Mad Margaret, in Gilbert and Sullivan's operetta Ruddigore, contemplates the notion of a "word that teems with hidden meaning - like `Basing stoke' ". There are many words like Basingstoke in meteorology: vorticity is one of them.
Vorticity engenders vortices, and most of us have some idea of what a vortex is.
In meteorology the term is applied to whirlpools of air embedded in the atmosphere, ranging in size from the large depressions of the North Atlantic to the tiny turbulent eddies of a gentle breeze. As the father of numerical weather prediction, L.F. Richardson, described it:
Great whirls have little whirls,
That feed on their velocity;
And little whirls have smaller whirls,
And so on to viscosity.
Vorticity is that which gives to a vortex its defining sense of twirl. The inclination comes about partly because the Earth itself rotates; in this context it is known to meteorologists as the Coriolis effect, after the French mathematician Gustave de Coriolis, and in the northern hemisphere it imparts to the air a kind of anti-clockwise twist, so that it swirls around a zone of low pressure in a great ascending anti-clockwise spiral. But apart from the Coriolis effect, there are other circumstances in which a volume of air may acquire a tendency to rotate.
It can happen, for example, if there is a marked change in wind speed over a relatively short distance. Imagine that you make a cross from two sticks tied together, and throw it into a flowing stream; as the cross is carried along by the current, it will rotate if the water near the centre of the stream is flowing faster than that near the bank.
The same thing happens in the atmosphere. If a westerly wind high over Belfast is blowing at, say, 40 m.p.h. and the corresponding wind over Dublin is 60 m.p.h., this difference will tend to impart to volumes of air caught in the flow an anticlockwise or cyclonic vorticity.
Now the importance of vorticity to the forecaster lies in the fact that it is known that when vorticity in the flow of air aloft is increasing over a given location, the result is very often falling barometric pressure at the surface, and a tendency for depressions to develop or become more active. This, of course, is helpful in anticipating deteriorating weather.
Before computers became all-powerful and ubiquitous, forecasters had clever little tricks for identifying areas of high vorticity on the weather chart.
Nowadays, computer models calculate the future shape of the atmosphere and identify bad weather zones directly, so forecasters pay less attention to vorticity than used to be the case.