METEOROLOGY. 



45] 



higher elevation. The epochs of the phases are 

 .d at about 0,000 feet above sea level as 

 oompared with those on the plains. The mini- 

 mum on the summits occurs about six o'clock 

 in i he morning, and in the valleys between three 

 tun! four o'clock in the afternoon. The double 

 daily amplitude shows, in relation to its ampli- 

 tude on t he summits nearly the normal decrease, 

 in proportion to the decreasing pressure, but the 

 epochs of the phases exhibit a retardation on 

 the summits 01 as much as one or two hours. 

 In the tropics, however, this retardation is very 

 small. The author then endeavors to show that 

 these modifications of the daily barometric range 

 on mountain summits are generally explained 

 by the differences of temperature in the lower 

 strata of air. 



A close coincidence has been shown, by Prof. 

 Hellmann, from observations taken at different 

 British, Continental, and American stations 

 where barographs are used, to exist between the 

 daily range of the monthly extremes and that 

 of the hourly values of the barometer. The 

 author finds that the hours of occurrence of the 

 highest and lowest readings of the barometer 

 during a month agree almost completely with 

 the times in which the normal daily range has 

 its maxima and minima, both curves being so 

 similar in shape that it may be possible to judge 

 of the general character of the daily range of 

 the barometer from knowing only the hours at 

 which the monthly extremes mostly occur. 

 Hence, as the lowest readings of the barometer 

 are accompanied by cloudy and stormy weather, 

 during which the effect of the solar radiation 

 upon the surface of the earth and the heating of 

 the lower strata of the atmosphere are insignifi- 

 cant. Prof. Hellmann concludes that Prof. 

 Hann and others are right in assuming that the 

 normal daily range of the barometer is chiefly 

 an effect of the absorption of the solar rays in 

 the upper strata of our atmosphere. Prof, fiann 

 has applied the harmonic analysis to the num- 

 bers furnished by Prof. Hellmann, and, by com- 

 bining several stations in a group, has found the 

 coefficients of the periodic formula to be practi- 

 cally the same as those for the normal daily 

 range. 



In a paper on the typical weather conditions 

 of winter, Dr. W. J. Van Bebber shows by means 

 of charts how the disposition of barometric 

 pressure over the Atlantic and the continent of 

 Asia regulates the weather over western Europe. 

 Nearly twenty years ago the synoptic charts 

 issued by Capt. Hoffmeyer, of the Danish Meteor- 

 ological Institute, showed that there were three 

 large areas of very low barometric pressure in 

 the Atlantic, the most important being south- 

 west of Iceland, and two smaller areas, one on 

 the eastern side toward the northern Arctic 

 Ocean, and another on the west side toward 

 Davis Straits. These areas cause the westerly 

 and southwesterly winds which carry the damp 

 and warm air over Europe. The shifting of 

 their positions causes the variations in the west- 

 ern European wind system. M. L. Teisserenc de 

 Sort has pointed out the important part also 

 played by areas of high barometer, which has 

 given greater significance to the Danish synoptic 

 charts. The most important area of high press- 

 ure for western Europe is that stretching east- 



ward over the Azores and Madeira. If this area 

 shifts northeastwardly toward France, it blocks 

 the way of the air over the ocean, and the 

 weather becomes foggy and cold. Another im- 

 portant barometric maximum persists over cen- 

 tral Asia. This maximum is subject to frequent 

 modifications; sometimes it splits up into two 

 parts, one of which often shifts as far westward 

 as Scandinavia, and produces a persistence of cold 

 easterly winds over western Europe especially 

 when the pressure over southern Ltirope is low. 

 Storms. The view has been propounded by 

 Dr. Hann that the storms of the temperate zone 

 originate not in the convectional ascent of 

 warm air, but in great vortical movements of the 

 upper air currents, which begin over the equator 

 as antitrades, and set continuously toward the 

 poles, being gradually diverted eastward in con- 

 sequence of the earth's rotation. Owing to the 

 spherical form of the earth's surface these cur- 

 rents become irregularly congested as they 

 necessarily converge on reaching higher latitudes, 

 and thus give rise to anticyclones, or tracts of 

 excessive accumulation and pressure, and to 

 cyclonic vortices in the intervals. Admittingthe 

 probability of this view for higher latitudes, H. 

 P. Blanford observes that in low latitudes those 

 causes which impede the even flow of upper 

 currents are at a minimum, for the tendency 

 to congestion must vary as the contraction of 

 the degrees of longitude in successive parallels 

 of latitude, and can only be insignificant in the 

 tropics. On the other hand, the supposed alter- 

 native cause of storms the production and 

 condensation of vapor is at a maximum in low 

 latitudes ; and the facts recorded by Eliot, Ped- 

 ler, and others who have traced out the early 

 history of cyclones in the Bay of Bengal, go to 

 show that their formation is determined by the 

 inrush of a saturated current from the equato- 

 rial sea, which is preceded by a day or two of 

 disturbed squally weather in the birthplace of 

 the storm. The relation of these storms to the 

 features of the terrestrial surface indicates that 

 they are primarily, at least, phenomena of 

 the lower atmospheric strata, even though at a 

 later period the vortical movement may be im- 

 parted to the greatly elevated antitrade, and so be 

 carried forward into high latitudes. The tem- 

 perature test, also, as applied to tropical cyclones, 

 is in favor of this view. After an elaborate dis- 

 cussion of these questions, Mr. Blanford decides 

 that " from every point of view, whether we re- 

 gard the place and circumstances of their origin, 

 their behavior after formation, their physical 

 constitution, or the relative activity of the 

 causes supposed to be concerned in their pro- 

 duction, the conclusion seems irresistible that 

 tropical cyclones originate in a manner quite 

 different from that ascribed to the storms of 

 the temperate zone ; that they are in their early 

 stages a disturbance of the lower atmosphere; 

 and that the primary impulse is given by the 

 ascent and condensation of vapor." These re- 

 marks apply only to the cyclones of the begin- 

 ning and end of the summer monsoon, and the 

 cyclonic storms of the summer months; while 

 the storms that traverse northern India in the 

 winter and early spring are of different charac- 

 ter, and may originate in the manner suggested 

 by Prof. Hann. 



