AEROGRAPHER'S MATE 3 & 2 



Frequent observation is necessary to evalu- 

 ate stratification. A series of observations 

 often show the existence of upper layers above 

 a lower layer. Through thin lower layers it 

 may be possible to observe higher layers. 

 Differences in the directions of cloud move- 

 ments are often a valuable aid in observing 

 and differentiating cloud stratification, par- 

 ticularly when haze, smoke, etc., render depth 

 perception difficult. 



CumuAo-type clouds developing below other 

 clouds may reach or penetrate them. Also, 

 by horizontal extension, swelling cumulus or 

 cumulonimbus may form stratocumulus, alto- 

 cumulus, or dense cirrus. When clouds formed 

 in this manner are attached to a parent cloud, 

 they are regarded as a separate layer only 

 if their bases appear horizontal and at a differ- 

 ent level from the parent cloud. Otherwise, 

 the entire cloud system is regarded as a 

 single layer at a height corresponding to that 

 of the base of the parent cloud. 



Sky Cover Amounts 



Sky cover amounts are evaluated with refer- 

 ence to the entire sky above the local (apparent) 

 horizon rather than the celestial horizon; in 

 terms of the amount of sky covered, or hidden 

 (to the nearest tenth) as viewed by an observer 

 on the earth's surface. 



Evaluations are made of the following: 



1. Amount of sky hidden by surface-based 

 atmospheric obscuring phenomena, such as fog, 

 smoke, haze, precipitation, etc. 



2. The amount of sky covered but not 

 necessarily hidden by clouds and/or obscuring 

 phenomena in each layer aloft. 



3. Amount covered or hidden by a combi- 

 nation of 1 and 2. 



4. Layers reportable as being thin. 



There are various methods of estimating 

 the amount of sky cover. The procedure is 

 simplified if the sky cover consists of an 

 advancing or receding layer, or a continuous 

 layer surrounding the station. 



To estimate the amount of an advancing 

 or receding layer, determine the angular 

 elevation above the horizon of the forward or 

 rear edge of the layer as seen against the 

 sky. Use a clinometer until experience is 

 gained in estimating vertical angles. Convert 

 the angle to tenths of sky cover. (See table 

 5-1.) When the layer does not extend to the 

 horizon, determine the angular elevation of 



Table 5-1. — Sky cover evaluation 



Angle of Advancing 



or Receding Layer Edge 



Tenths of 

 Sky Cover 



Angular Elevation of 

 Layer Surrounding Station 



0° to 25° 

 26° to 45° 





 1 



0° to 2° 

 3° to 8° 



46° to 59° 

 60° to 72° 



2 

 3 



9° to 14° 

 15° to 20° 



73° to 84° 

 85° to 95° 



4 

 5 



21° to 26° 

 27° to 33° 



96° to 107° 

 108° to 119° 



6 



7 



34° to 40° 

 41° to 48° 



120° to 134° 

 135° to 154° 



8 

 9 



49° to 58° 

 59° to 71° 



155° to 180° 



10 



• 72° to 90° 



72 



