468 T. Russell — Prediction of Cold-waves. 



3. The extent of cold-wave being known, which is the con- 

 tents of a cone, and the altitude being known, which is the 

 maximum fall of temperature expressed in units of ten de- 

 grees, then from suitably prepared tables, the areas included by 

 the ten and twenty degree temperature-fall lines to be taken, 

 these lines being the sections of planes with the cone. 



4. The various shapes that the twenty degree temperature- 

 fall areas take with different types of high and low pressure 

 will be determined. The shape of the areas will be taken as 

 exactly elliptical, with varying ratio of axes in different cases. 

 This is not strictly the shape of temperature- fall areas in actual 

 cold-waves, but it is sufficiently near for practical purposes, 

 and the best that can be adopted. In more than 90 per cent 

 of the cases, a regular ellipse will represent actual temperature- 

 falls with errors not greater than six degrees. 



5. The location of point of greatest temperature-fall will be 

 determined, and the position of the longer axis of the twenty- 

 degree fall area. 



6. A previously prepared piece of card-board of the size 

 and shape of the twenty-degree fall area will be placed on a 

 map in its proper position and a line drawn around it. The 

 thirty degree, forty degree, etc., temperature-fall lines will be 

 drawn in with regard to this twenty degree fall line, and the 

 point of maximum fall. 



7. From these curves the falls at various stations in the 

 region covered can be estimated, and the isothermal lines 

 drawn for the day on which the cold-wave is to prevail. The 

 isothermal lines in a region where a cold-wave prevails, always 

 have a certain smoothness and definiteness of sweep. If the 

 predicted isothermal lines are crooked and irregular, some 

 slight adjustment can be made, and new isotherms put in, so as 

 to represent the average position of the ones tirst drawn. 



• Extent of Cold- Wave. 



The extent of a cold-wave has been taken, as proportional to 

 the extent of the area of low pressure multiplied by an 

 unknown factor, plus the extent of the high area multiplied 

 by another unknown factor, plus another term, composed of 

 the product of an unknown factor by the extent of low area 

 of pressure, and a number expressing the density of the iso- 

 thermal lines throughout the region of the high and low areas. 

 The unit of deficiency of pressure in the low area, below a 

 pressure of 30 inches and excess in the high, is taken as one 

 inch over an area of 100,000 square miles. In different cases 

 the number expressing this excess or deficiency of pressure 

 varies from a small fraction of a unit to as much as ten units. 

 Observation equations were formed on this plan, of the form, 



