T. Russell — Prediction of Cold-waves. 467 



When both a low and high pressure occur together, the cold 

 wave is apt to be very great. The surest indication of a com- 

 ing great fall of temperature, both deep and extensive, is a 

 crowded condition of the isothermal lines to the northwest of 

 an area of low pressure. This condition is usually the result 

 of a great fall of temperature in the preceding twenty- four 

 hours, in a district to the west and north. Sometimes, how- 

 ever, this crowded condition of the isotherms is the result of a 

 slow cooling over a wide area of country, lasting several clays. 

 Then a low area of pressure putting in an appearance to the 

 southeast of it, the next day there follows a great and exten- 

 sive fall of temperature, without any very great fall the day 

 preceding. At times, there is a regular progression of the 

 areas of fall from west to east, or southeast. Areas of low 

 pressure appearing in the vicinity of Lake Superior, with a 

 high area to the west, have temperature-falls on the next day 

 at places east of the Missouri River and skirting along as far 

 south as the Ohio River. 



One of the most important types of map is that of a low 

 area of pressure in Texas and a great high to the north. Areas 

 of great temperature-fall always follow this type, with the long 

 axis extending north to south. The greatest falls occur in the 

 southwest. The areas of fall on the next day are farther to 

 the east, and later, falls follow to the north, in Maryland, 

 Pennsylvania, New Jersey and ]N"ew York. This is a type for 

 which it is possible, when the high and low areas are large, to 

 make successful predictions of cold- waves for the eastern States 

 two days in advance. 



Inasmuch as the extent and depth of temperature-fall depend 

 on the extent of the areas of high and low pressure, it was 

 decided to ascertain how far this was true, and to determine if 

 possible the numerical relation between them. The areas 

 enclosed by the temperature fall lines of ten, twenty, thirty, 

 forty degrees, etc., in the various cold-waves were measured by 

 means of a planimeter on maps of the United States on a scale 

 °^ tto.toV.tw The areas between the isobars of high and 

 low areas of pressure on the maps preceding cold-waves were 

 measured in the same way. The areas between the isothermal 

 lines in the region covered by the areas of high and low 

 pressure were also measured. 



The method proposed for the prediction of cold-wave, is as 

 follows : 



1. From the measured extent of the high and low area of 

 pressure always preceding cold-waves to determine what the 

 total extent of the fall in the cold- wave will be. 



2. To determine the maximum fall of temperature that 

 is to take place in- a cold-wave. 



