Everyone I meet is asking: What will the weather be like for the eclipse? The prospects, as I write, are not encouraging, and of course, from our earthly viewpoint, the effect of weather on the spectacle is obvious: any cloud, other then the very thinnest layer of cirrus, will greatly diminish the drama of the great event.
There is another side to the coin, though, another question one could reasonably ask: what effect will the eclipse have on the weather?
If the weather is cloudy, wet, or even relatively fine with broken skies, the answer is that the eclipse will have no impact at all, but in places where the skies are clear, and temperatures have risen during the earlier part of the day, there may well be a small but measurable, and very temporary, effect upon the local weather.
As we all know by now, 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 which skims rapidly from west to east across the landscape.
The principal consequence of interest to meteorologists 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, is considerably less.
This cooling has an effect upon the wind, particularly sea breezes, which tend to 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 summer breezes tend to die down 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. The same phenomenon also gives rise to the "eclipse gust front", a sharp gust of wind of perhaps 40 m.p.h. which sometimes occurs between half-an-hour and an hour before the shadow comes, and the same length of time after it has gone.