Chapter 5 — CLOUDS AND VISIBILITY 



Replace defective lamps promptly whenever 

 the lamp has begun to blacken or the filament 

 sags to a noticeable extent. Lamp life varies 

 considerably due to local conditions and is 

 highly critical with respect to overvoltage or 

 excess voltage fluctuations. If short lamp life 

 or a reduction in lamp intensity is noted, notify 

 your supervisor. 



The lamp and the reflector are properly 

 focused by the manufacturer and ordinarily 

 should not need adjustment. When the focus 

 goes out of adjustment, notify your supervisor. 



The light beam should be checked frequently 

 for verticality. It may be checked with a 

 theodolite and a spirit level. Use the spirit 

 level to check on the levelness of the projector. 

 If the projector is level but the beam is not 

 vertical, the trouble is in the focusing assembly, 

 and the beam needs vertical alignment. Notify 

 your supervisor when the light beam is not 

 vertical. 



CAUTION: Always wear dark glasses when 

 working near the light beam. Also, never attempt 

 to work on the projector with the power on. 



CLINOMETER ML-119 

 (SHORE TYPE) 



The Clinometer ML-119 is the portable hand 

 instrument used to measure the angular ele- 

 vation of a projected light "spot" on the base 

 of a cloud. The clinometer consists of a sighting 

 tube nearly 3 inches in diameter at its outer 

 end. This size is necessary in order that not 

 only the light spot on the cloud, but a portion 

 of the surrounding dark sky as well, may 

 be included for contrast in the field of view. 

 A pair of cross wires aids the eye in centering 

 on the light spot. A quadrant with a scale from 

 0° to 90°, in whole degrees graduations, is rigidly 

 attached to the underside of the tube, and a 

 weighted pendant is pivoted so as to hang verti- 

 cally of its own weight when the tube is 

 sighted on an object. The reference line on the 

 pendant coincides with the 90° line on the 

 quadrant when it is sighted on the zenith. A 

 clutch, operated with the left hand by means 

 of a milled-head thumbscrew, clamps the 

 pendant in position when a sight is made. 

 (Refer to fig. 5-5.) 



To determine the cloud height, loosen the 

 pointer thumbscrew. Sight the clinometer with 



PROJECTOR 



SIGHTING 



TUBE PEEP 



SIGHT 



Figure 5-5. 



QUADRANT PLATE 



QUADRANT PLATE 

 COVER 



PENDANT CLUTCH 

 pcx ASSEMBLY 



PENDANT 



CLINOMETER 



209.111 

 — Ceiling light projector and clino- 

 meter ML-119. 



the cross wires centered upon the lower part 

 of the most clearly defined light spot on the 

 cloud; tighten the clutch by means of the thumb- 

 screw. Read the elevation angle to the nearest 

 whole degree from the quadrant. The average 

 elevation angle obtained by three sightings 

 should be used to obtain the cloud height. 

 Simple triangulation then enables the cloud 

 height to be computed by using the known 

 horizontal distance from the observer to the 

 projector as a baseline. The height in feet 

 equals the distance (length of the baseline 

 in feet) multiplied by the natural tangent of the 

 elevation angle. The height so determined is 



83 



