Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/165

Rh STORMS.] METEOKOLOGY 155 metric gradient,&quot; a term introduced by Stevenson in 1867. Cyclones have diameters seldom less than GOO, and they occasionally exceed 3000 miles; the cyclone of fig. 19 had a diameter of about 1200 miles. The cyclones of the Mediterranean are usually of smaller dimen sions than those of north western Europe and America. The rates at which cyclones advance over the earth s surface vary greatly, the average in America being 24 miles an hour, in the Atlantic 20 miles, and in Europe 26 miles. A rate as high as 70 miles an hour has occurred in the British Islands ; sometimes they remain stationary, and more rarely their course is for a time retrograde. The temperature and humidity increase at those places towards and over which the front part of the storm is advancing, and fall at those places over which the front part of the storm has already passed. In other words, the temperature and humidity rise as pressure falls and fall as pressure rises. This is the important climatic significance of cyclones. Thus a succession of low pressures passing eastwards in courses lying to northward of the British Islands are the essential conditions of open winters ; whereas, if the cyclones follow courses lying to southward, the winters are severe. In a cyclone the broadest feature of weather is an area of rain about or rather somewhat in front of the centre, sur rounded by a ring of cloud, outside which the sky is clear. The precise form and position of these areas have been shown by Abercrombie to vary with the type of pres sure distribution, with the intensity of the cyclone, and with the rate of its progress, and they are also influenced by local, diurnal, and seasonal variations. The chief point of difference between American and European storms is essentially the result of the mean winter pressures to the west and north-west of their respec tive storm-tracks. Owing to the high winter pressure in the interior of America, the barometer rises in the wake of the storms of the United States more rapidly, the wind veers round more quickly and more uniformly to N.W., N.N.W., and N. and keeps longer in these directions, and the temperature and humidity fall to a greater degree, than happens in Europe. In the New England States and Canada the easterly winds of the storms, coming as they do from the Atlantic, are disagreeably cold, damp, and misty in a degree and with a frequency much greater than occurs with the same winds in the British Islands. The chief points of difference between the hurricanes and typhoons of the tropics and the cyclones of higher latitudes are these : tropical cyclones are of smaller dimensions, show steeper barometric gradients and therefore stronger winds, and advance at a slower rate over the earth s surface. Another point of difference is that a large number of the hurricanes of the West Indies and the typhoons of eastern Asia first pursue a westerly course, which gradually becomes north-westerly, and on arriving at about lat. 30 they recurve and thereafter pursue a course to north-east wards. The tropical cyclones of the Indian Ocean south of the equator also first pursue a westerly course, which gradually changes to south-west, and often on arriving about I lat. 30 recurve to the south-east. Many of the cyclones of India have their origin to westwards of the Nicobar Islands, pursue a course to north-westward, and die out in the valley of the Ganges ; and, similarly, a considerable number of the cyclones of the West Indies pursue a westerly course through the Gulf of Mexico, and several die out in the States. The most dreadful attendant on tropical cyclones is the storm-wave, caused by the in-blowing winds and the low pressure of the centre of the storm. When this wave is unusually high and is hurled forward on a low-lying coast at high water it becomes one of the most destructive agents known. The Bakarganj cyclone of October 31, 1876, was accompanied by a wave which flooded the low grounds to the east of the delta of the Ganges to heights varying from 10 to 45 feet, by which more than 100,000 human beings perished. Tracks of Cyclones of North America, Atlantic, and Europe. In the Physical Atlas of the Atlantic Ocean, issued under the direction of Dr Neumayer of the Deutsche Seewarte, plate 28 shows by shadings the mean positions of the centres of cyclones and by lines their mean tracks. The following are the regions where the lowest barometer of storms has been most frequently found: the region to west-south-west of the lakes of the United States ; the Gulf of St Lawrence ; mid- Atlantic about lat. 35 long. 52; to the south-west of Greenland ; to the south-west of Iceland, which is by far the most important of the whole ; to the south-west of the Lofoten Isles ; the region embracing Denmark, the south of Scandinavia, and Finland ; and, as secondary centres of frequency, the south of the British Islands, Corsica and part of Italy adjoining, and the north east of the Adriatic. The great importance of these centres, where the lowest barometers are most frequently found, consists in the indication they give of the precise regions either where many storms originate or where they are either retarded or arrested in their course. As regards the origin of storms, the centre west of the Mississippi is the region where most of the United States storms originate, the centre in the Gulf of St Lawrence is where many of the great Atlantic storms have their origin, and the centres in mid-Atlantic and to the south-west of Iceland are the regions where the storms of north-western Europe chiefly originate. The centres on the south-west of Greenland, the Lofoten Isles, Denmark, and the south of the British Islands, all appear to suggest that storms are retarded in their onward courses on coming up against large masses of land, which may, in part at least, be occasioned by the heavy rainfalls that mark these parts of their courses. Of all storm tracks the most frequently taken is that by the storms of the United States, which pursue an easterly course through the lakes to the Gulf of St Lawrence. A considerable number of storms follow a course from Nova Scotia to Davis Straits ; but the larger number take a north-easterly course through the Atlantic towards Iceland and thence past the north of Norway. Among the less frequent but important tracks are these : from near New Orleans along the east coast of the States towards Nova Scotia ; from mid- Atlantic to south of Ireland and thence through France to the north of the Mediterranean; and from the Atlantic about lat. 42 long. 40 in a north easterly course quite outside but at no great distance from the British Islands, and thence towards the North Cape. Of the tracks more immediately affecting British weather are one from Iceland in a south-easterly direction through the North Sea and Germany, and four tracks which start from near Scilly: (1) to the south-east as already described; (2) eastward through the north of Germany ; (3) north-east to Christiania; and (4) north through Ireland and the Hebrides. These are the storm tracks which chiefly give the United Kingdom its easterly and northerly winds.