This page archived in 2008
The diagram above is of a well developed tropical depression, which is not the main focus of this page. We will be looking at the sort of mid-latitude depressions which are common over the British Isles. They develop over the north Atlantic and are then steered over the British Isles by upper westerly winds. The pressure, wind speed and rainfall amounts are of a very different magnitude to that of their tropical cousins, although hurricanes start out as simple storms.
There have been several more well developed depressions. A depression is an area of low atmospheric pressure. It tends to be accompanied by a frontal system: warm, cold and occluded fronts.
For the 'AS' exam, you will need to know about the sequence of weather which is associated with the passage of a depression. This is related to the Bergen school model, which was developed by Bjerknes and partners. The term FRONT came out of the First World War where armies met along a 'front' and fought it out. Warm and cold air were considered to be doing a similar thing.
There is an excellent set of resources on the MET OFFICE site which covers the historic storm of October 2000. This includes some readings taken at Heathrow Airport, and questions which go with it. There is a graph which is constructed, which then has the passage of the fronts marked on.
Students need to be aware of synoptic charts: identifying low and high pressure systems, troughs and cols. The systems are identified by the shapes created by isobars: lines joining places of equal pressure. These tend to have intervals of 4mb (or sometimes 6mb or 8mb).
LOW PRESSURE
Results from air being uplifted in one of 3 ways:
a) free convection - heat
b) forced uplift - orographic uplift when the air encounters a relief barrier
c) convergence of air and divergence aloft...
CHARACTERISTICS
Start with a WAVE on a FRONT.
Near to UK is POLAR FRONT with associated PFJS or PFJ (POLAR FRONT JET STREAM)
Front is a zone between 2 air masses of different temperatures (and therefore different densities..)
CYCLOGENESIS is the term for the formation of a low pressure area.
Begin with an EMBRYO stage.
JUVENILE
MATURE
POST-MATURITY
OCCLUSION / Filling
Need to add some flesh onto these bones...