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0.328 inch between the winter and summer pressure at St Petersburg. In Western Asia the difference is very much greater. This fact may be made more impressive by saying that there is conveyed away from St Petersburg during summer a stratum of air about 320 feet in thickness. When the sun has begun markedly to heat the air in the northern hemisphere in spring, part flows back into the southern hemisphere. But what influences the east winds more immediately is the heating of the north of Africa and south of Europe and Asia, by which the superincumbent air expanding flows upwards, thus setting in motion an indraught of air from the north of Russia to take its place. This northerly current marked out by a high atmospheric pressure generally holds its course through the centre of Europe ; and it is a tributary, so to speak, from this current, pouring its dry pestiferous air westward over Great Britain, which constitutes the east wind.
488. The Isobarometric Charts of Europe for the spring months exhibit a smaller difference of mean pressure between Great Britain and the north of Europe, than during the other months, except November, when east winds also to some extent prevail. This smaller difference arises from the more frequent occurrence during these months of higher pressures to the north and north-east of Great Britain than in Great Britain itself; and it is from the repeated occurrence of these throughout the year that next to the S.W. or W.S.W. the N.E. wind is the most prevalent. Sometimes, and in some years, notably during 1867, high pressures prevail to the north-west of Great Britain, in which case the polar current comes to us, not as an east wind, but as a north, north-west, or even west wind. But it matters not from what direction, or in what disguise, the polar current comes to us, its noxious characteristics are the same, and it is nearly as injurious to health and to vegetation as when it comes out of the “horrid east.”
489. Simoom (otherwise written simoun, semoun, samoun, samun), or sambuli, a name derived from the Arabic samma,
signifying hot, poisonous, or generally whatever is disagreeable or dangerous, is applied to the hot suffocating winds which are peculiar to the sandy deserts of Africa and Western Asia. In Egypt it is called khamsin (Ar. fifty), because it generally continues to blow for fifty days, from the end of April to the time of the inundation of the Nile, in June.
490. Owing to the great power of the sun's rays, the extreme dryness of the air, and the small conducting power of sand, thus causing the accumulation of heat on the surface, the superficial layers of sand in the deserts of Africa and Arabia often become heated to 200° F. to a depth of several inches. The air resting on this hot sand becomes also highly heated, thus giving rise to ascending currents; air consequently flows towards these heated places from all sides, and, the different currents meeting, small cyclones or whirling masses of air are formed, which are swept onward by the wind prevailing at the time. Since the temperature, originally high, is still further raised by the heated grains of sand with which the air is loaded, it rapidly increases to a degree almost intolerable. In the shade, it was observed by Burckhardt in 1813 to have risen to 122°; and by the British Embassy to Abyssinia in 1841, to 126°. It is to the parching dryness of this wind, its glowing heat, and its choking dust, and not to any really poisonous qualities it possesses, that its destructive effects on animal life are to be ascribed.
491. The approach of the simoom is first indicated by a thin haze along the horizon, which rapidly becomes denser, and quickly overspreads the whole sky. Fierce gusts of wind follow, accompanied with clouds of red burning sand, which often present the appearance of huge columns of dust whirling forward. Thus vast mounds of sand are transported from place to place by the terrible energy of the tempest. By these mounds of sand, large caravans are frequently destroyed. The destruction of Sennacherib's army is supposed to have been caused by the simoom. The simoom generally lasts from six to twenty-four hours, but sometimes for a shorter period.
492. Hot winds from Africa are felt in neighbouring regions, where they are known under different names. They are subject to important modifications by the nature of the earth's surface over which they pass. The Sirocco blows occasionally over Sicily, South Italy, and adjoining districts. It is a hot moist wind, receiving its heat from the Sahara, and acquiring its moisture in its passage northward over the Mediterranean. It is the plague of the Two Sicilies; and while it lasts, a haze obscures the atmosphere, and so great is the fatigue which it occasions, that the streets of Palermo become quite deserted. The wind sometimes extends to the shores of the Black and Caspian Seas, and to the steppes beyond the Volga, the seat of the dreaded rinderpest, where, by its blighting touch, vegetation withers and dries up, and thousands of cattle are cut off. It is called the Samiel in Turkey, from its reputed poisonous qualities.
493. The Solano of Spain is a south-east wind, extremely hot, and loaded with fine dust. It prevails at certain seasons in the plains of Mancha and Andalusia, particularly at Seville and Cadiz. It produces dizziness, and heats the blood to an unusual degree, causing great uneasiness and irritation; hence the Spanish proverb, “Ask no favour during the Solano."
494. The Harmattan of Guinea and Senegambia belongs to the same class of winds. It is a periodical wind blowing from the dry desert of Africa to the Atlantic, from N. lat. 15° to S. lat. 1°, during December, January, and February. It blows with moderate force, is often highly charged with fine particles of dust, and since under its influence no dew falls, vegetation grows languid and withers.
495. The Pampero is a wind which blows chiefly in the summer season from the Andes across the pampas of Buenos Ayres to the sea-coast. It is thus a north-west wind, or part of the anti-trade of the southern hemisphere, and in this respect it is analogous to the stormy winds which sweep over Europe from the south-west. But since it blows from the Andes over the South American continent, it is a dry wind, frequently darkening the sky with clouds of dust, and drying up the vegetation of the pampas. The cause of its prevalence in the summer season will be evident on examining the isobarometric lines on Plate II.
496. The Nortes, or “Northers,” are dry cold winds which frequently prevail from September to March in the regions bordering on the Gulf of Mexico. They result from the high pressure of the interior of North America during winter, taken in connection with the low pressure to the south (Plate II.) In his 'Climate of America,' R. Russell gives an instance of the temperature falling in Southern Texas, with a "norther," from 81° to 18° in 41 hours, and adds that “such great and sudden changes are rendered still more disagreeable by the 'northers' frequently blowing with extreme violence.” The influence of these cold winds on the vegetation of the Southern States is very deleterious. A temperature of 18° with a violent wind is almost unknown in Great Britain.
497. In the south of Europe north winds are notorious for their violence. The great differences of the temperature of the Alps, the Mediterranean, and Africa explain them; and when the polar current, with a high atmospheric pressure accompanying it, is descending at the same time over Europe, the effect is greatly heightened. Of these the most notorious is the Bora, which, descending from the Julian Alps, sweeps over the Adriatic—the bitterly cold tempestuous wind of that much-vexed sea. It is probably the Euroclydon mentioned in the Acts of the Apostles. The Mistral is a steady violent north-west wind which blows from France down on the Gulf of Lyons. It is immediately caused by a low atmospheric pressure in the Gulf of Lyons, as compared with the pressure to the north, and is most severe when at the same time very high pressures occur from France northwards towards the Arctic regions. The great cold which prevailed in the north of Italy and south of France in the beginning of last January (1868) arose in this way. While it lasted, atmospheric pressure was very low in the north of the Mediterranean, 29.450 inches, whence it rapidly rose in advancing northwards to the almost unprecedented height of 30.905
inches in the north of Russia, thus drawing over the northern shores of the Mediterranean the polar current in its full strength, which became still colder and drier in crossing the Alps in its southward course. Atmospheric pressure was at the same time high in Scotland, but as this country lay to the west of, and beyond, the cold polar current which set in from Russia towards the Mediterranean, the temperature was only slightly under the average of the season; in other words, no unusual cold occurred in Scotland. There are other winds of a stormy character peculiar to different parts of the Mediterranean, such as the Levanter in the east. The heating of the Sahara in summer causes a general and continued flow of the cooler air of the Mediterranean to the south to take the place of the heated air which rises from the sandy desert. These are the Etesian winds of Southern Europe.