Chacun a son gout, as the French used to say: "Everyone to his own taste". Thus it is with those bold curved lines which dominate our weather charts - the ones adorned with little flags like bunting, that are wafted skilfully across the country every night with often ominous gesticulation by the forecaster. What we call a "front" may be thought of in either of two ways.
Meteorologists have a complex view of these phenomena. In their minds' eyes they see a front as the sharp dividing line between two masses of air of differing characteristics: the air on one side of a front is usually warm and humid, and that on the other side cooler and relatively dry, and the boundary between the two is surprisingly well defined. But from a purely practical point of view, a front can be thought of simply as an elongated zone of rain which moves steadily, and usually from west to east, across the surface of the globe.
If one were to judge by the daily weather, it might appear as if there were an infinite supply of these troublesome rain belts, but this is not strictly so. In fact there are very few fronts; it is usually the same front which returns time after time, bestowing on us its unwelcome attention.
The cold, or second, front of one low often trails away to the south-west, before curving northwards to become the leading warm front of the next depression. This sequence often stretches right across the Atlantic, with depressions strung along a single, continuous, undulating front for 3,000 miles or more.
There are three principal fronts in our part of the world. The most important is the Polar Front, which generally stretches from south-west to north-east across the Atlantic, although its average position varies with the seasons.
In winter it normally runs from the West Indies to Portugal, while in summer it is much farther north - it stretches from the Great Lakes towards Iceland. Most of our unsettled weather in Ireland results from depressions associated with this Polar Front.
But there are two other important fronts in our vicinity. The normal position of the Arctic Front is well to the north of Ireland, and its depressions generally move from Iceland over the northern part of Norway into the Barents Sea.
The Mediterranean Front is where its name implies, extending from there over the Middle East into the southern part of Russia. Although relatively inactive for much of the time, its depressions, when they do form, bring much-needed rain to the south-western parts of the Asian continent.