Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/142

Rh 132 METEOKOLOGY [HAIL. happens in the warmer months of the year, anticyclones remain practically stationary for some time, the lowermost strata of the air become abnormally heated, thus bringing about a vertical disturbance of the equilibrium of the atmo sphere out of which whirlwinds originate. It is under these conditions that white squalls or fair-weather whirlwinds occur, the originating cause of this special form being the great dryness of the air due to its place in the anticyclone, and the abnormally rapid diminution of temperature and humidity with height owing to the strong insolation through the clear dry atmosphere. The clouds accompanying the white squalls are at a great height, but the commotion and boiling of the sea under them and following them as they drift onwards show that the squalls are true whirlwinds, the vapour column of the waterspout not being formed solely on account of the extreme dryness of the air which ascends the columns. The white squall accompanies fine weather, and its appearance is sudden, its duration brief, and its destructive power at times so dreadful that it has been known to strip a ship of every sail and mast in a few seconds, and leave it rolling a helpless log amidst the tremendous sea which follows it. In sailing through such regions a close lookout should be made, particularly when the weather looks singularly fine, the skies beautifully clear, the air calm or nearly so, and the temperature and moisture of the air on board the vessel noticeably high. Diurnal Period of Hail. The hail here referred to is round, hard, and compact, and in the form of clear or granular ice, the hailstones sometimes being found when broken across to be composed of alternate layers of these two states of ice. The following figures show the number of times it has occurred during the different hours of the day at Coimbra during the last six years : Midt. to 2 A.M. 2 A.M. ,, 4 ,, 4, ,, 6 , 8 A.M. to 10 A.M. 3 10 ,, ,, Noon 20 Noon ,, 2 P.M. 15 2p.M.,, 4 ,, 13 4 P. M. to 6 p. M. 3 6 ,, 8 ,, 1 8 ,, 10 ,,, , Midt. A diurnal period is thus well-marked at Coimbra, where forty-eight out of the fifty-nine cases have occurred from 10 A.M. to 4 P.M. This period is essentially the same as those calculated for a large number of places in representa tive climates, care having been taken to limit the inquiry to the particular hail described above. The important point to be noticed in the diurnal period of hail is that the time of maximum is about two hours earlier than the maximum period of thunderstorms. The maximum period for the thunderstorm is when the ascending current from the heated land is at its greatest force for the day ; but the maximum period for hail is some time before the ascending current has fully established itself, or at that time of the day when the vertical disturbance of the atmosphere is greatest, in other words, when atmospheric temperature and vapour fall with height at a much greater rate than the normal. In the higher latitudes hail falls almost exclusively during the warmer months of the year. In regions where the summer climate is practically rainless no hail falls ; and where the rainfall is small and at distant intervals few cases of hail occur. Thus at Coimbra, where little rain falls in summer, hail was recorded as having fallen only once in the six years during the four dry hot months from June to September. All hail is probably connected immediately with whirl winds, more or less developed; and it is when the hail storm is one of the phenomena attendant on the tornado or on a great thunderstorm that it assumes its most destruc tive form. The theory of the formation of hail has been stated by Ferrel in his Meteorological Researches for the Use of the Coast Pilot, part ii. p. 85. The vapour carried aloft by the gyrations of the tornado is below a certain height condensed into cloud and rain, but above that height into snow. Let the raindrops formed below be carried up into the snow region by the powerful ascending currents of the tornado and be kept suspended there a little while, and they become frozen into hail. If now these be thrown quite outside the gyrations of the tornado, they fall to the earth as a shower of compact homogeneous hailstones of clear ice of ordinary size. If, however, they are caught in the descent and carried in toward the vortex by the inflowing currents on all sides, they are again rapidly carried aloft into the freezing region. A number of such revolutions of ascent and descent may be made before they fall to the earth. While high up in the snow region, the hailstones receive a coating of snow ; but, while traversing the region lower down where rain yet unfrozen is carried up, they receive a coating of solid ice. Thus alternate coatings of snow and ice are received, and the number of each sort indicates the number of revolutions described before the hailstones fell to the ground. When the nucleus is com posed of compact snow, as is generally the case, the hailstone had its origin high up in the snow region as a small ball of snow, or soft hail (Graupel in German and gresil in French) ; but when it is composed of clear ice throughout it was formed in the rain region, carried up into the snow region and there frozen, and immediately afterwards fell to the ground. MONTHLY, ANNUAL, AND IRREGULARLY KECURRING PHENOMENA. The Temperature of the Sea. Figs. 8 and 9, represent ing the distribution of the temperature of the surface water of the ocean for the two extreme months February and August, are reproduced chiefly from The Wind and Current Charts for Pacific, Atlantic, and Indian Oceans, published by the British Admiralty in 1872. In February (fig. 8) the temperature of the surface of the sea falls to the annual minimum over the northern hemisphere, and rises to the maximum in the southern hemisphere. The course of the isothermals more closely follows the latitudes in the Pacific, Indian, and South Atlantic Oceans ; but the divergence from the latitudes is great and striking over the North Atlantic. The wider and more open the ocean the more does the distribution of the temperature approach the normal; and the more confined the ocean the greater is the divergence from the normal. The key to the anomalous distribution of the temperature of the ocean is furnished by the charts of the distribution of atmospheric pressure and the prevailing winds of the globe. So far as observation has gone it would appear that the surface currents are practically altogether caused by the prevailing winds over the respective oceans, subject to such deilexions in their courses as are occasioned by the land. In the southern hemisphere the currents on the west side of the Indian Ocean flow southwards along the east coast of Africa, and, since the currents here pass from lower to higher latitudes, tiie temperature along the whole extent of this coast is raised consider ably above the normal. On the other hand, since the currents on the west coast of Africa flow from south to north in other words, from higher to lower latitudes the ocean currents which impinge on this coast have a temperature much under the normal. The winds and currents on the coasts of South America are precisely analogous to those of Africa, and the distribution of the temperature of the sea is also similar. The temperature of the ocean on the east coast of that continent is for the same latitudes everywhere higher than on the west coast. Even in the smaller continent of Australia the same law holds good. In the northern hemisphere a different distribution of the tem perature of the sea is seen at this season. In the Atlantic the tem perature is very much higher on the west of Europe than on the east of America. On the east of America from Wilmington to Boston occur the most rapid transitions in the mean temperature of the ocean anywhere on the globe, the temperature falling in that short distance from 70 to 30, whereas on the eastern side of the Atlantic these isothermals pass Cape Yerd Islands and Spitzbergen respectively. In the winter months the prevailing winds of the east side of North America are north-westerly, whilst in the central and eastern portion of the Atlantic they are south-westerly, thus pouring along the east coast of America the icy currents of the Arctic regions, but over the central Atlantic and along the western shores of Europe the warm waters of southern climates. The