464 T. Russell — Prediction of Cold-waves. 



maximum fall between forty and fifty degrees ; two hundred 

 and sixty-two cases between thirty and forty degrees ; and two 

 hundred and sixty-four between twenty and thirty. 



The Signal Service weather-maps show that these cold-waves 

 always occur over the country covered on the preceding day by 

 an area of low barometric pressure, or over the country covered 

 by an area of high pressure. Where both occur, the cold-waves 

 attain their greatest extent. The cold-waves occur from the 

 center of the areas of low pressure towards the west. There 

 are a few cases where low pressure areas have not been fol- 

 lowed by a fall in temperature at their centers. There are 

 twelve of these in ten years where there have been rises in 

 temperature instead of falls around the centers of low pressure. 

 On the other hand, no cold-waves ever occur without the pres- 

 ence of an area of high or low pressure. Why there are 

 exceptions is not known. 



The Signal Service observations made at 8 A. M. (at 7 A. M., 

 previous to July 1, 1888) are given on the weather-maps issued 

 daily. The barometric pressure at the various stations reduced 

 to sea-level are generalized by the isobars. The temperatures 

 are shown by the isotherms drawn through places having the 

 same temperature. 



A typical case of weather-map preceding a cold-wave is that 

 of February 9, 1885. The map shows a characteristic feature 

 of the isotherms preceding a cold-wave. They run from south- 

 west to northeast in the country covered by the low pressure, 

 and after passing the center bend towards the southeast and 

 east. To the west of the region of high pressure the isotherms 

 bend to the northwest. The general appearance of the isotherms 

 would seem to indicate that the air in the low pressure has pro- 

 gressed from the south, and that in the high from the north, 

 carrying with it the isothermal lines peculiar to those regions. 

 The temparature-falls on February 10, 1885, shown on a map 

 by lines joining the points of equal fall, give areas which are 

 quite regular, having usually the appearance of ellipses one 

 within the other. The total temperature-fall on a map has 

 graphically the semblance of a cone. The temperature-fall 

 lines are sections of planes with the cone. The maximum or 

 greatest fall of temperature in the center is the altitude of the 

 cone, and the area enclosed by the lines of no change is its base. 



An examination of the maps preceding cold-waves shows that 

 the extent of cold-wave is dependent on the extent of the area 

 of low pressure and the area of high pressure on the day pre- 

 ceding it. The extent of cold-wave depends very greatly on 

 the density or sparseness of the isothermal lines in the region 

 of the low area and to the west of it. When they are numer- 

 ous and close together, the falls will be very great and will 



