AEROGRAPHER'S MATE 3 & 2 



the forward and rear edges and the tenths of 

 sky corresponding to each elevation. The 

 difference equals the sky cover. For example: 

 forward edge 78° = 0.4 sky cover; rear edge 

 53° = 0.2 sky cover. Total sky cover is the 

 difference between the two, or 0.2 sky cover. 



When a continuous layer surrounds the 

 station and extends to the horizon, determine 

 the angular elevation of the edge, and convert 

 to tenths of sky cover. (See table 5-1.) Since 

 such a distribution is improbable, the table 

 serves only as a guide in estimating amounts 

 in situations that approach such a configuration. 



At night, the use of a ceilometer or ceiling 

 light aids the weather observer greatly in 

 estimating the amount of sky cover of layers 

 of clouds. 



After the observer has determined the 

 amount of sky cover and assigned a value 

 to the various layers present, a classification 

 for each layer can now be made in accordance 

 with table 5-2. 



Variable Sky Cover 



"Variable sky condition" refers to a sky 

 condition which has varied between reportable 

 conditions (e.g., SCT to BKN, etc.) during 

 the period of observation (normally the past 

 15 minutes). 



When a ceiling is variable and is less 

 than 3000 feet, it must be reported. Ceilings 

 above 3000 feet that are variable may be 

 reported if considered operationally significant 

 by the observer. The average of all observed 

 values is used as the ceiling. 



(or a list) showing objects suitable for reference 

 heights. 



Aircraft Determinations (PIREPS) 



Many pilot reports (PIREPS) of sky con- 

 ditions are received daily from aircraft. Prior 

 to incorporating these reports into an aviation 

 hourly weather report, the observer should 

 be sure that certain rules are met and under- 

 stood. These rules are as follows: 



1„ All heights are reported from aircraft 

 in hundreds of feet above MSL (mean sea level) , 

 and a correction is applied to the height to 

 report the bases of cloud layers AGL (above 

 ground level) over your station. 



2. Heights of low and middle clouds are 

 reported within 1 l/2 nautical miles of a 

 runway of the airport and within 15 minutes of 

 the actual time of observation. 



3. Heights of cirriform clouds are reported 

 within 50 nautical miles of a runway of the 

 airport and during the past hour preceding 

 the observation. 



Regardless of the method used, heights 

 of layers are determined and reported in terms 

 of feet above the surface of the station and are 

 rounded to the reportable values in accordance 

 with table 5-3. 



NOTE: For cloud heights that are halfway 

 between reportable values use the smaller 

 of the two values. 



Determining Cloud Heights 



There are many methods that may be used 

 to determine the heights of cloud bases. These 

 include the use of balloons; radar height data; 

 known heights of landmarks; convective cloud 

 height diagrams; pilot reports; and ceiling 

 lights and ceilometers. (Refer to FMH No. 1 

 for guidance in selecting the most reliable 

 method.) If the above methods cannot be used, 

 the Aerographer's Mate can estimate the heights 

 relying on his experience and knowledge of 

 cloud forms, and from a comparison with 

 previous observations. 



Landmarks, including mountains, trees, 

 buildings, etc., may be used as reference where 

 the heights of the objects above the surrounding 

 terrain and observation point are known. 

 Normally, each weather unit maintains a chart 



Table 5-3. — Sky cover height 



values 





Feet 



Reportable values 



(coded in hundreds 



of feet) 



Entries 



5.000 or 

 less. 



5.001 to 

 10,000. 



Above 

 10,000. 



To nearest 100 ft. 

 To nearest 500 ft. 

 To nearest 1,000 ft. 



1, 10, 50, 

 etc. 



55, 75. 

 100, etc. 



140, 180, 

 200, etc. 



74 



