28 



MEASURING THE RADIO REFRACTIVE INDEX 



the uniformity of American radiosondes has been conducted by both the 

 U. S. Air Force and the U. S. Army Signal Corps. ResuUs indicate a 

 standard deviation for the temperature sensor of 0.8 °C to 6,000 m and 

 1.0 °C above that ahitude. The standard deviation for the pressure 

 determination was 2.2 mbar below 9,000 m and 1.1 mbar above 9,000 m. 

 Under ideal conditions the humidity sensor (lithium chloride) exhibited 

 a standard deviation of 5 percent. This accuracy is possible only if the 

 element is not subjected to high humidity (95 to 100 percent), or satura- 

 tion by liquid water, and if the temperature is above °C. The response 

 of the element is especially poor where both temperature and humidity 

 are low. The measurement means little if the relative humidity is below 

 15 percent at a temperature of 20 °C, 20 percent at a temperature of 

 °C, or 30 percent at a temperature of —30 °C. 



Hence, "under ideal conditions" at sea level, the standard deviation 

 in the determination of the refractivity from radiosonde data is approxi- 

 mately 3 A^ units; at 1 km, the standard deviation would be approxi- 



16.7% 



33.3% 



50% 



66.7% 



DECREASING 



CHANGE IN RH 



100— 50 % - 



83.3-33.3% 



100-33.3% 



< 400 

 if) 



g 300 



< 200 



16.7% 



50% 



66.7% 



INCREASING 



CHANGE IN RH 



50-100% 



:/t33.3-IOO°/ 



% 



-20 -40 -20 -40 -20 -40 -20 -40 

 TEMPERATURE (°C) 



Figure 2.3. Factors affecting the lag constant of the lithium chloride humidity sensor. 



(.\fter Wexler.) 



