INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 45 



BAROMETRIC PRESSURE 



111 the column headed "Barometer as read", which is fomid near 

 the left margin of Form 1210A, the observer should record the ba- 

 rometer exactly as read (i. e., without application of any corrections 

 v/hatever) to the nearest hundredth of an inch, tenth of a millimeter, 

 or tenth of a millibar. Particular care must be taken to make cer- 

 tain that the reading obtained is correct, as even the most consci- 

 entious observer is prone on occasions to misread the barometer by 

 0.10 inch or 10 millibars (depending upon the scale of the barometer) 

 or multiples thereof. Errors of this sort, usually committed when a 

 hurried observation is made, can be eliminated if the observer will 

 make it a practice to verify the original observation by taking a 

 check reading. Since the accurate reading of the barometer depends 

 largely upon a familiarity w^ith the construction and adjustment of 

 the instrument, detailed instructions for its reading have been 

 included in Part III, 



When the barometer is pumping the observer should take two or 

 three pairs of readings. Each pair should contain one of the highest 

 and one of the lowest readings obtainable. The reading to be 

 recorded is that which is obtained from averaging the whole set. 



Attached thermometer. — If the reading is obtained from a mer- 

 curial barometer the temperature indicated by the small thermometer 

 that is affixed to the barometer must be entered in the adjacent col- 

 umn to the left, headed "Attached thermometer" ; this is necessary in 

 order that the height of the mercury in the barometer tube may be 

 properly corrected for temperature. The reading shown by the at- 

 tached thermometer represents the temperature which the instru- 

 ment, as a whole, has assumed. It should be obtained before the 

 actual barometer reading is made, because heat from the observer's 

 body may, after a moment or two, cause the attached thermometer 

 to rise slightly and to indicate a temperature somewhat higher than 

 that of the mercury in the barometer. 



If the barometer is an aneroid, with attached thermometer, the 

 temperature should be entered. If there is no attached thermometer 

 it is desired, when practicable, to make an entry in the "Attached 

 thermometer" column, of the reading of a thermometer in the same 

 room. 



Barometer corrected. — A third column is provided near the left 

 margin in which should be entered the reading of the barometer after 

 all necessary corrections have been applied, including the correction 

 for instrumental error furnished by the Weather Bureau, This last 

 correction includes reduction to sea level Avhen the barometer em- 

 ployed is an aneroid. The observer need not make an entry in this 

 column, unless the observation is to be transmitted by radio, in which 

 case he must, of course, apply the corrections himself. However, 

 when a mercurial barometer equipped with a Gold scale is used in tak- 

 ing the observations, the corrected as well as the uncorrected readings 

 of the instrument should always be entered in the spaces provided on 

 Form 1210A, as the Weather Bureau does not attempt to establish 

 individual corrections for mercurial barometers with Gold scales. 



Barometer as coded (PP) . — When the barometer reading is to be 

 reported by radio, the observer must use the code table for PP 



