The Dark Before the Dawn

Abstract

For 2,000 years after Aristotle, there was very little progress in meteorology. Considerable efforts were devoted to atmospheric optics, but most of the scant attention given to meteorology consisted of commentaries on Aristotle’s treatise. Most of his distinguished successors added little to the perfection of his system. There were a few, however, who did expand on his theories, especially in those areas to which he had paid slight attention. One was his pupil, Theophrastus of Eresos, who wrote “De Signis Tempestatum” (On Weather Signs) and a treatise, “De Vends,” on winds. The practice of weather prognostication by empirical rules dates from these treatises. Theophrastus gives some eighty different signs of rain, forty-five of wind, fifty of storm, twenty-four of fair weather, and seven signs of weather for periods of a year or less. In looking at the overall weather picture, Theophrastus advocates a general principle:¹

Now the first point to be seized is that the various periods are all divided in half, so that one’s study of the year the month or the day should take account of those divisions. The year is divided in half by the setting and rising of the Pleiad; for from the setting to the rising is a half year. So that to begin with the whole period is divided into halves: and a like division is effected by the solstices and equinoxes. From which it follows that, whatever is the condition of the atmosphere when the Pleiad sets, that it continues in general to be till the winter solstice, and, if it does change, the change only takes place after the solstice: while, if it does not change it continues the same till the spring equinox: the same principle holds good from that time to the rising of the Pleiad, from that again to the summer solstice, from that to the setting of the Pleiad.

So too is it with each month; the full moon and the eighth and the fourth days make divisions into halves, so that one should make the new moon the starting point of one’s survey. A change most often takes place on the fourth day, or, failing that, on the eighth, or, failing that, at the full moon; after that the periods are from the full moon to the eighth day from the end of the month, from that to the fourth day from the end, and from that to the new moon.

The divisions of the day follow in general, the same principle: there is the sunrise, the mid-morning, noon, mid-afternoon, and sunset; and the corresponding divisions of the night have like effects in the matter of wind storms and fair weather; that is to say, if there is to be a change, it will generally occur at one of these divisions.