Chapter 5 — CLOUDS AND VISIBILITY 



OBSERVER CAN SEE 

 UPWARD !00' INTO FOG 

 BUT CANNOT SEE SKY ABOVE 



OBSERVER CAN SEE 

 SKY THROUGH FOG 



7. Ceiling. The ceiling is the height as- 

 cribed to the lowest opaque layer of clouds 

 or obscuring phenomena aloft that is reported 

 as "broken" or "overcast." 



For an obscuration, a ceiling value repre- 

 sents vertical visibility in a surface-based 

 phenomenon associated with an obscured sky. 



The ceiling is termed "unlimited' when 

 the foregoing conditions are not satisfied. 



8. Vertical visibility. Vertical visibility is 

 a ceiling value used to express the distance 

 that an observer at the ground in an obscuring 

 medium can see vertically upward into the 

 medium, or the maximum vertical height above 

 the ground at which a pilot in surfaced-based 

 obscuring medium can recognize the ground. 



9. Variable ceiling. A term that describes 

 a condition in which the ceiling rapidly in- 

 creases and decreases by one or more re- 

 portable values while the ceiling observation 

 is being taken. 



10. Visibility is defined as the greatest dis- 

 tance at which selected objects (visibility mark- 

 ers — dark or nearly dark objects viewed against 

 the horizon sky) during the day, or unfocused 

 lights of moderate intensity (approximately 25cd) 

 during the night, can be seen and identified. 



DETERMINING CLOUDS AND VISIBILITY 



Clouds 



209.213 

 Figure 5-1. — (A) Sky completely hidden by fog; 

 (B) sky partially obscured by fog. 



at and below the level of the layer) of trans- 

 parent to total sky cover is l/2 or more. 



6. Surface. For height determinations, the 

 term "surface" means the horizontal plane 

 whose elevation above sea level equals field 

 elevation. At a station where the field ele- 

 vation has not been established, the term 

 "surface" refers to ground elevation at the 

 point of observation. 



The weather observer should take several 

 factors into consideration when evaluating sky 

 cover (clouds and obscuring phenomena) — 

 determination of the sky cover's stratification; 

 amount of sky cover; direction of movement 

 of the clouds; height of the bases of the clouds; 

 and the effect of obscuring phenomena on the 

 vertical visibility. Observation of these ele- 

 ments should be taken from as many points 

 as necessary to view the entire sky. 



Determination of Stratification 



The observer should first determine how 

 many layers of clouds or obscuring phenomena 

 are present at the time of the observation. 



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