A solar eclipse occurs when the moon passes directly between our planet and the sun. It obscures for a time our view of the solar disc or, viewed from space, the moon casts a shadow on the surface of the Earth. Since the moon is tiny by comparison, the shadow is only very small - a circle of darkness about 100 miles in diameter that skims rapidly from west to east across the landscape.
Such an event will occur on August 11th, 1999, but we will not see it here in Ireland. The shadow of the moon will start near Nova Scotia, cross the Atlantic to pass just south of Ireland, and zip across the pointy end of England before proceeding over Europe to die in the Bay of Bengal, off the coast of India.
Hundreds of thousands of people are expected to congregate in Devon and Cornwall on August 11th to witness the phenomenon, and the weather at the time, of course, will be of obvious importance.
The odds are not encouraging: statistics suggest that on only one day in three in the average August in those parts would the sky be clear enough to give a satisfactory view. What is less well known, however, is that the eclipse itself will influence the weather in a minor way.
The principal effect is that the shadow temporarily lowers the local temperature by several degrees. In very warm areas of the world an eclipse has been known to cause a drop of 10 degrees or more, but in the temperate latitudes, and near the sea, the fall, though noticeable, would be considerably less.
The cooling also has an effect upon the wind, particularly sea breezes which fade away as totality approaches. The lowest wind is usually found to correspond to the time of minimum temperature, which mirrors the familiar way in which the wind tends to die as the air cools at dusk on summer evenings.
There are also recorded instances of what has come to be called an "eclipse cyclone", a phenomenon which occurs when the temperature decrease causes a slight drop in local atmospheric pressure, which in turn sets up a local circulation of the air around the shadow.
Eclipse winds of this kind show a definite pattern, with a reversal of direction before and after the event. And the same phenomenon gives rise to the "eclipse gust front", a sharp gust of wind of perhaps 40m.p.h. that occurs between half an hour and an hour before the shadow comes, and the same length of time after it has gone.