Chapter 5 — CLOUDS AND VISIBILITY 



After determining the height of all of 

 the cloud layers, the observer can now assign 

 a ceiling designator to the ceiling, if one 

 is present. Table 5-4 lists the various ceil- 

 ing designators. However, for complete in- 

 formation on ceiling designators, refer to 

 FMH No. 1. 



Visibility 



Visibility observations are taken from as 

 many points as necessary to view as much 

 of the horizon as practicable. 



Observing Aids 



Visibility charts, lists of visibility mark- 

 ers, or other positive means of identifying 

 a representative sample of lights or objects 

 used in determining visibility at the station 

 are maintained near the observing position. 

 These include lights and objects for both 

 day and night use; at local discretion, sepa- 

 rate lists or charts for daytime or nighttime 

 use may be maintained. See figure 5-2 for 

 a typical daytime visibility marker chart main- 

 tained at a weather station. Panoramic photo- 

 graphs may be used to supplement visibility 

 charts. Distances and cardinal compass points 

 are entered on such photographs. Where control 

 tower visibility is observed, separate charts 

 or lists using the control tower as the point 

 of observation are normally needed. 



Refer to FMH No. 1 for complete instructions 

 for construction of visibility marker charts 

 or lists and for the selection of daytime and 

 nighttime markers. 



Determining Visibility 



Using all available visibility markers, 

 determine the greatest distance that you can 

 see in all directions around the horizon circle. 

 When the visibility is greater than the distance 

 to the farthest markers, estimate the greatest 

 distance you can see in each direction. Base 

 this estimation on the appearance of your mark- 

 ers. If they are visible with sharp outlines 

 and little blurring of color, the visibility 

 is much greater than the distance of the markers. 

 However, if they can barely be seen and identi- 

 fied, the visibility is about the same as the 



distance to the markers. Prior to taking a 

 visibility observation at night, spend as much 

 time as practicable in the darkness to allow 

 the eyes to be accustomed to the limited light. 



Reportable Values 



Visibility is reported at land stations in 

 statute miles, and at ocean stations in nautical 

 miles, using the values given in table 5-5. 

 If the visibility is halfway between two re- 

 portable values, select the lower. 



Prevailing Visibility 



Prevailing visibility is the greatest visi- 

 bility which is attained or surpassed throughout 

 at least half of the horizon circle not necessarily 

 continuous. This term is synonymous with the 

 term "horizontal visibility" as used in the 

 synoptic code. If the visibility is variable 

 (i.e., the prevailing visibility rapidly increases 

 and decreases by one or more reportable values 

 during the period of the observation), the 

 average of all observed values is the prevailing 

 visibility. 



In uniform conditions the determination is 

 relatively simple because the prevailing visi- 

 bility will be the same as the visibility in 

 any direction. 



In nonuniform conditions one aid for 

 determining prevailing visibility is to divide 

 the horizon circle into several sectors, each 

 of which has substantially uniform visibility. 

 The observer then must determine the visibility 

 value that equals or surpasses at least one-half 

 of the horizon circle. (See fig. 5-3 for various 

 examples.) 



Sector Visibility 



Sector visibility is the visibility within a 

 specified sector of the horizon circle having 

 essentially uniform visibility. As in the case 

 of prevailing visibility, it represents the great- 

 est horizontal distance that an observer can 

 see and identify dark objects against the horizon 

 sky in the daytime, or see and identify moderate 

 intensity, unfocused lights at night. 



Transmissometer data (discussed later in 

 the chapter) may also be used as an aid in 



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