INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 57 



C haracteHstic of changes of the harometeT in the last 3 hours 

 (a). — The character of the variation of pressure during the 3-hour 

 period preceding the observation is expressed by a single figure in 

 the radio message. It must be determined from a barograph that 

 is properly compensated for temperature. The barograph should be 

 elastically suspended in order that it may not be affected by shocks 

 and vibration incident to the motion of the ship. Singularities in 

 the run of the curve, obviously due to defects in the instrument 

 (friction, backlash, etc.) or to external effects other than rapid 

 changes in atmospheric pressure, should be disregarded. 



The use of barograph sheets with 3-hour divisions is recommended. 



The character of the 3-hour pressure change is described in the 

 table below, and graphically illustrated in figure 13. The appro- 

 priate code number for each distinctive type of curve is also given. 



Symbol a — Characteristic of changes of barometer in the last 3 hours 



Code 

 figures 



Description 



Rising, then falling 



Rising, then steady, or rising, then rising more slowly 



Unsteady ■ 



Steady or rising 



Falling or steady, then rising; or rising, then rising more 



quickly 



Falling, then rising - J 



Falling, then steady; or falling, then falling more slowly.. 



Unsteady.- 



Falling-.. _ 



Steady or rising, then falling; or falling, then falling more 



quickly _-_ 



Barometer now higher 

 than or the same as 3 

 hours ago. 



Barometer now lower 

 than 3 hours ago. 



In regions where there is a 

 marked regular diurnal varia- 

 tion of the barometer the 

 figures 2 and T may be used 

 when the regular diurnal vari- 

 ation is interrupted in such a 

 way that the term "unsteady" 

 gives the best description of 

 the character of the curve. 



When unusually r a p i d 

 changes of the barometer 

 occur between regular Green- 

 wich noon observations, a note 

 is desired in the Daily Journal describing the changes. If an unusual 

 barometric change is accompanied by winds of force 8 or higher 

 (force 6 or higher in tropical seas), the facts should be incorporated 

 in the space headed "Gale or Storm Eeports." 



Barometer change (pp). — This quantity, which represents .the 

 exact amount of the variation of pressure during the 3-liour period 

 preceding the observation, is expressed in fifths of a millihar accord- 

 ing to the scale 00 to 99. Thus, if the pressure variation were 3.4 

 millibars, "pp" would be coded as IT; if it were 11.6 millibars, "pp" 

 would be coded as 58. The conversion table, giving the code num- 

 bers for different values of j)ressure variation, follows. 



Figure 13. — Characteristics of changes of the 

 barometer in the last three hours. 



