Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/159

Rh PREVAILING WINDS.] METEOROLOGY 149 N.W. and S.W. in Utah, N. and S. at Fort Yuma (California), E.S.E. and N.W. at Portland (Oregon), and S. and E.N.E. at Fort York, Hudson Bay. These winds are readily accounted for by the distribution of pressure over the continent in winter and summer. On the Atlantic seaboard of the United States the prevailing winds of winter vary from N.W. in the New England States to W. in South Carolina ; whereas in summer they vary generally from S.S.W. in South Carolina to S.W. in the New Eng land States. Hence over the eastern States the summer winds are not directed towards the low-pressure region of the interior of the continent, but are determined by the relations of their pressure to the high pressure of the Atlantic to the eastward, and to the lower pressure over spreading the Atlantic to the N.E. This influence of the Atlantic may be considered as felt westward through the States as far as the Mississippi. Though not so decidedly marked, the winds of Europe, except the extreme south, are also monsoonal. In winter they flow from the land towards the region of low pressure in the north of the Atlantic j but in summer the arrows, representing the prevailing winds, show that all but the ex treme south of Europe is swept by westerly winds, which flow in a vast continuous stream from the Atlantic towards the central regions of the Old Continent, and which deposit in their course the rains they have brought from the ocean. Similarly, monsoons prevail on the coasts of Brazil, Peru, North Africa, and many other regions which happen to lie between other regions whose temperatures, and therefore pressures, differ markedly from each other at different times of the year. These are the chief prevailing winds of the globe when the differences of the normal atmospheric pressure are such as to cause a decided and steady movement of the atmo sphere over a large portion of the earth s surface, resulting in well-marked prevailing winds. But there are other winds which are greatly influenced by local causes, such as the nature of the ground, whether covered with vegetation or bare ; the physical configuration of the surface, whether level or mountainous ; and the vicinity of extensive sheets of fresh or salt water. An important characteristic of winds in their practical relations to climate is their quality, they being warm or cold, dry or moist, according to their direction and the nature of the earth s surface over which they have just passed. Thus in the northern hemisphere southerly winds are warm and moist, while northerly winds are cold and dry. In Europe south westerly winds are moist and easterly winds dry, while in the New England States and Canada north-easterly winds are cold and raw and north-westerly winds cold and dry. In particular regions certain meteorological conditions occur at stated seasons intensifying these effects, resulting in excessive drought, heavy rains, intense or great heat, thus giving rise to the following among other well-known winds. The east winds of the British Islands occur chiefly in spring, but also in a less degree in November, being in the latter case often accompanied with fog. The winds here referred to are dry and parching, and their deleterious influence on the health is seen, not merely in the discomfort and uneasiness they impart to the less robust of the popu lation, but also in the largely increased mortality which they cause from consumption and all other diseases more or less connected with the nervous system. In the countries bordering on the north of the Atlantic, atmo spheric pressure reaches the annual maximum in May, and it is above the average during the other two spring months. In these months the normal pressure approaches nearer to what obtains farther south, and an examination of daily weather maps shows that this is due to the repeated occurrence in spring of very high pressures in the north of Europe while pressures much lower prevail to southward. Now these east winds are simply the outflow from these regions of high pressure to northward. Northerly and even westerly winds which are truly outflows from what may be styled Arctic anticyclonic areas bring with them qualities as noxious as those of the east wind itself, and prove as injuri ous to health and vegetation. The cold dry wind of April 29, 1868, which blasted and shrivelled up vegetation in Scot land, particularly in the western counties, as effectually as if a scorching fire had passed across them, was a west wind. In the south of Europe, during the winter and early spring, peculiarly dry, cold, and violent northerly winds are of occasional occurrence. Of these winds the &quot;mistral&quot; is one of the most notorious, which is a steady, violent, and cold north-west wind blowing from central and eastern France down on the Gulf of Lyons. It is particularly trying while it lasts to invalids who are spending the winter at the various popular sanataria which are scattered along this part of the Mediterranean coast. The great cold that took place in the north of Italy and south of France in the beginning of 1868 was a good example of the mistral. The meteorological conditions under which it occurred were unusually low pressure over the Mediterranean to southward (29 450 inches), whilst at the same time pressure rose steadily and rapidly on pro ceeding northward to 30 905 inches in the north of Russia. From this geographical distribution of the pressure, northerly winds swept southwards over Europe, carrying with them the low temperatures of the higher latitudes, and became still colder and drier on crossing the Alps before they made the descent on the shores of the Mediter ranean. The cold tempestuous winds which descend from the Julian Alps and sweep over the Adriatic, and the dreaded &quot;gregale&quot; of Malta, which is a dry cold north-east wind, are in their character and origin quite analogous to the mistral. The &quot; northers,&quot; or &quot; uortes,&quot; are peculiarly dry cold strong winds which repeatedly occur from September to March in the States bordering on the Gulf of Mexico, and are perfectly analogous to the mistral. The conditions under which they occur are a pressure lower than usual to the south or south-east over the Gulf of Mexico, together with a pressure even higher than the high normal which is so marked a feature of the meteorology of the Rocky Mountains during the colder months. When, as most frequently happens, they occur in the wake of a storm, their disagreeable qualities of extreme dryness, cold, and violence are all intensified. From a temperature of upwards of 80 experienced as the storm comes up the thermometer rapidly falls to 18 or even lower ; and, as the low temperature often occurs with a wind blowing with great violence, the northers prove most deleterious. A violent wind with a tem perature of 18 is altogether unknown in the British Islands. The pampero &quot; is a strong, dry, cold wind which blows across the pampas of the River Plate of South America, occurring at all seasons, but most frequently during the spring and summer from October to January. They are preceded by easterly winds, a falling pressure, a rising temperature, and increased moisture. A pampero is de scribed by Dr D. Christison, and its appearance figured, in the Journal of the Scottish Meteorological Society, vol. v. p. 342, as seen advancing on the morning of November 28, 1867, in central Uruguay. In the early morning the wind blew rather strongly from north-east, and by and by clouds were seen moving very slowly from the west, throwing out long streamers eastwards. As they advanced, two dense and perfectly regular cloud-masses appeared in front, one behind the other, in close contact yet not intermingling, the one being of a uniform leaden grey, while the other was as black as the smoke of a steamer. On arriving over head, the front, though slightly wavy in appearance, was seen to be quite straight in its general direction, and the bands were of uniform breadth. They rushed forward at great speed under the other clouds without uniting with them, preserving their forms unbroken, being borne onward by an apparently irresistible force, as if com posed of some solid material rather than vapour. They extended probably 50 miles in length, but as they took only a few minutes to pass their breadth was not great, and they appeared to diminish to mere lines in the distant horizon. At the instant the first cloud- band arrived overhead, the wind chopped round from north-east to north and then to south-west ; a strong cold blast at the same time seemed to fall from the leaden cloud, and continued to blow till both bands had passed. No rain or thunder occurred at this time, but in the confused rabble of clouds which followed low thunder continued to roll, and in a quarter of an hour rain fell, and for some hours thereafter wind, rain, and thunder continued, but only to a moderate degree. The low temperature and rising barometer and change of wind are the constant and most striking characteristics of the pampero. On one occasion the temperature fell 44 in four teen hours, and on another occasion the fall was only 4. Rain is a usual accompaniment, but on rare occasions the pampero passes off and no rain falls.