March 29, 1889.] 



SCIENCE. 



247 



■can be calculated. From measurements made at Upsala and else- 

 where, it seems safe to assume that the average height of the 

 highest form of clouds, which are kaown as cirrus and cirro-stratus, 

 is 25,000 feet as a minimum estimate ; and on this basis it is cal- 

 culated by the writer, according to known geometrical principles, 

 that, if the relative velocities obtained by him for this form of cloud 

 be multiplied by ten, they will represent approximate absolute ve- 

 locities. This, however, does not alter, but merely increases, the 

 number of divisions in the scale used for the observed relative 

 velocities ; and, even if the attempt to express the results in ap- 

 proximate absolute velocities is erroneous, it does not alter the 

 main conclusions of the following article, which are based on the 

 measured apparent velocities of the cirrus as seen from the earth's 

 surface. 



Observations on the relative velocity of the clouds were begun 

 in February, 1887 ; and during the two years 1887 and 1888 there 

 were obtained 1,821 observations of the relative velocity of the 

 cirrus level of clouds, distributed as follows : — 



January. . . 

 February.. 



March 



April 



May 



June 



July 



August 



September. 

 October..., 

 November. 

 December. 



Year 



No. Days. No Ob: 



To correct for irregularities in the intervals between observa- 

 tions, the months were divided into six periods of about five days 

 each, and averages for each period determined. From the aver- 

 age of these the monthly average was obtained. During the two 

 years there were only three of these periods of five days within 

 which no observations of the cirrus velocity were obtained. 



The following numbers obtained as stated above express ap- 

 proximately in miles per hour the average monthly velocity of the 

 cirrus obtained during two years (February, 1887, to January, 

 1889): January, 120; February, 106; March, 80; April, 85; May, 

 67; June, 58; July, 57; August, 64; September, 60; October, 81 ; 

 November, 81 ; December, 102 ; year, 80. 



Individual velocities exceeding 200 miles per hour were not un- 

 common in the winter months ; and, even if these very rapidly 

 moving cirri did not exceed 20,000 feet in height, their velocities 

 must have been greater than 150 miles. In the accompanying 

 diagram are plotted for each month during nearly two years the 

 following data : the average monthly storm-velocities in the United 

 States, as obtained from the United States Signal Service Weather 

 Review ; the average monthly velocity of the cirrus observed at 

 Blue Hill Observatory ; the average wind-velocity obtained during 

 a part of this time on Mount Washington ; the average wind- 

 velocity obtained at Blue Hill Observatory ; and the average west- 

 erly component of the Blue Hill wind-velocity. The westerly com- 

 ponent was calculated by multiplying the north-west and south- 

 west winds observed at Blue Hill by cos 45°, and adding the results 

 to the wind-movement from the west, then subtracting from this 

 the easterly wind-movements treated in the same manner. The 

 result gave the excess of the westerly component of the atmos- 

 pheric movement. 



The observed direction of the cirrus movement was almost in- 

 variably from some westerly point, movements from the east only 



being observed on about a dozen days during the two years : hence 

 no correction for direction was attempted. 



It is seen that all of the curves follow the same general sweep, 

 indicating that the velocity of storms is intimately related to the 

 velocity of movement of the general atmosphere ; but the most in- 

 timate relation between the two is evidently in the cirrus region. 

 The curves show that almost every increase or decrease in cirrus- 

 velocity was coincident with a corresponding increase or decrease 

 of storm-velocity. The first letters of the months are placed along 

 the curves so that the corresponding parts can be more easily fol- 

 lowed. In general, the minor oscillations of the curves represent- 

 ing the lower winds were in opposite direction to those of the 

 cirrus ; but it seems worthy of notice that the only part of the 

 storm-curve which differed from the cirrus-curve followed very 

 closely the curve showmg the westerly movement of the wind at 

 Blue Hill. 



In order to determine whether the relation between the upper air 

 movements and the storm-velocity was a constant, the months 

 which showed average cirrus-velocities between 30 and 50 miles 

 per hour, were combined and averaged, as were also the average 

 storm-velocities for these same months. In the same manner the 

 cirrus-velocities between 50 and 70 miles, and the storm-velocities 

 for the same months were averaged ; and so for each 20 miles of 

 increased velocity of the cirrus-level. The following table gives the 

 results : — 



Cirrus-velocity. 



Average 



Storm-velocity. 

 Ratio 



130-150 



136 



This shows that the cirrus-velocities increased more rapidly than 

 the storm-velocities. A more detailed study, which is not given 

 here, showed that this held true for winter as well as summer, and 

 also showed that the most frequently observed cirrus-velocity was 

 about 60 miles per hour. 



A similarly prepared table showed that the storm-velocity in- 

 creased more rapidly than the wind- velocity at Blue Hill ; virhile the 

 ratio between the two at the height of Mount Washington seems 

 to be almost a constant (see American Meteorological Jour7ial iox 

 December, 1888. 



The following table, prepared in the same manner as that above, 

 shows how intimately also the variability of the weather is con- 

 nected with the velocity of the cirrus. The variability of the press- 

 ure and temperature at Blue Hill Observatory was found by ascer- 

 taining how much the means of consecutive days differed from 

 each other, and averaging the results without regard to sign. The 

 variability of rain was calculated on the basis that rain on every 

 alternate day would make 100 per cent : — 



Cirrus-velocity 



Average (in miles) .... 



Mean daily change in pressure 



Mean daily change in temper 



Rain variability (in per cent) 



130-150 



.36 



There has been a striking contrast between the velocity of the 

 cirrus observed during the winter of 1887-88, and during the win- 

 ter of 1 888-89 up to the present time ; and this is no doubt correlated 

 with a striking contrast in the distribution of temperature during 

 the two winters. During the winter of 1887-S8 the temperature in 

 the northern part of the United States was decidedly below nor- 

 mal, while in the southern part it was above. This, no doubt, very 

 much increased the normal pressure-gradient from the equator 

 toward the pole in the upper air ; and as a consequence the upper- 

 air movement was very rapid, carrying the cyclonic eddies along 

 with exceptional rapidity, and causing rapid and violent fluctua- 

 tions in the temperature, rainfall, humidity, etc., over the entire 

 United States except the Pacific coast. On the other hand, during 



