by agitating the water with the propeller mentioned above. The basin was put outside our 

 three-storied Oceanography Building and the sensor was mounted at the rooftop, the third 

 and second floor window, and on the tripod at ground level. The result is shown in Figure 3. 

 Air temperature changed from 38.0°F for the rooftop mount to 33.0°F for the tripod mount. 

 This figure indicates that the IRT readings again are lower than the true values and the 

 difference becomes large as the distance increases and as the water-air temperature dif- 

 ference increases. However, there is an indication that the effect of the distance might be 

 diminished as the distance increases further. 



The third series of experiments were made to determine the effect of angle of the 

 sensor window from the vertical. The sensor was mounted on a rod at two meters above 

 the water surface. Then the rod was rotated to about ten degrees from the hroizontal while 

 the sensor was always pointed at the same point on the water surface at the constant distance 

 away. Each curve in Figure 4 represents a different situation, which includes both indoor 

 and outdoor experiments. The outdoor experiments were made on a sunny day with snow on 

 the ground and on an overcast day with snow falling. On each day the differences between 

 the IRT and a mercury thermometer reading were varied according to the directions pointed 

 by the sensor, that is, toward the building, the sky or the shade of a large tree. The effect 

 of falling snow on the overcast day was so large in lowering the IRT readings that the 

 effect of angles of the sensor seemed to be obscured. However, in all these situations angles 

 of the sensor from the vertical less than 40° do not seriously influence the IRT readings. 





40 



50 



^° T. CFJ^^ 



80 



90 



100 



Figure 3. Results of tests with varying distance between IRT sensor and 

 target; A - rooftop, B - third floor, C - second floor, 

 D - ground level. See text for details 



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