[PATTERSON 



CANADIAN STANDARD ANEMOMETER 



87 



/J 



^2' 



'■Cfi. 



SO 



60 



70 SO 



Ml3 per ,hr 



The pressure tube velocities below 8 miles per hour are not 

 trustworthy, for it is in this range that most of the defects of the 

 pressure tube are found; one of them is due to the head requiring a 

 wind velocity of about seven or eight miles an hour to turn it, and if 

 the wind is from some other direction than that to which the head is 

 pointing the registered velocity will be too small. In this region also, 

 the pressure of the wind is very small and the pen is liable to stick. 

 Sometimes during a heavy snow storm in winter the tubes become 

 blocked with snow, causing the instrument to register too low a velo- 

 city, but in all other respects the instrument worked very well. 

 Omitting then the results for velocities below 10 miles an hour, it 

 would appear that the factor "3" for the cup anemometer is approxi- 

 mately correct for velocities under 15 miles per hour, but for higher 

 velocities the readings are too high. There are too few observations 

 however above 30 miles an hour to enable one to determine with 

 accuracy the value of the factor from the pressure tube instrument, 

 but by employing the method of least squares a probable curve for 

 the correction factor, as given in Table III, is 



Y = 1-24- -25 log Vr 

 where Y is the correction factor and Vr is the recorded velocity. This 

 equation is plotted in Fig. 3 together with the values of the factor 

 obtained in Table III, and it will be seen that the agreement is fairly 

 good. The actual wind velocities corresponding to the recorded 

 velocities as calculated from this equation are given in Table IV, and 

 the values obtained by Marvin's equation 



log V= -509+ -9012 log V (3) 



are also tabulated. The two results agree very closely. 



Table IV 



54-6 



Recorded Velocity 



Actual Velocity 



Actual Velocity from Marvin's 

 equation 



53-2 



