T. Russell — Prediction of Cold-waves. 465 



cover a wide extent of country. The maps preceding cold- 

 waves vary very much as regards isotherms. There is always 

 some diminution of temperature to the northwest of a low area. 

 Sometimes it is not more than twenty degrees in a distance of 

 500 miles from the center, and sometimes eighty degrees in the 

 same distance. 



To show to what extent the amount of temperature-fall and 

 the area of fall depend on the variation of barometric pres- 

 sure and contrast of temperature, the means in a number of 

 selected cases are shown in Table II. 



Table II. 

 Summary of Pressure and Temperature Gradients, and Mean Temperature Falls. 



Mean Mean Mean Mean 



Temperature Falls. Temp. Grad'nt Pres. Grad'nt extent of 



per 500 miles. per 500 miles. cold- wave. 

 Inches. 



53°-6 16 cases 55° 0-66 26-1 



43°-3 60 cases 47° 0-56 15- 1 



33°-5 " 44° 0-59 100 



24°-0 " 34° 0-46 3'T 



14 c -3 " 23° 040 



In forming the means in Table II, sixty cases each of falls 

 between thirty and forty, twenty and thirty, and ten to twenty 

 were selected, one from each month, October to March, during 

 the ten years. The changes less than twenty degrees do not 

 class as cold-waves, but are given, to show the dependence in 

 magnitude of the temperature-fall on the pressure and temper- 

 ature gradients in the case of small changes. The last 

 column of Table II, contains the extent of cold-wave, the unit 

 being a fall of twenty degrees over an area of 50,000 square 

 miles, or ten degrees over an area of 100,000. The extent of 

 fall in cold-waves, including all the fall greater than ten 

 degrees, but not including any less than ten degrees, varies in 

 different cases from 5 to 60 on this basis. The temperature- 

 fall is computed as the contents of a rough cone. The areas 

 of the falls are measured with a plani meter and the altitude 

 expressed in units of ten degrees of the greatest fall in temper- 

 ature. 



The shapes and relative positions of high and low areas of 

 pressure preceding cold-waves are very various. The principal 

 types most frequently recurring are as follows : 



1. A low pressure without any accompanying high pressure. 

 Cold-waves with this type are not apt to be important or 

 extensive, unless the area is continental in extent. With a 

 central pressure as low as 29 "3 inches, and a distance across the 

 thirty-inch isobar of 1,600 miles, the cold-wave is apt to be 

 considerable. 



2. A low pressure area with a high to the northwest of it. 

 This is the most frequently occurring type. The cold-waves 



