90 



N CLIMATOLOGY 



more than a few parts in 10^. it is convenient to consider the chmatic 

 variation of n in terms of 



N = {n - 1)10«, 



(4.1) 



as defined in chapter I. The notation N s is used to indicate that (4.1) 

 has been evaluated from standard surface weather observation. 



To obtain long-term average values of A^, one should properly average 

 individual observations over many years. This is difficult to do since, 

 in general, only summaries of weather observations are readily available. 

 However, long-term average values of temperature, pressure, and 

 humidity are available and may be converted into an "average" value 

 of A''. This "average" N differs from the true average since the inter- 

 correlation of pressure, temperature, and humidity is neglected. This 

 difference was examined by an analysis of 2 years of weather records of 

 the months of February and August at an arctic location (Fairbanks, 

 Alaska), a temperate zone location (Washington, D.C.), and a tropical 

 location (Swan Island, W.I.). These data, given in table 4.1, indicate 

 that the difference between the two methods was never more than 1.5 A'' 

 units and that the average difference was less than 1 A'' unit, which is small 

 compared to commonly observed seasonal and geographic variations of 

 20 to 100 A^ units. 



Table 4.L Two-year average value of Ns versus the value of Ns calculated from average 

 temperature, pressure, and humidity 



Fairbanks; 



February. 



August 



Washington: 



February. 



August 



Swan Island: 



February. 



August 



Average. 



0.83 



On this basis it was decided to use the long-term means given in the 

 United Nations' monthly publication, Climatic Data for the World. This 

 publication is particularly advantageous for our present study since it re- 

 ports the fictitious value of the relative humidity needed to obtain the 

 actual average vapor pressure from the saturated vapor pressure of the 

 reported mean temperature [1].^ 



^Figures in brackets indicate the literature references on p. 170. 



