Popular Science Monthly/Volume 46/February 1895/Windmills and Meteorology
| WINDMILLS AND METEOROLOGY. |
By P. J. DE RIDDER.
THE credit that has been given in all ages to the spirit of observation of sailors is only justice. There are other observers, however, no less sagacious and no less assiduous than sailors, whose powers have not been so conspicuously published. These are the millers of windmills. The number of these observers is necessarily diminishing rapidly in our days in consequence of the progressive disappearance of windmills before the advance of steam mills. Yet there are still in the lands of Holland and Flanders a considerable number of these old-fashioned millers, and it has occurred to me to give a few lines to the consideration of the way in which they observe the phenomena of the atmosphere.
They are real observers, as we shall show. The most intelligent of them observe, according to their own rules, all the changes of the weather. Those of a lesser degree of intelligence are satisfied with noticing the movements of the mills situated farthest in the direction whence the wind comes, and thus regulating the management of their own mills after the example set them by their fellows. These prepossessions of millers concerning the weather should not surprise us, for it is in the line of their direct interest to prognosticate atmospheric changes, and especially to be able to foresee how strong the wind will be, as much for the safety of their mills, which are exposed to all the storms, as in view of keeping on good terms with their customers by properly executing their orders.
So, after a considerable duration of calm weather, the miller, seeing the cirrus and the cirro-stratus appearing in the southwestern horizon, joyfully exclaims:
"To-morrow the wind will blow
And the mill will turn."
Modern meteorology was still in an embryological stage when the millers had attributed their true signification to the cirro-stratus clouds which stretched out in long, narrow bands, sometimes from the horizon to the zenith. They called them "wind trees." They were as well acquainted with the cirro-cumulus, the alto-stratus, and the cumulus. Proof of this is found in the enigmatical and obscure language of an old miller who declared he had seen in the sky "a shepherd under the shadow of a tree while the sheep were pasturing in the field."
Besides looking into the nature and meaning of the cirrus and the cirro-cumulus, the miller tried to calculate the force of the wind in distant storms; he observed the direction and velocity of the lower clouds; he estimated at sight the volume and density of the storm clouds; and if the wind fell off before the rain came, he recollected the old saying, "A calm comes before a storm."
He especially displays extraordinary vigilance in times of heavy showers. He observes, among other things, if the mills farthest away in the direction of the rain have kept their sails unfolded; if they have, it is a good sign. He scrutinizes the sky at every moment, from the zenith to the horizon; he measures the curvature of the forward part of the precipitation or of the storm. The lines of rain or hail that escape from it show him by their length and their approach to the perpendicular how intense the precipitations are. If the lines run obliquely, he is shown the direction of the dominant wind in the squall—in short, no sign indicating the force and direction of the wind passes unperceived by him. He knows likewise that these showers are often accompanied by tempestuous, plunging gusts which seem to come out of the clouds; and frequently, before the most advanced flecks of the storm cloud have reached the zenith, the sails of the mill are rolled up around the arms so as to give the squall free passage.
Except by meteorologists, it is still not generally known that the air is urged on more violently on the right of the squall than on the left; but the old miller knew it long ago by experience. The storms that passed by on his left, from west-by-southwest to south, never gave him any fear, and he confidently left his sails all unfurled in the wind; but whenever he was directly threatened with a storm which would pass over his zenith, or which was coming from the right—that is, from north-by-northeast to east, at a distance of less than five kilometres—he foresaw the possibility of a strong blow, and took his measures accordingly. I never knew of a miller who could account for the squalls from his left being less formidable than those from his right. The explanation of the phenomenon was reserved for modern meteorology. which has taught us that squalls and thundershowers constitute depressions in miniature, or at least weak secondary depressions dependent upon a principal depression and formed under its immediate influence.
The old miller, an observer by virtue of his profession and then an observer by the force of circumstances, was obliged to study the wind in all its manifestations of direction and force; he was thus aware that its maximum velocity corresponded with the maximum temperature of the day. Although this rule is not without exceptions, the miller was rarely mistaken. Proof of this is given in the habitual and reassuring response to the farmers who came to the mill in the afternoon, when the wind had fallen so low that no grinding could be done:
"Come to-morrow noon again,
And then I will grind your grain."
Everyday observation has taught us that things really go thus when the wind originates under anticyclonic conditions; in the opposite case, if the wind rises in the evening and gains force during the night, the meteorologist concludes that its origin is cyclonic, and a change of weather is probable.—Translated for The Popular Science Monthly from Ciel et Terre.