INTRODUCTION 5 



tion. 15 The relation of the International Steam Tables calorie to the 15° 

 gram-calorie is : 16 



1 ITcal. = 1.00032 cal. 15 



Meteorological practice. — From the relationship above, it is evident that 

 for most ordinary meteorological purposes the difference between the IT 

 calorie and the 15° calorie is negligible. However, it is still customary to use 

 the 15° calorie in radiation and associated fields of meteorology, while the IT 

 calorie has become the standard for use in thermodynamic calculations. This 

 dual usage has been adopted in general throughout this volume. In some 

 instances the type of calorie is unspecified ; here the nature of the data is 

 such that the difference is immaterial. 



15 Int. Meteorol. Org. Twelfth Conference of Directors, Resolution 164, Washington, 

 1947. 



16 See Birge, R. T., Rev. Mod. Phys., vol. 13, p. 233, 1941. 



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