nation of lj. and Q T values are read from each of the tables 3 to 12. A 

 plot of ice thickness vs degree-days of frost is made from each set of 

 values obtained from the graphs, and a smooth curve is drawn for each 

 value of snow depth (l g ). An example of a completed forecast is given 

 in Figure 3. In Figure 3,0™= 1 corresponds to an ice thickness of 27 

 cm. and Qp a 2 to an ice thickness of 72 cm. on the ice potential curve 

 of Figure 2. To convert the completed forecast into a time forecast the 

 meteorologist must provide temperature and snow depth forecasts in order 

 to secure an ice thickness forecast of 120 to 150 days for this station. 



Now let us suppose that the date of first ice formation at Station 

 #37 is 12 November and that the meteorologist provides a forecast, 

 including the following data for the succeeding months s 



Mean Snow 



p , . Temperature Degree-Dy Depth 



Period ^ 0c) of Frost (cm j 



12 - 15 Nov. 



-5.8 



16 







16 - 30 Nov. 



-4.8 



45 







1-15 Dec. 



-8.8 



105 



2.5 



16 - 31 Dec. 



-7.8 



96 



2,5 



1-15 Jan. 



-9.8 



120 



7.5 



16 - 31 Jan. 



-18.8 



272 



7.5 



The ice thickness for any given date may be found from the above data 

 and the forecast curve. 



During the period when the snow depth is zero, the growth of ice 

 will be along the upper curve on Figure 3} on 30 November, the final 

 day with no snow, the total of degree-days of frost is 16 4- 45 or 61 

 and the ice thickness will be 17 cm. (point A on Figure 3). 



During the month of December the ice grows under a snow depth of 

 2.5 cm. from a starting point of 17 cm. This point is marked A*. During 

 December a total of 105 plus 96 or 201 degree-days of frost is counted^ 

 so that it is necessary to follow the curve l g = 2 5 for 201 degree-days 

 of frost beyond point A 1 , or to point B. At point B, reached 31 December , 

 the ice thickness will be 35 cm. 



Similarly, during January the ice grows uhder a snow depth of 7.5 

 cm. for 120 plus 272 or 392 degree-days beyond the point B 1 corresponding 

 to an ice thickness of 35 cm. On 31 January the point G is reached, and 

 the ice thickness will be 53 cm. This constitutes an operational fore- 

 cast, as the user of the forecast can determine the ice thickness on any 

 day between 12 November and 31 January by counting the number of degree- 

 days of frost and noting the snow depth applicable to the situation,, 



11 



