws and the rotation of the Earth.
For the moment, the following images show the relationship of the 300mb contour chart with convergence and divergence. The fact that air cannot be created or destroyed in the flow means that:
- if upper level air is in CONVERGENCE the air is forced downwards causing subsidence, a build up of pressure and good weather, and,
- conversely, if upper level winds are in DIVERGENCE air is sucked upwards causing condensation, droping of pressure and bad weather.
Applying the rules in these images to the latest 250 mb flow (300mb not available, but 250 mb close) and surface flow form Earth.nullschool.net and then looking at the latest forecast sequence from the UK Meteorological Office (weathercharts.org) will give you an insight into current atmospheric dynamics and weather development.
The Convergence / Divergence Rules