STORMS 



753 



proceeded east to Copenhagen. By the time it 

 arrived at Copenhagen its extent was only a fourth 

 of what it had been the previous day, and the 

 central depression half an inch less. Twelve hours 

 later the atmospheric equilibrium was restored, 

 the storm having died out on reaching the Baltic 

 Sea. The storms of the Mediterranean follow a 

 different course. While a number take the general 

 easterly course of European storms, a larger num- 

 ber originate in the gulfs of Lyons and Genoa, and 

 pursue devious courses over this northern extension 

 of the Mediterranean, till they die out ; several 

 advance from Turkey and Greece towards the 

 Alps ; and others, comparatively few, advance in 

 an easterly course towards the Levant. A marked 

 feature of these Mediterranean storms, including 

 those of the Adriatic, is their slow, frequently 

 retrograde, and erratic courses, and the small 

 space traversed by them ; and while they last the 

 mistral, with dry cutting wind, sweeps down from 

 the Alps on the nealth- resorts of southern France. 

 By far the greater number of the storms of North 

 America take their rise in the vast plain which 

 lies to the east of the Rocky Mountains, and 

 thence advance in an eastern direction over the 

 United States, their course being largely deter- 

 mined by the great lakes ; some of them cross the 

 Atlantic, and burst on the western shores of 

 Europe. But the connection of the American with 

 the European storms is not even yet well estab- 

 lished. The storms of the West Indies generally 

 take their rise somewhere north of the region of 

 calms, and, tracing out a parabolic course, proceed 

 first towards the west-north-west, and then turn to 

 the north-east about 30 N. lat. , not a few travers- 

 ing the east coasto of North America as far as 

 Nova Scotia. South of the equator they follow an 

 opposite course. Thus, in the Indian Ocean they 

 first proceed toward the south-west, and then 

 gradually curve round to the south-east. The 

 liiirrii'iiin-t of India usually pursue a parabolic 

 path, first traversing the eastern coast towards 

 Calcutta, and then turning to the north-west up 

 the valley of the Ganges. The typhoons of the 

 Chinese Seas resemble, in the course they take, 

 the hurricanes of the West Indies. 



Probably the course tracked out by storms is 

 determined by the general system of winds which 

 prevail, modified by the unequal distribution of 

 land and water on the surface of the globe, the 

 diffusion of the aqueous vapour, and its concentra- 

 tion over the regions traversed by the storms. Facts 

 seem at present to point to the general conclusion 

 that storms follow the course of the atmospheric 

 current in which the condensation of the vapour 

 into the rain which accompanies them takes place. 



Rate at which Storms travel. If the position of 

 the centre of storm I. in chart B be compared 

 with its position on the 1st on the charts, it 

 will be found to have travelled 420 miles in 24 

 hours, or at the rate of 17J miles an hour. Similarly 

 storm II. will be found to have travelled in the 

 same time 400 miles, or at the rate of 16 miles an 

 hour. This is not far from the average rate of the 

 progressive movement of European storms. From 

 an examination of extensive series of storms Pro- 

 fessor Loomis has shown that the average rates of 

 progress of storm centres are in miles per hour 28 

 for the United States, 18 for the middle latitudes 

 of the Atlantic Ocean, 17 for Europe, 15 for the 

 West Indies, and 9 for the Bay of Bengal and 

 China Sea. On January 7-8th 1877 a storm 

 travelled in 24 hours from Indianola (Texas) to 

 Eastport (Maine) 1872 miles, or 78 miles an hour. 

 On tne other hand, the rate of progress is, particu- 

 larly in the tropics, sometimes so slow as to be 

 virtually stationary ; and, as already stated, they 

 occasionally recurve on their paths. 

 46> 



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I; 



Relations of Temperature, Rain, and Cloud to 

 Storms. Temperature increases at places toward 

 which and over which the front part of the storm 

 is advancing, and falls at those places over which 

 the front part of the storm has already passed. 

 In other words, the temperature rises as the baro- 

 meter falls, and falls as the barometer rises. When 

 the barometer has been falling for some time 

 clouds begin to overspread the sky, and rain to 

 fall at intervals ; as the central depression ap- 

 proaches the rain becomes more general, heavy, 

 and continuous. After the centre of the storm 

 approaches, or shortly before the barometer begins 

 to rise, the rain becomes less heavy, falling more 

 in showers than continuously ; the clouds break 

 up when the centre has passed, and fine weather, 

 ushered in with cold breezes, ultimately prevails. 

 If the temperature begins to rise soon and markedly 

 after the storm has passed, a second storm may be 

 shortly expected. 



Direction of the Wind. If the winds in storm II. 

 in chart B be examined, they will be observed 

 whirling round the area of low barometer in a 

 circular manner, and in a direction contrary to 

 the motion of the hands of a watch, with a con- 

 stant tendency to turn inwards towards the centre 

 of lowest pressure (i.e. in the manner formulated in 

 Buys-Ballot's law). The wind in storms neither 

 blows round the centre of lowest pressure in circles, 

 nor does it blow directly towards that centre, but 

 takes a direction nearly intermediate, approach- 

 ing, however, nearer to the direction and course 

 of the circular curves than of the radii to the 

 centre. In the front of the storm the winds blow 

 more towards the centre, but in the rear they blow 

 more closely approximate to the circular isobaric 

 lines. Where the direction of the wind differs to 

 any material degree from the above it is light, and 

 consequently more under local influences, which 

 turn it from its true course. Thus, the centre of 

 the storm being near Liverpool, the direction of 

 the wind is south-west at Paris, south at Yarmouth, 

 north-east at Silloth, north at Dublin, and north- 

 west at Cork instead of south at Paris, south-east 

 at Yarmouth, north at Silloth, north-west at 

 Dublin, and west at Cork, if it had blown directly 

 to the area of lowest pressure, and west at Paris, 

 south-west at Yarmouth, east at Silloth, north- 

 east at Dublin, and north at Cork, if it had circu- 

 lated in the direction of the isobaric curves. Hence 

 in this storm the winds circulate round the centre 

 of least pressure, or, to speak more accurately, the 

 whole atmospheric system flows in upon the centre 

 in a vorticose manner. This peculiarity is com- 

 mon to all storms in the northern hemisphere that 

 have yet been examined. In the southern hemi- 

 sphere a rotatory motion is also observed round the 

 centre of storms, but it takes place in a contrary 

 direction, or in the direction of the motion of the 

 hands of a watch, instead of contrary to that direc- 

 tion, as obtains north of the equator. 



Professor Taylor first applied Dove's law of 

 rotation to explain the direction of the rotation 

 of storms round their centre. This may be 

 explained by referring to storm II. in chart B. 

 On that morning the pressure over England being 

 much less than in surrounding countries, if the 

 earth had been at rest air-currents would have 

 flowed from all directions to England, to fill up 

 the deficiency, in straight lines. The earth, how- 

 ever, is not at rest, but revolves from west to 

 east ; and, as the velocity of rotation diminishes as 

 the latitude increases, it is evident that the current, 

 which set out say from Lyons to the north, would, 

 on account of its greater initial velocity when it 

 arrived at Paris, blow no longer directly to the 

 north, but to a point a little to the east of north ; 

 in other words it would no longer be a south, but 



