AMOUNT AND DIRECTION OF MOTION 



An observation of the state of the sky should also contam an esti- 

 mate of the cloudmess; both the total cloud amount, that is the 

 amount of the sky in tenth parts covered by all the clouds, and the 

 partial cloud amounts for each genus of cloud. 



In estimating the cloud amount it is best to ignore the part of the 

 sky close to the horizon, for there the clouds seem to be packed 

 together owing to perspective, so that the cloud amount seems greater 

 than it really is. Wlien the amount of cloud is large it is easier to 

 estimate the amount of the sky that is free from cloud, and from this, 

 by subtraction, to obtain the cloud amount. 



In entering cloud amounts in record forms the observer will be 

 guided by instructions prmted on the forms or special instructions 

 accompanying them. 



The direction of motion of clouds may often be easily observed by 

 sighting on a steeple, tower, pole, or other structure in an open place. 

 If the motion of the cloud be very slow, the head should be steadied 

 by using a rest. This method is readily applied only to clouds that 

 are well above the horizon, preferably near the zenith. When a 

 nephoscope is available it should be used at or near the time of obser- 

 vation and if feasible, at some period prior to the observation when 

 plenty of time is available. Wliether with or without a nephoscope, 

 the observer should take note of the kind of clouds and their direction 

 of movement during a period of 2 or 3 hours prior to the observation 

 whenever practicable so that the information obtained will be of 

 assistance at the time of observation. 



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