Mountain and valley breezes furnish another example of diurnally reversed winds. Relatively cold and heavy air drains down from the upper slopes by night, constituting the mountain breeze. By day the air in the valley is warmed and expanded, and as it is confined laterally by the sides of the valley it flows up the slopes, constituting the valley breeze. Long before meteorologists undertook to classify the winds of the globe, these mountain air currents attracted attention and acquired local names. Among the Alps, alone, we find scores of such names. In many cases, too, the breezes have acquired a legendry of their own. Thus the pontias, a cold, nocturnal wind that blows out of a narrow valley opening upon the plains of the Rhône near the town of Nyons, is said to have been brought thither in a glove by St. Cæsarius, Archbishop of Aries, for the purpose of improving the fertility of the valley! There is a quaint little book about the pontias, published by Gabriel Boule in 1647, in which the author sets forth at length the arguments for and against the miraculous origin of this wind. The Italian lakes are especially rich in locally named mountain and valley breezes.
Parenthetically it may be remarked that wind nomenclature in general is a vast subject, owing to the habit that prevailed in prescientific times, and still prevails to some extent among nonscientific people, of giving individual names to the winds characteristic of different localities. As a matter of curious interest, we set down here some of these names (a small fraction of the total number):
Khamsin, leveche, leste, levanter, pampero, zonda, papagayo, buran, purga, brickfielder, southerly burster, williwaw, willy-willy, pontias, vésine, solore, joran, morget, rebat, vaudaire, breva, tivano, ora, Wisperwind, Erlerwind, Rotenturmwind, vent du Mont Blanc, vent d’aloup, autan, tramontana, gregale, imbat, kite and junk winds, bad-i-sad-o-bist roz (the furious “wind of 120 days” of Persia and Afghanistan).
The present writer has collected several hundred local wind names—and is constantly adding to the list.
There are several other types of wind peculiar to mountains besides the alternating mountain and valley breezes. Most of these are descending winds, or “fall winds,” which may blow by day as well as by night. Thus a strong daytime wind sometimes blows down from lofty snowfields and glaciers. A foehn (pronounced like “fern,” but without the r) is a wind that has been robbed of most of its moisture through precipitation on the windward slope of mountains and which is further dried, and also strongly heated by compression, in descending the leeward slope. Winds of this type are common in the Alps, where they were first described and named, and their heat and aridity led to the belief that they came by way of the upper atmosphere from the distant deserts of Africa. Now that their origin is better understood, we find that foehns prevail in many other mountainous countries throughout the world, including the western United States, where they are called chinooks. When the foehn blows in winter, it causes snow to disappear with amazing rapidity—not only melting it, but speedily drying the ground—whence it has earned the name of “snow eater” in America, and “Schneefresser,” which means the same thing and a little more, in Switzerland.
The bora of the Adriatic and the mistral of southern France are winds that blow from a cold, mountainous interior down to a warm coastal region, where they arrive as relatively cold winds, in spite of the dynamic heating they have undergone in their descent. The bora is sometimes moderate (borina) and at other times a tremendous gale (boraccia), while the mistral has been known to blow a railway train from the track in the valley of the Rhône.
The blizzard is a wind of which Americans once thought they had almost a monopoly until Sir Douglas Mawson located the “home of the blizzard” on the shores of the Antarctic continent. The true blizzard, whether American or Antarctic, is a violent, intensely cold wind, heavily charged with snow. Such winds are a characteristic feature of the winter climate of our Middle Western States. Although the name of this wind first became current as recently as the seventies of the last century, nobody knows its origin. Nowadays the name is often loosely applied to big snowstorms that are not really blizzardlike.
The dry “hot winds” that sometimes wither the crops of our western plains are the American antithesis of the blizzard. These winds belong to the great sirocco family—the name “sirocco” having become, in recent scientific usage, a generic designation for extensive hot winds, whether dry or moist, as distinguished from local hot winds, such as the foehn.
The harmattan of West Africa is a dry, dusty wind from the Sahara, and one that feels relatively cool; perhaps on account of causing rapid evaporation from the skin. The simoom (with a final m), especially the variety blowing in southern Asia, is perhaps the hottest and most parching of all winds—judging from its effects on animal life.
The great majority of the winds above enumerated are merely minor features of what are called cyclonic and anticyclonic wind systems. Reverting to what has been said about weather maps and Buys Ballot’s Law—if the reader will examine a series of maps for successive days, he will notice that the areas of high pressure and low pressure are not stationary, but show a more or less rapid displacement, the general direction of which, in our latitudes, is from west to east. The fact that there are great traveling vortices or swirls in the atmosphere, which, in whatever regions they occur, partake of the general drift of the air around the globe, has been known for about a century, and constitutes the corner stone of practical meteorology. In the temperate zone, where these swirls are sometimes of moderate force and sometimes very stormy, they are the chief factor in controlling changes of weather from day to day, and their observation is the basis of weather forecasting. Within the tropics, where they are much less frequent and are confined to a few restricted regions, they are always violent storms.