CHAPTER 5 



CLOUDS AND VISIBILITY 



This chapter will familiarize the Aerogra- 

 pher's Mate with the definitions of clouds and 

 visibility, including the methods and equipment 

 used to determine these important weather 

 elements. Detailed descriptions on types of 

 clouds and their structure is presented in 

 chapter 15. The basic reference for cloud 

 observations, classification, and identification, 

 is the International Cloud Atlas, NA50-1D-509. 

 Frequent reference should be made to this 

 atlas when determining cloud data. 



CLOUDS AND VISIBILITY 



Of all the weather conditions adversely 

 affecting aircraft operations, low ceilings and 

 low visibilities are by far the most common. 

 They are the cause of most of the flight 

 delays and cancellations due to weather. 



"Ceiling" and "visibility" are two terms 

 that are fundamental in aviation terminology 

 and are probably used more than any others 

 in describing flying weather. Seldom does a 

 pilot check on the flying weather without 

 paying particular attention to these conditions 

 since visibility, together with ceiling, holds 

 the answer to many flight problems. 



DEFINITIONS 



Clouds are defined as a visible aggregate 

 of minute particles of water or ice, or of 

 both, in the free air. It may also include larger 

 particles of water or ice, and particles 

 such as those present in fumes, smoke, or dust. 

 Clouds are further defined in ceiling and sky 

 conditions as follows: 



1. Layer. Clouds or obscuring phenomena 

 whose bases are at approximately the same 

 level are regarded as a layer. The layer 

 may be continuous, or it may be composed 



of detached elements. The term "layer" 

 does not imply that a clear space exists 

 vertically between layers or that clouds or 

 obscuring phenomena composing them are of the 

 same type. The only requirement of compo- 

 sition is that all the elements of the layer 

 are based at approximately the same level. 



2. Obscuration. The term ' obscuration" is 

 used to denote that an observer at the surface 

 is unable to evaluate the sky condition aloft 

 in the usual manner because surface-based 

 obscuring phenomena (fog, smoke, etc.) hide 

 more than 9/l0 of the sky, as determined to 

 the nearest tenth. (See fig. 5-1 (A).) 



3. Partial obscuration. The term partial 

 obscuration is used to denote that l/lO or 

 more of the sky (to nearest tenth), but not 

 all of the sky is hidden by surface-based 

 obscuring phenomena. Normally the phenomena 

 is uniformly distributed. (See fig. 5-1 (B).) 



4. Transparency and opacity. As used in 

 this training manual, transparency and opacity 

 of cloud layers or obscuring phenomena are 

 defined as follows: 



a. Transparent sky cover. Those portions 

 of cloud layers or obscurations which do 

 not hide the sky. Blue sky or higher clouds 

 can be discerned through these portions during 

 daylight, and the moon and brighter stars may 

 be discerned at night. 



b. Opaque sky cover. Those portions of 

 cloud layers or obscurations which hide the 

 sky and/or higher clouds. Translucent sky 

 cover which hides the sky but through which 

 the sun and moon (not stars) may be dimly 

 visible will be considered as opaque. 



5. Thin sky cover. A term applied to a 

 layer when the ratio (of summation amounts 



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