NA TURE 



{May 5, 1887 



curvature. The isobars for January 14, 1876, show between 

 one widespread low-pressure area over the North Atlan- 

 tic and Arctic Oceans, and another over the Mediterra- 

 nean, a belt of high pressure, with nearly straight and 

 parallel isobars of 30-3 and 30-5 inches, extending from 

 south-west to north-east for about 3000 miles. 



With the view of throwing some light on the origin of 

 anticyclones, all those cases in the States in which the 

 barometer rose to 30-85 inches or upwards during the 

 twelve years ending with 1884 were examined. Of such 

 anticyclones there were fifty-two cases, distributed 

 as follows : in October one, November ten, December eight, 

 January fourteen, February sixteen, and March three ; the 

 earliest having occurred on October 26, and the latest 

 on March 12. The majority of the whole number of 

 cases occurred north of lat. 46°, only two being south 

 of lat. 40° ; and the influence of longitude was equally 

 strongly marked : 82 per cent, of the cases occurred west 

 of long. 90° W., and nearly half of the number west of 

 long. 100° W. These areas of high pressure indicate a 

 general movement towards the south-east, the prevalent 

 direction being S. 40° E. at a mean velocity of twenty-one 

 miles an hour. 



An instructive point in the history of anticyclones is 

 the low temperature which accompanies them. It is 

 shown from a somewhat exhaustive examination that, when 

 pressure is unusually high, temperature is generally very 

 much below the mean, and that the amount of the de- 

 pression increases with the height of the barometer ; and 

 it is further shown that with a given barometric pressure 

 the temperature depression is greatest in the neighbour- 

 hood of Manitoba, or approximately in the centre of the 

 continent. These relations are sometimes shown on the 

 weather-maps with a surprising closeness, of which an 

 instructive instance is figured on Plate XXV., which gives 

 the isobars and thermic isabnormals for December 24, 1872. 

 In the case of the more strongly pronounced anticyclones 

 the depression of the temperature below the mean for 

 the season is on the average 38°7. The maximum tem- 

 perature depression may occur in any direction from the 

 place of highest pressure, but it is generally about 400 miles 

 distant from it, and most frequently on its northern side. 

 Indeed, in only 12 per cent, of the cases examined did 

 the place of greatest temperature depression correspond 

 exactly with the place of highest pressure. Similar rela- 

 tions exist as regards the summer anticyclones, with the 

 important differences that their maximum pressure is 

 generally about half an inch less than that of winter 

 cyclones, and their temperature depression below the 

 mean of the season only about 8°'o. 



The average breadth of the American anticyclones is 

 2587 miles, which is nearly equal to the breadth of the 

 continent in lat. 40°, and an examination of the maps of 

 the hiternational Bulletin show that a large number of 

 high-pressure areas are 4000 miles in length. The 

 average distance from the centre of an anticyclone to the 

 centre of the cyclone on its east side is 2371 miles, and 

 to the centre of the cyclone on its west is 2381 miles, 

 being thus nearly equal. Since, however, the average 

 value of the lowest isobars of the cyclones on the east 

 side is 29-190 inches, and on the- west 29*570 inches, it 

 follows that the gradients on the east side of the United 

 States anticyclones are about twice as great as those on 



the west side. This is an extremely valuable result, as 

 showing the powerful influence of the Atlantic in lower- 

 ing atmospheric pressure, and thus intensifying storms 

 as they near its seaboard in their course eastwards. 

 Indeed, as regards those cyclones which show a pressure 

 not exceeding 29-00 inches at their centre — in other 

 words, the more important storms— five-sixths of the 

 number occurred near the Atlantic. The cases examined 

 numbered 131, and were distributed through the months 

 as follows, beginning with January : 14, 16, 25 ; 10, 4, i ; 

 o, 2, 2 ; 7, 26, and 24 ; there having thus occurred 

 105 during the five months from November to March, 

 and only 5 during the four months from June to 

 September. 



It is shown that, while an area of extraordinarily high 

 pressure uniformly has an area of low pressure both on 

 its east and west sides, the barometer in these areas 

 seldom sinks very low, and when it does sink very low 

 the centre of the low pressure is very remote ; and on the 

 other hand that an area of very low pressure has an area 

 of high pressure both on its east and west sides, but in 

 these areas of high pressure the barometer seldom rises 

 very high. 



As regards anticyclones, Prof. Loomis makes it abund- 

 antly clear that the height to which pressure rises in their 

 centres is approximately proportioned to the depression 

 of temperature over the region at the time. In these cir- 

 cumstances, the air, being condensed by the cold, sinks 

 more and more into the lower regions of the atmosphere, 

 thus leaving a less pressure in the upper regions than pre- 

 vails all round at these heights, and consequently upper 

 currents set in all round towards and upon the anti- 

 cyclonic region, by which its high pressure is further 

 increased. Hence it follows that it is over one and the 

 same region, viz. the more inland portions of the Europ- 

 asian continent, where depressions of temperature are 

 greatest, most widespread, and most persistent, and 

 where the best-pronounced anticyclones occur in the 

 colder months of the year. This state of things is entirely 

 due to the great extent of the Old continent, by which a 

 very large portion of it is remote from the disturbing 

 influence of the ocean. The same cause explains a 

 singular feature of the anticyclones of this part of the 

 globe, viz. the absence they frequently show, sometimes 

 for weeks together, of any movement of translation over 

 the earth's surface, being so different in this respect from 

 the anticyclones of the comparatively narrow continent 

 of America, which have a distinctly progressive move- 

 ment towards the south-east, already referred to. 



On the other hand, low pressures are distinguished by 

 the relatively high temperature which accompanies them. 

 The highest temperature occurs about 300 miles on the 

 south or east side of the low centre of the cyclone, the 

 average excess above the mean temperature being 22°*3. 

 No inconsiderable part of the low pressure of cyclones is 

 occasioned by the high temperature and excessive 

 moisture which accompanies them. But, while all, or 

 nearly all, of the high pressure of anticyclones may be 

 accounted for by the very low temperatures which over- 

 spread the same region at the same time along with the 

 resulting upper currents concentrating upon them from 

 adjoining cyclonic regions, it is quite different with the 

 low pressures of cyclones. In the case of cyclones the 



