■±66 T. Russell — Prediction of Cold-ioaves. 



accompanying it may be of any extent from the smallest to 

 the largest. 



3. A great area of high pressure, with a slight or very ill- 

 defined low pressure, irregular in shape to the southeast of it, 

 or very far to the east. The cold-waves are not apt to be 

 great in this case. The area of temperature-fall is a long nar- 

 row strip extending from southwest to northeast, and never 

 reaching more than 300 miles from the southeastern edge of 

 the high area. 



4. An area of low pressure with an area of high- southwest 

 of it. The temperature-fall area is a long narrow strip in this 

 case. This type is usually a sequence of type 2 and always 

 follows after a severe cold-wave has prevailed the day before 

 in country farther to the west and north. 



5. A double Y-shaped area of low pressure, one in the region 

 of the Great Lakes open to the northeast, the other in Louisiana 

 or Texas and open to the southwest, a great "area of high pres- 

 sure in between the two, to the northwest. The cold-waves of 

 greatest extent have occurred with this type. 



6. A double low, one in the Lake Region and the other on 

 the Atlantic coast. The cold- waves with this type are always 

 extensive, but keep well towards the north. 



These varieties may be still further subdivided according to 

 the shape and position of the isobars in the area of low pres- 

 sure. The low may have closed isobars, or it may be open in 

 any direction. The closed isobars may be circular or elliptical. 

 When elliptical, the long axis may be from northeast to south- 

 west, from north to south, east to west, or northwest to south- 

 east. The latter variety is very unusual. The cold-waves 

 following each of these varieties have distinctive features. 

 These various types have distinctive features in the areas of 

 twenty-degree fall following them. The longer axis of the 

 area of temperature fall extends in the direction in which the 

 isobars are open, etc. 



The method given here for the prediction of cold-waves, 

 does not give a correct result at all times. It represents very 

 fairly the average of cases that occur ; though in a few cases, 

 it gives largely erroneous results. It is purely empirical. 

 What follows certain combinations of isobars and isotherms 

 is seen from past weather-maps, and it becomes a question how 

 to formulate the conditions and use them, for judging what 

 may occur in any special case in the future. 



From an examination of the charts of temperature-fall in 

 connection with the weather-map twenty-four hours preceding, 

 it will appear that the extent of the cold wave depends on the 

 extent and depth of the area of low pressure. It likewise 

 depends on the extent and height of the area of high pressure. 



