In textbooks on the weather the classic depression is a series of concentric, almost circular, isobars. It corresponds to a great "bowl" of low pressure, a giant whirlpool of air blowing anti-clockwise around the centre, and it follows a well-defined track.
It forms off the eastern seaboard of the US, moves at about 30 or 40 m.p.h. in a north-easterly direction across the Atlantic and passes close to the northwest coast of Ireland, before disappearing somewhere between Iceland and the Scandinavian countries.
In real life, depressions often come along in groups, following each other across the Atlantic at something like 36-hour intervals. Meteorologists refer to these groups as "families" of depressions, and as Tolstoy put it in the opening lines of Anna Karenina: "All happy families resemble one another, but each unhappy family is unhappy in its own individual way."
Much the same can be said about these storms. There is a classic textbook structure, yet each one has its own characteristics.
If depressions all followed the same path, for example, or if there were any means of ascertaining beyond doubt the track an individual might follow, the forecaster's lot would be a happier one.
Many depressions, however, sail in from the Atlantic and disappear, almost ineffectually, on a course that takes them towards the north of Norway. Others, like those associated with the storms recently, come much closer to us here in Ireland.
For a forecaster, the path is critical in determining the strength of the expected wind; generally, the closer the depression comes, the stronger the associated gales will be.
Lows are also idiosyncratic in intensity. The classic depression 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. However, some peak prematurely, becoming mere shadows of their former selves by the time they reach Irish waters.
Others, like some recent ones, are slow developers and reserve the worst of their fury for the regions to the east of us.
And finally there is what we might call the eccentricity of the individual low. Few depressions are exactly symmetrical; the low centre is usually closer to one edge than to the other, and as a consequence the isobars on that side are "squashed" closer together, which means stronger winds.
Very often the strongest winds are on the southern side of the low, but sometimes the "squeeze" is in the north-westerly flow of air behind the depression and at other times in the southerly winds along its leading edge.