AEROGRAPHER'S MATE 3 & 2 



INCH 

 SCALE 



Vernier 



reading 



0.042 inch 



^ MB 

 ^ASCALE 



— — — 1020 



040 



1000 



29 

 Scale 

 reading 

 29.250 inches 



980 



OBSERVED 



BAROMETER 



READING 



29.292 INCHES 



Figure 2-1. — Reading the Mercurial barometer. 



constant additive correction. This constant ad- 

 ditive correction is a permanent value to be 

 added (algebraically) to the station pressure. 

 Each station authorized to use the constant 

 additive correction has been assigned this value 

 by Naval Weather Service Detachment (NWSD), 

 Asheville, N.C. 



For instance, Naval Station Midway Island 

 would add +0.034 to the station pressure in 

 order to obtain sea level pressure. 



REDUCTION BY COMPUTER.— Shore activ- 

 ities not authorized to use the constant additive 

 correction, use Meteorological Pressure Reduc- 

 tion Computer CP-402/UM, together with the 

 tables of "r" values. 



The "r M value is a ratio of sea level pressure 

 to station pressure for each degree of temper- 

 ature. As this ratio is always equal to or greater 

 than unity (1), the figure "1" preceding the 

 decimal point has been omitted from the "r" 

 tables. No interpolation is necessary when using 

 the table of "r" values. 



Complete instructions for the use of the 

 computer are printed on the computer. Instruc- 

 tions for the "r" tables are given with the 

 tables. The "r" table must be appropriate for 

 the station elevation and location. To obtain 

 "r" tables, a request should be sent to the 

 Naval Weather Service Detachment (NWSD) Ashe- 

 ville, North Carolina 28801. 



Altimeter Setting 



Altimeter settings are computed for all obser- 

 vations with the exception of a single element 

 special. They are recomputed when necessary to 

 meet local requirements or upon request. Values 

 are determined at stations equipped with a 

 mercurial barometer that is used routinely as 

 a comparison standard. Barometric instruments 

 used to obtain altimeter settings include precision 

 aneroid barometers, barographs, mercurial baro- 

 meters, and altimeter setting indicators. 



Careless and hasty altimeter settings contrib- 

 ute to potential accidents of aircraft;; therefore, 

 use extreme care in computing them — as you would 

 in all phases of weather observations. 



Altimeter settings may be computed by use of 

 the Pressure Reduction Computer (CP-402/UM), 

 specially prepared altimeter- setting tables, or in 

 the case of ships, by applying a constant correction 

 to barometric data. 



Stations authorized to use the pressure re- 

 duction computer use the computer to determine 

 the altimeter setting from station pressure. 

 Instructions for these steps are printed on the 

 computer. Basically, the operation consists of 

 setting the station pressure on one index and 

 reading the altimeter setting on another scale. 



Most stations have tables of altimeter settings 

 already prepared. Some stations may have tables 



18 



