Interactions 



An examination of the data in figure 2 shows that the tem- 

 perature variation with latitude differs from one longitude to 

 another, indicating that there are interactions between latitude 

 and longitude. In addition, there are indications that there are 

 interactions between latitude and day, and longitude and day. It 

 is necessary, therefore, to include such interactions in the model 

 If the model is to be realistic. 



A trial -and-error approach was used to obtain information 

 on the characteristics of the interactions to be included in the 

 model. Many combinations of the main-effect terms were tried 

 and discarded. The following terms appear to be the most 

 important for the area under consideration: 



"12^0 ■*- "13^0' + "i^LD''^ latitude by day 



longitude by day 



latitude b.\' longitude 



35 



