32 



NA TURE 



\_May ii, 1882 



different latitudes, in order that the navigator may b e 

 able to modify his rules accordingly, and so avoid such a 

 fatal error as that of running gradually into the centre 

 of a storm, which a rigid adherence to Piddington's rules 

 would be certain to entail. 



So far we have not considered what effect is produced 

 on the inclination by the progressive motion of a cyclone. 

 Were a cyclone regular in form and stationary, the in- 

 clination should obviously be the same at every point on 

 the surrounding isobars. When, as is generally the case, 

 it has a progressive motion, this — on the supposition that 

 it is mainly due to the general motions of the atmosphere 

 — should, by a simple application of the parallelogram of 

 velocities, alter not only the velocity but the direction of 

 the inflowing winds, increasing their inclination in the 

 rear, and diminishing it in the front part of the cyclone. 



This conclusion tallies remarkably well with the ob- 

 servations of Padre Vines in the Antilles, where the 

 cyclones travel westwards, as well as with those of Prof. 

 Loornis in North America, where they travel towards the 

 east. When we come to Europe, however, a remarkable 

 exception to this rule occurs, since here the inclination 

 is much greater on the east or front side (especially south- 

 east) of a cyclone than on the west or rear side. 1 The 

 author attempts an explanation of this fact on p. 40, but a 

 better one both of this and of the equally enormous 

 though opposite difference found by Loomis between the 

 inclination in the rear and front parts of cyclones in the 

 United States, which can hardly be altogether due to the 

 admittedly small velocity of the general motion of the air 

 over America from west to east is given by Lieut. Spindler, 

 of the Russian Navy, in a recent number of the Reper- 

 toriumj- where he considers it to be mostly due to the 

 friction encountered by the wind on the Continental side 

 of cyclones, increasing the inclination in the west and 

 therefore rear part of the American cyclones, and that in 

 the cast or front part of the European cyclones. 



If the general truth of the difference between the incli- 

 nation in the front and rear of cyclones at sea, due to their 

 progressive motion, be admitted, the case of the poor 

 navigator becomes still more complicated, as in addition 

 to considering latitude, distance from the centre, and 

 velocity of the wind, he must likewise consider in what 

 quadrant of the cyclone he is situated, since the direction 

 of the vortex with reference to that of the wind, is so 

 different in different quadrants. Fortunately it is just in 

 front of a cyclone — the most dangerous position for a 

 ship to be in — where the old circular rules are least at 

 fault, since it is precisely here where theoretically the in- 

 clination should be least. 



With respect to upper currents, the results derived 

 from theory are remarkably confirmed by those from 

 observation. In general, since the air flows in towards 

 the centre below, it must flow out from it above, and also 

 somewhat across the current which flows below, so that 

 if we stand facing the wind at the earth's surface, and at 

 no very great distance from the low centre, the upper 

 current should almost invariably flow from some point to 

 our right. This agrees with observation, since, accord- 

 ing to Clement Ley, the average direction of the upper 

 currents is 44 to the right of the direction from which 

 the surface-winds blow. The general conclusion arrived 

 at by Mr. Ley regarding upper currents, that they "mani- 

 fest a centrifugal tendency over areas of low pressure, 

 and a centripetal over those of high," is identical with 

 that arrived at by Dr. Hildebrandsson, and with the 

 author's theory as far as it applies to ordinary, or as he 

 calU them, warm-centred cyclones. 



Cold-centred cyclones, to which we alluded in our 

 previous article, do not seem to have been identified by 



according to Ley is only e/, 



1 In the north-west quadrant the inclii 

 while i!i 1 1 ■ Irant it is 35 . 



2 Ueber die Abhangigkcit der Surke und Richtung des Winde: 



iradienten an den Kiisten des Baltischei 

 " Rep. fur Met , torn. vii. No 5. St. Petersburg. 1S80. 



the author except in a stationary form surrounding either 

 pole. The observations of the upper currents hitherto 

 made indeed argue powerfully against their existence at 

 all in the progressive form, since in their case the upper 

 currents should flow in towards the low centre, accom- 

 panied by a gentle outflow below, a state of things dia- 

 metrically opposed to all present experience in connection 

 with a central area of low barometer. 



The author next discusses the effect of the general pro- 

 gressive motion of the atmosphere on the upper currents, 

 which is similar to that on the lower currents, but larger in 

 consequence of its increase with the altitude. 1 In winter 

 when the progressive motion is theoretically larger than in 

 summer (Part I.), the upper currents in our region should 

 in nearly all cases move from some westerly point, ac- 

 quiring their greatest velocity on the south side of the 

 low centre. In summer the directions should be more 

 variable, and the wind's velocity less. Observation verifies 

 both these conclusions ; thus Clement Ley found the 

 greatest velocities of the upper currents, such as 1 20 miles 

 an hour, to occur generally in the winter, when the cyclone 

 centre was to the north or north-east, and it was travelling 

 eastwards. On the other hand the cases in which the 

 upper clouds were found to be stationary most commonly 

 occurred in summer, and near the centre of areas of high 

 pressure. These facts, both as regards the strength and 

 direction of the upper currents, are confirmed by Prof. 

 Loomis's observations of the winds on Mount Washington. 

 The author then proceeds to show how the upper currents 

 may be employed to indicate through the medium of their 

 visible accompaniments — the cirrus clouds — the approach 

 and direction of a distant cyclone ; a point most valuable 

 to the seaman, who cannot command a daily weather 

 chart. He says : " The almost universal precursor of a 

 distant storm is the appearance of more cirrus-clouds 

 than usual, not only differing from those of the general 

 currents in form, but also in the direction of the currents 

 indicated by those clouds." 



In low latitudes, where according to theory the upper 

 and lower currents are more nearly radial to and from 

 the centre respectively, the direction from which these 

 clouds come, especially while the storm is still at some 

 considerable distance, is found to indicate very nearly 

 the direction of its vortex. In higher latitudes matters 

 are more complicated, since the currents are more tan- 

 gential, the upper currents flowing anticyclonically at 

 great distances from the centre, but even here, the ob- 

 server will not, as a rule, be far wrong according to the 

 author's diagrams, and those of Clement Ley, if he places 

 the centre of the approaching storm a little to the right 

 of the direction from which the cirrus advances. 



Regarding the existence of an anti-cyclone in connection 

 with every cyclone, and the broad annulus of high baro- 

 meter with its maximum at the dividing limit between 

 them, the author finds ample confirmatory evidence ; 

 though from the fact that the depression at the centre is 

 more marked than the rise of pressure near the border of 

 the cyclone, the latter is often so masked by other irregu- 

 larities as not to be readily discernible on a synoptic chart. 

 In our own islands, where we frequently encounter a string 

 of small cyclones travelling over us from the Atlantic, the 

 barometer rises briskly after the passage of each low centre 

 only to warn us of the approach of its successor, from 

 whence no doubt arose the old maxim, " Quick rise after 

 low foretells stronger blow." The author mentions that 

 the approach of the hurricane of September, 1875, was 

 indicated at Havana by a sudden rise of the barometer, 



the velocity of the general atmospheric drift with the 

 altitude is shown in Part I. p. 45, to result from the relations between the 

 velocity of the wind and the observed barometric pressures and temperatures 



in different parts of the 



ward component of vel 



cirrus-clouds — is estimated 



January, twenty-nine miles per hour In July, and throughout'the yearabou 



fifty miles per hour. At the surface, the mean velocity is calculated 



four miles per hour in January, and two and a half nvles in July. 



Id. For the latitude of the British Isles the east- 

 the elevation of five miles— the height of the 

 average about sixty-three miles per hour f 



