490 



METEOROLOGY. 



place in hot weather, also during the morning and 

 early afternoon, while the lowest altitudes are 

 found during cyclones. 



A series of cloud photographs taken by Mr. 

 Alfred J. Henry, of the United States Weather 

 Bureau, is valuable for giving a succession of 

 pictures of the same clouds, showing their vari- 

 ations during the interval, and taken in various 

 azimuths at different stations, so that we get 

 the same formation viewed from different posi- 

 tions. They present the clouds in the form of 

 parallel bands, resembling waves or billows, and 

 record the slight changes in forms or groupings 

 which they undergo from time to time as they 

 are acted upon by slight variations in the lighter 

 air currents, till* the billowy appearance finally 

 passes away and gives place to a sky about half 

 covered with cirrus and cirro stratus. Nature 

 observes concerning this method of observation 

 that, "the study of such a cloud formation as 

 that pictured here goes a step beyond the read- 

 ing of instruments, and places in our hands a 

 powerful means by which to investigate the mo- 

 tion of the atmosphere. It can not have escaped 

 general notice that this regular arrangement of 

 .streaks presents the peculiarity of covering a con- 

 siderable extent of the sky almost simultaneous- 

 ly. On a comparatively* clear sky these strips 

 o*f cloud are suddenly formed, and, on the other 

 hand, a sky uniformly covered can in a very 

 short time break up and offer the appearance of 

 these billow waves. This sudden origin of paral- 

 lel streaks finds a complete analogy in the forma- 

 tion of waves over still water when a slight 

 wind agitates the surface and it is seen to break 

 into ripples over a considerable area. Von Helm- 

 holtz, working on this suggestion, has shown con- 

 clusively that these billow waves are due to the 

 existence of air strata of different temperatures 

 moving with different velocities, and are produced 

 at the surfaces of separation of these various 

 strata. Travelers in balloons have confirmed this 

 theory from actual experiment, and have shown 

 that the peculiar formation described is encoun- 

 tered at very various altitudes. It may be that 

 the billow clouds are visible to us only under 

 peculiar circumstances of moisture, but the wave 

 motion in the invisible air is probably a most 

 common phenomenon, and one that plays a large 

 part in determining our weather conditions." 



In a study of the amount of cloud in Europe 

 during cyclonic and anticyclonic days Dr. C. 

 Kessner has investigated the cloud observations 

 at five principal stations for twenty years (1871- 

 '00), and has followed the plan of selecting the 

 days in each month when the readings of the 

 barometer were lowest or highest. These days, 

 including the days preceding and following that 

 on which the extreme reading occurred, are those 

 called respectively cyclonic or anticyclonic peri- 

 ods. The author finds that in cyclonic periods 

 the maximum amount of cloud occurs only on 

 the principal day in summer and autumn, while 

 in winter and spring a large amount of cloud oc- 

 curs in the evening of the preceding day as well 

 as on the morning of the principal day. The 

 preceding day has generally somewhat less cloud 

 than the principal day, and almost always more 

 than the following day. This result agrees with 

 that deduced by the late Mr. Ley and by the 

 Deutsche Seewarte with respect to the distribu- 

 tion of cloud in cyclones. In anticyclonic periods 

 the author finds that the least cloud frequency 

 occurs not on the principal day, but on the pre- 

 ceding or following day; this is especially the 

 case at Christiania and Pavlovsk, where the* least 

 loud occurs before the passage of the highest 



barometric pressure, and then the cloudiness 

 gradually increases. Generally speaking, how- 

 ever, the principal day is clearest, and next to 

 this the preceding day: but not always, for at 

 Buda-Pesth and Tiflis the day following that of 

 the maximum barometric pressure has less cloud 

 than the day preceding. 



Winds. Besides the mathematical discussions 

 and physical researches, a considerable proportion 

 of the labor of the International Cloud Commis- 

 sion in the United States in 1896 and 1897 was 

 expended upon the determination of the stream 

 lines and vectors of motion which occur through- 

 out anticyclonic and cyclonic regions. An ac- 

 count of this work is given by Prof. Frank H. 

 Bigelow in the American Journal of Science. The 

 wind and the lower cloud circulation up to the 

 strato-cumulus type were found to be quite the 

 same in form, though the cloud level is rather 

 more rounded. This movement is very independ- 

 ent of that of the upper cloud region, which is 

 due eastward, or only a little sinuous over the 

 highs and the lows. This is true of ordinary 

 cyclones, but in the case of hurricanes for the 

 South Atlantic States the penetration of the 

 lower circulation into the higher is very pro- 

 nounced, showing a much deeper disturbance of 

 the air. Cyclones are very thin only 2 or 3 

 miles deep while hurricanes are certainly 5 or 

 6 miles deep. The anticyclonic and cyclonic areas 

 are hardly to be considered centers of motion 

 except in the very lowest strata, since currents 

 of air blow across them from west to east, even 

 in the cumulus region of the Rocky mountain 

 districts. It is shown that remarkably long 

 streams of air, as from the north Pacific to the 

 lake region and from the Gulf of Mexico to the 

 lakes, counterflow against each other to form the 

 cyclonic circulations. We can not consider these 

 to be due to vertical convections drawing in these 

 masses of air by indraught, since the vertical com- 

 ponent ceases at 2 or 3 miles high. Rather the 

 great horizontal convections of the lower strata, 

 caused by the interchange of air between the polar 

 and the tropic zones, produce counter-currents 

 at the cyclone centers, which develop vortices 

 discharging upward into the permanent eastward 

 drift. The fact is cited that a strong and warm 

 current in the cumulus region blows directly from 

 the Pacific Ocean eastward across a cold-wave 

 area, as showing that cold waves are thin masses 

 of air hardly 1 mile thick, produced by surface 

 radiation on the eastern or lee side of the moun- 

 tains. It is also remarkable to find that the cen- 

 ter of the high areas formed by the isobars 

 drawn from reductions made by the Hazen method 

 now employed by the Weather Bureau is often 

 500 miles distant from that indicated by the vec- 

 tors of motion. 



The results of the nephoscope observations show 

 that a slightly sinuous eastward movement pre- 

 vails over the high and low areas, as in the cir- 

 rus stratum, gradually deepening as the surface 

 is approached, till in the strato-cumulus the gyra*- 

 tory movement is very marked, and in the cumu- 

 lus, stratus, and wind levels it is predominant. 

 In the cyclone the local gyratory vectors show 

 an inward radial component from the bottom to 

 the top, and nothing outward in the upper strata, 

 as Ferrel's circulation requires. They do not show 

 a maximum velocity at a certain distance from 

 the center with a falling off near it, as Over- 

 beck's solution demands, but they increase from 

 the outside up to the center. The components are 

 strongest in the strato-cumulus region, and di- 

 minish above and below; they show a continu- 

 ous inflow everywhere, together with a strong 



