Chapter 2— PRESSURE 



that you enter with the station pressure (rounded 

 off) and emerge with the altimeter setting. Other 

 stations may have tables which require you to make 

 the 0.01 correction to station pressure and then 

 enter the table. Refer to FMH No. 1 for the use 

 of altimeter- setting tables. The altimeter setting 

 is determined for all aviation observations; it 

 is redetermined upon request, and when necessary 

 to meet locally established requirements. 



Pressure Tendency 



The barometric pressure tendency comprises 

 the net change within a specified time and the 

 characteristic of the change during that time. 

 Pressure tendencies are determined at stations 

 equipped with a microbarograph. Stations not 

 equipped with a barograph will utilize the trend 

 of the altimeter settings entered in column 12 of 

 MF1-10 or column 15 of NWSC 3140/8 to determine 

 the pressure tendency. The pressure tendency is 

 determined for the full 3-hour period ending at 

 the actual time of the observation. 



Classify the characteristic of the barograph 

 trace for the 3-hour period, using the code 

 figures prescribed in FMH-1 table A3-9, cor- 

 responding to the same general pattern. When 

 the tendency of the observed trace is incompatible 

 with the sign of the net change, select the tendency 

 that is most nearly representative and still 

 compatible with this sign. 



FORMS 



Several types of forms are commonly used 

 in the recording of pressure elements. These 

 forms are known as; MF1-10, Surface Weather 

 Observations (Abridged for Naval Weather Use); 

 NWSC 3140/8, Surface Weather Observations 

 (Ship); MF1-13, Barometer Comparisons; and 

 Barograms that are appropriate to the barograph 

 being used. (See appendixes, IV, V) 



MF1-10 Entries 



Although the following description of pressure 

 element entries on MF1-10 is correct, it is 

 brief. The Aerographer's Mate should refer to 

 the FMH-1 for a more complete and detailed 

 description of the proper procedures. 



SEA LEVEL PRESSURE (COL. 6).— Sea level 

 pressure is entered in tens, units, and tenths 

 of millibars. If the pressure is estimated, prefix 

 the value with an "E". To obtain the sea level 

 pressure value, take the station pressure (col. 



17) and reduce this value to sea level by use 

 of a computer, constant, or table. 



ALTIMETER SETTING (COL. 12).— The alti- 

 meter setting is entered in column 12 in units, 

 tenths, and hundredths of an inch, omitting 

 the decimal point. An altimeter setting of 29.98 

 inches is logged as 998. Altimeter settings deter- 

 mined from pressure instruments of doubtful 

 accuracy or which are not routinely compared 

 with a mercurial barometer are prefixed with an 

 "E". Compute the altimeter setting value from 

 the station pressure (col. 17) by using a computer, 

 constant, or table. 



MANDATORY REMARKS (COL. 13).— These 

 remarks include the pressure tendency at 3-hour 

 intervals. Other data includes pressure that 

 is rapidly rising or falling; barogram "V"; 

 unsteady pressure; and pressure jumps. 



STATION PRESSURE (COL. 17).— Enter sta- 

 tion pressure to the nearest 0.005 inch in column 

 17. 



NWSC 3140/8 



SEA LEVEL PRESSURE (COL. 9).— Enter sea 

 level pressure in the same manner as that for 

 MF1-10. 



Aboard Navy ships, sea level pressure is 

 obtained by adding a constant pressure- reduction 

 factor to the station pressure as entered in 

 column 23. This constant is the product obtained 

 by multiplying the height (in feet) of the precision 

 aneroid barometer above the loadline by either 

 0.001 inch or 0.037 mb, depending on the markings 

 of the barometer. 



ALTIMETER SETTING (COL. 15).— Enter the 

 altimeter setting in the same manner as on 

 MF1-10. Ordinarily, altimeter settings are com- 

 puted and entered only on naval vessels from which 

 aircraft are operated. 



Shipboard altimeter settings are computed by 

 converting sea level pressure to inches. When 

 estimated, prefix the setting with an "E". Some 

 ship6 may have altimeter setting indicators which 

 are direct- reading instruments requiring only 

 instrument error corrections. 



REMARKS (COL. 16).— Enter appropriate re- 

 marks in the same manner as in column 13 of 

 MF1-10. The major exception to this is that 

 additive data referred to in sections covering 



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