276 Table 67 (continued) 



CORRECTION OF ALTIMETER READINGS FOR MEAN TEMPERATURE OF 



THE AIR COLUMN 



using Table 67 A. If the altimeter setting used is higher than the true altimeter setting 

 the indicated altitude should be decreased. 



Examples. — Case I (altimeter setting 29.92 in. Hg. or 1013.2 mb.). 



What is the actual altitude A of flight relative to sea level when the elevation of the 

 surface E is 5,000 feet above sea level, the pressure altitude at the surface H P is 5,500 

 feet, the pressure altitude at flight level I P is 12,000 feet, and the value of t mv is — 10 °C? 



Referring to Table 67 A with /„ = 12,000 ft. and H P = 5,500 ft. as arguments, 



( Ip ~ Hp \=24.0. Then referring to Table 67 B with ( /p ~ Hp ) =24.0 and t mv = 



\ Tm* / \ T ma / 



— 10 °C. as arguments we find T mv ( Ip ~~ Hp \= 6310 ft. (to the nearest 10 ft.). 



\ 1 ma / 



It follows from equation (3) that A = 5.0U0 + 6,310 = 11,310 ft. 



Case II (current, local altimeter setting being used). 



What is the actual altitude A of flight relative to sea level when the elevation of 

 the surface E is 5,000 feet above sea level, the indicated altitude / at flight level is 12,000 

 feet, the altimeter setting at the surface is 29.38 in. Hg., and the value of tmv is — 10 °C. ? 



= 25.8. 



Referring to Table 67 A with / = 12,000 ft. and E = 5,000 ft. as arguments, (i—M\ 



\ 1 ma / 



25.8. 



Then referring to Table 67 B with (LjpJPl =25.8 and t mv = — 10 °C. as arguments 



\ i- ma / 



we find Tmv (^-— -)=6790 ft. (to the nearest 10 ft.). It follows from equation (7) 



\ 1 mx / 



that A = 5,000 + 6,790 = 11,790 ft. 



Allowance for perturbations of the pressure field caused by mountains. — Great caution 

 must be used in applying the above procedures to aircraft operation in regions of marked 

 perturbations of the pressure field. An aircraft flying in the vicinity of a mountain during 

 periods of strong winds and using an altimeter setting appropriate to a nearby valley 

 station may have an indicated altitude several hundred feet too high. This error is 

 caused by the lowering of the constant pressure surfaces in the vicinity of mountains 

 by the action of strong winds. The error is numerically equal to the depression of the 

 constant pressure surface in which the aircraft is flying from its location over the valley 

 station furnishing the altimeter setting to its location over the mountain. In planning 

 operations in such circumstance, equation (3) and (7) can be rewritten 



Am + C^E + Tmv 



or Am + C^E + T 



where 



C't,"' )~ d (9) 



Am = maximum height of the terrain to be cleared, 

 C = minimum vertical clearance relative to the terrain, 



D = estimated maximum depression of the constant-pressure surface being flown 

 between the valley and the mountain. 



Example.— Suppose A m — 9,200 ft., C = 2,000 ft., D = 500 ft, £ = 5,000 ft., and H P — 

 5,500 ft. for valley station, and tmv = — 10 °C. What are the minimum values of l v or / 

 which satisfy equations (9) and (9a), respectively? Use of Tables 67 A and 67 B 

 yields /, = 12,390 ft. or / = 11,920 ft. 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



