The mid-latitude depressions, those all-too-common atmospheric monsters we experience as storms, have a classic text-book structure. Yet, each one has an individual nature.
The classic depression appears on the weather map as a series of concentric, almost circular, isobars. It corresponds in real life to a great "bowl" of ever-decreasing pressure, which generates a giant whirlpool of air blowing anticlockwise towards the centre.
It follows a well-defined track: it forms off the eastern seaboard of the United States; it moves at about 30 or 40 miles per hour in a north-easterly direction across the Atlantic; and it passes close to the north-west coast of Ireland before disappearing for ever somewhere between Iceland and Scandinavia.
But depressions do not read the textbooks, and each one is, in its own peculiar way, unique. The most obvious way is in the path it may choose to follow.
Many proceed impeccably along the track prescribed in the preceding paragraph; but some come further south, others head north, and the odd maverick may venture where no depression has ever dared before.
Lows are also idiosyncratic in their volatility. The classic example begins as a very small feature, develops into a full-blooded depression over the ocean to the west of Ireland, and then begins to decay.
But some may peak perversely prematurely, and be a mere shadow of their former selves by the time they reach our coastline; others are slow developers, and reserve the worst of their fury for regions to the east of us; some die in infancy, while others survive to seven-day senility, expanding in mid-Atlantic to cover the ocean and make the weather map look like a dart-board.
When Milton, in Paradise Lost, summed up the cosmic traffic lanes of the solar system as
. . . mazes intricate,
Eccentric, intervolved, yet regular
When most irregular they seem,
he might well have been talking about mid-latitude depressions and the way they behave.
And speaking of eccentricity, depressions merit that description in a literal sense. If you look closely at a random selection of weather maps, you will see that no depression is exactly symmetrical; the low centre is usually closer to one edge than the other, and as a consequence the isobars on that side are "squashed" closer together than they are elsewhere.
This translates into stronger winds in that vicinity. Often the strongest gales are on the southern side, but sometimes the squeeze is in the north-westerly flow of air behind the low, and at other times in the southerly winds that mark its leading edge.