54 MANUAL OF MARINE METEOROLOGICAL OBSERVATIONS 



6300. THREE-HOUR PRESSURE TENDENCY 



6310. GENERAL. The pressure tendency is composed of three ele- 

 ments: 



6311. The net change in barometric pressure within the 3-hour period 

 before an observation. 



6312. Indication as to whether the barometric pressure is higher or 

 lower at the end of the period than at the beginning of the period. 



6313. The characteristic of the change during the period. 



6320. DETERMINATION OF TENDENCY FROM THE BAROGRAPH. The 

 pressure tendency may be determined from the barograph as described 

 below: 



6321. Find the net amount of pressure change over the 3-hour period by 

 determining to the nearest 0.1 mb., the difference in pressure between the 

 beginning and the end of the period. No barometer correction (see 316020) 

 is to be applied to the value of the trace at these points. 



632ii. Observe whether the pressure is the same as, higher, or lower 

 than it was at the beginning of the period. 



6323. Determine the characteristic of the trace by observing whether the 

 trace shows a falling or rising, steady or unsteady character, or a combina- 

 tion of these. If the characteristic is so variable over the 3-hour period that 

 it cannot be identified, determine it for the 1-hour period immediately 

 preceding the observation. Code Table 15 indicates the 10 possible charac- 

 teristics. 



6330. DETERMINATION OF PRESSURE TENDENCY FROM THE BAROM- 

 ETER. If the ship does not have a barograph, determine to the nearest 

 0.1 mb. the difference in pressure between the barometric reading at the 

 time of the observation and the pressure that existed three hours prior to the 

 observation. These data may be obtained from the ship's log (by inter- 

 polation if necessary). If the pressure has increased or remained unchanged 

 during the preceding 3 hours, record the characteristic as code figure "2" 

 from Code Table 15, and as "7" if the pressure has decreased. Other code 

 figures listed in Code Table 15 will not be used when the tendency is deter- 

 mined from the barometer. 



6400. CODING AND ENTRIES OF PRESSURE DATA ON FORM 615-5 



6410. BAROMETRIC PRESSURE (PPP). Enter the barometric pres- 

 sure exactly as read (in the original units and before any corrections have 

 been applied) in column 13, "Barometer as read." Apply the correction 



