418 RECORD OB^ SCIENCE FOR 1887 AND 1888. 



tain whether a given storm will increase or diminish, and especially 

 whether it will follow some one of the average storm tracks, or whether 

 it will make special deviation from them. The general paths pursued 

 by storm centers and their average velocities can be used only for 

 general climatic indications, and not for special storm predictions. 

 Detailed maps of the direction and velocity of the movements of storm 

 centers in the United States have been jirepared; a general view of the 

 so-called storm frequency, or, more properly, the frequency with which 

 barometric minima (sometimes without violent winds) pass over each 

 section of the country, is given by a chart designed and prepared by me 

 in 1874, and published in Walker's Statistical Atlas of the United States. 

 With these barometric minima generally come clouds,rain and snow, high 

 winds, suddenly shifting from south and east to west and north, warm, 

 moist, followed by colder, dry weatliei-, and all the characteristics of 

 very variable severe climates. A more complete chart, showing the 

 total number of times that such barometric minima have dnring the ten 

 years October, 1872, to September, 1882, passed over each degree square 

 of the territory east of the meridian of 102° west longitude, has been 

 prepared, and gives a basis for many climatic generalities. 



The prediction of a special storm path, or its deviation from a normal 

 path (if such a term is allowable) must be based upon a detailed study 

 of all the above mentioned disturbing or controlling influences. Some 

 of the average effects of these influences are shown in the following par- 

 agraphs : 



(«) Maydell, from the study of the storms of Northern Europe, showed 

 in 1873 that the storm path for the coming twenty-four hours forms a 

 determinate angle with the line connecting the present place of the bar- 

 ometric minimum with that of the greatest rise of temperature in the 

 past twenty-four hours. This angle is always formed on the left hand 

 of that connecting line, supposing the face to be turned towards the 

 point of rising temperature and the back towards the pouit of minimum 

 barometer. This angle varies between zero and 90 degrees, as the ex- 

 treme cases, while it is on the average in winter about 00 degrees ; the 

 summer storm tracks are classified in two groups, for which this angle 

 is respectively 40 and 70 degrees. The angle depends to a certain ex- 

 tent upon the position of the center of highest temperature, being 

 smaller according as the point lies more to the east of the barometric 

 minimum. (See Maydell's appendix to the Bulletin for December, 1873, 

 of the Physical Observatory at St. Petersburg.) 



INl ay deli's rules accord with my own experience ; the elevation of tem- 

 l)erature befoie a storm had been long known both for England and for 

 the United States, but the amount of angular deviation of the storm- 

 center to the left of the rising temperature is, I think, very little in 

 tlie eastern part of the United States. 



(/>) The region over which the greatest rain fall will take place is 

 that towards which the storm center will have a strong tendency to 



