Of yesterday's Weather Eye it might be said, as Joyce remarked about his Ulysses, "I've put in so many enigmas and puzzles that it will keep the professors busy for centuries over what I meant, and that's the only way of ensuring one's immortality."
Even so, the perceptive reader may have left the page with at least an inkling of the importance of Greek prefixes to the art of weather forecasting.
The piece concerned itself, you may recall, with kat- and an-, which to the ancient Greeks meant "down" and "up" respectively. Thus anallobars, you learned, are lines joining points with equal rates of rising atmospheric pressure, and katallobars do the same over areas where the pressure happens to be falling. But let us look at another use for kat- and an-, this time as prefixes denoting certain kinds of wind.
On a warm and sunny afternoon, in otherwise calm conditions, a breeze may suddenly begin to blow uphill. This occurs when a stretch of sloping ground is heated by the sun, causing the air immediately above it to become lighter than elsewhere in the surrounding atmosphere; this warm air drifts steadily up the incline and is felt as a warm breeze blowing gently up the hill; it is called an anabatic wind.
The reverse, a katabatic wind, blows down a hill. It is usually in evidence on a quiet, clear night, when the ground loses heat by radiation after dark; if the temperature of a volume of air in contact with a patch of sloping ground falls rapidly in this way, it becomes denser and heavier than the surrounding atmosphere, and the cold air slides down the slope.
Katabatic winds in Ireland are, as a rule, of a rather gentle nature, but in some parts of the world they achieve a velocity that is quite astonishing. In steep mountainous areas the cold air near the summit flows from little channels into larger gullies, and thence down deep ravines before perhaps plunging headlong over a precipice - a turbulent cataract whose ferocity is further exacerbated by constriction as it is channelled down narrow mountain passes towards the sea.
The bora of the Adriatic, and the mistral of the Rhone Valley in the south of France, are each a case in point.
Having taken such care to define anabatic winds, that blow uphill, and katabatic winds that blow downwards from the summit, one could suggest that the winds that swirl and frolic around a mountain peak might be known as acrobatic winds. But meteorologists have not, as yet, usurped the term; such winds are nameless.