CHAPTER 2 



PRESSURE 



Taking weather observations is one of the 

 primary duties of the Aerographer's Mate 3 & 2. 

 Since life, property, and successful military 

 operations depend on reliable forecasts, it is 

 essential that the forecasts are based on accurate 

 observations. Therefore, it requires that the 

 Aeorgrapher's Mate be able to define the various 

 weather elements, know the equipment utilized 

 to sense, detect, and measure these elements, 

 including the proper safety and maintenance pro- 

 cedures of each, along with the correct pro- 

 cedures and form to be used in recording the 

 elements. 



The treatment of observational procedures 

 in this manual begins by dividing meteorological 

 instruments into chapters according to the ele- 

 ments being sensed or detected or measured 

 (Chapters 2 through 6). These chapters are 

 followed by two chapters on communications and 

 office equipment. (Chapters 7 & 8). Chapter 9, 

 specialized meteorological equipment, concludes 

 these chapters and treats with that equipment 

 which is only used in limited numbers or for 

 special observational requirements. 



PRESSURE 



Pressure is of vital interest to meteor- 

 ologists. This section is designed to give you 

 various pressure definitions, observation and 

 computational procedures, and their proper 

 entries on the MF1-10 (NWSC 3140/7) and 

 NWSC Form 3140/8. 



DEFINITIONS 



Pressure definitions are as follows: 



1. Atmospheric pressure. The pressure ex- 

 erted by the atmosphere at a given point. 



2. Station pressure. The atmospheric pres- 

 sure at the assigned station elevation. 



3. Station elevation. The officially desig- 

 nated height above sea level to which station 

 pressure pertains. 



4. Sea-level pressure. A pressure value 

 obtained by the theoretical reduction of station 

 pressure to sea level. 



5. Altimeter setting. That pressure value 

 to which an aircraft altimeter scale is set so 

 that it will indicate the altitude above mean sea 

 level of an aircraft on the ground at the location 

 for which the value was determined. 



6. Pressure change. The net difference be- 

 tween the barometric pressure at the beginning 

 and ending of a specified interval of time, 

 usually the 3-hour period preceding an obser- 

 vation. 



7. Pressure characteristic. The pattern of 

 the pressure change, as would have been in- 

 dicated by a barograph trace, during the speci- 

 fied period of time, usually the 3-hour period 

 preceding an observation. 



8. Pressure tendency. The pressure char- 

 acteristic and amount of pressure change during 

 a specified period of time, usually the 3-hour 

 period preceding an observation. 



9. Pressure altitude. The altitude, in the 

 standard atmosphere, at which a given pressure 

 will be observed. It is the indicated altitude 

 of a pressure altimeter at an altitude setting 

 of 29.92 inches of mercury and is therefore the 

 indicated altitude above the 29.92 constant-pres- 

 sure surface. 



10. Density altitude. The pressure altitude 

 corrected for temperature deviations from the 

 standard atmosphere. 



DETERMINING PRESSURES 



Weather observations require observations 

 of station pressure, sea level pressure, and 

 altimeter setting. Also required at certain 

 standard times is the pressure tendency. 



When properly calibrated and compared, the 

 precision aneroid barometer is normally used 



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