Chapter 5 — CLOUDS AND VISIBILITY 



is used for measurement, the instrument is 

 calibrated to indicate values statistically 

 comparable to those that would be observed 

 by a human observer, using as targets either 

 dark objects against the horizon sky during 

 daylight or moderately Intense unfocused lights 

 at night; and are applicable only to the specified 

 runway or runways near which the instrument 

 is located. When the instrument is defective, 

 or when the observer has reliable reports 

 or has otherwise determined that the trans- 

 missometer indication is not a representative 

 value for the runway, transmissometer data 

 will not be used. 



Report runway visibility when it is less 

 than 2 miles along the identified runway, or 

 the prevailing visibility is less than the high- 

 est instrument minimum for the identified 

 runway. 



Most transmissometer equipment used by 

 Naval Weather Service units have visibility 

 meters and recorders that are equipped with 

 day and night scales. The weather observer 

 has only then to read the correct scale, de- 

 pendent upon the time of the day, to the nearest 

 reportable visibility value. 



Runway Visual Range (RVR) 



In the United States, runway visual range 

 is an instrumentally derived value from equip- 

 ment located alongside and about 14 feet 

 higher than the center line of the runway 

 and clibrated with reference to the sighting 

 of high- intensity runway lights or the visual 

 contrast of other targets, whichever yields 

 the greater value. This value represents the 

 horizontal distance a pilot will see down the 

 runway on takeoff or landing. Navy stations 

 are not equipped to report this value. 



FORMS 



The same forms (MF1-10 and NWSC 3140/8) 

 are used to record cloud and visibility data 

 as mentioned previously, along with the record- 

 ing rolls used in the Cloud Height Set GMQ-13 

 ( ) and Transmissometer Set GMQ-10 ( ). 



MF1-10 Entries 



The entries pertaining to ceiling, sky con- 

 dition, and visibility are described in the follow- 

 ing sections. Again, when in doubt, consult 

 FMH-1 as your final authority. 



SKY AND CEILING (COL. 3). — For each 

 layer visible at the station at and below the 

 lowest overcast layer not classified as thin, 

 enter sky-cover data (in ascending order of 

 the height of the bases of layers), the ceiling 

 classification, if the layer is a ceiling layer 

 (table 5-4); the height of the layer, in accordance 

 with table 5-3 followed by the suffix "V" 

 for variable ceilings, if applicable; and the 

 sky cover contraction appropriate to the layer, 

 (table 5-2) based on the amount of sky cover 

 at and below the level reported (summation 

 amount), and on the transparency of visible 

 portions of the layer. 



Height values are not given to surface-based 

 layers classified as a partial obscuration 

 "-X". The height ascribed to an obscuration 

 "X" represents vertical visibility and is 

 always preceded by the ceiling designator "W". 



VISIBILITY (COL. 4). — Enter theprevailing 

 visibility as determined from the weather 

 station's usual visual point(s) of observation 

 using the reportable values (table 5-5). When 

 visibility is less than 3 miles and fluctuating 

 rapidly between one or more reportable values, 

 suffix the average with a "V" and enter the 

 range of variability in the remarks section. 



VISIBILITY (COL. 4a). — Enter the tower's 

 prevailing visibility using table 5-5 whenever 

 the reported prevailing visibility at the usual 

 point of observation is less than 4 miles and 

 the reported tower visibility is more than 

 half the value entered in column 4, but less 

 than 7 miles. Refer to FMH-1 for the proper 

 procedures for entry into the transmitted re- 

 port. 



REMARKS (COI. 13). — There are many 

 mandatory and optional remarks made in 

 column 13 which relate to ceiling, sky condition, 

 and visibility. These remarks serve the purpose 

 of explaining certain column 3 and 4 entries, 

 or data concerning sky condition and visibility 

 criteria that cannot be entered in columns 

 3 and 4 for various reasons. 



CEILING AND SKY CONDITION RE-| 

 MARKS. — These remarks should include base 

 and top reports by pilots of layers not visible 

 at the station. Other remarks would include 

 breaks in the overcast (BINOVC), higher clouds 



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