AEROGRAPHER'S MATE 3 & 2 



or above, expose the psychrometer to ambient 

 air conditions without the ventilating fan run- 

 ning. When the outside air temperature is below 

 50° F, expose the psychrometer to ambient 

 air with the ventilating fan running. 



Thoroughly saturate the wet-bulb wick, with 

 pure water, taking every precaution to prevent 

 water from contacting either the thermometer 

 tube or the dry bulb. Any moisture which may 

 have contacted the dry bulb must be removed. 



After observing the wet- and dry-bulb tem- 

 peratures, calculate the difference between the 

 dry-bulb and the wet-bulb temperature read- 

 ings. After the difference or wet-bulb depres- 

 sion is obtained, the next step is to compute 

 the dew point and relative humidity by use of 

 Psychrometric Computer CP-165/UM. The in- 

 structions are printed on the computer. The 

 psychrometric computer is used, utilizing the 

 scale closest to the NORMAL station pressure 

 (e.g., sea-level stations will use a 30-inch 

 scale for all observations). If the normal sta- 

 tion pressure is not known, refer to FMH No. 1 

 to determine the appropriate scale to use based 

 on the elevation of the station. 



Precipitation 



For determining the amount of precipita- 

 tion, most Naval Weather Service units employ 

 the 4-inch rain gage or the tipping-bucket rain 

 gage ML- 558, which is an integral part of 

 the AN/GMQ-29( ) and AN/GMQ-14( ). 



Precipitation is measured on the basis of 

 the vertical depth of water, or water equivalent, 

 which would accumulate within a specified time 

 on a level surface. 



The inch is the unit of measurement for 

 precipitation. The vertical depth of water, or 

 water equivalent, is expressed to the nearest 

 0.01 inch; less than 0.005 inch is called a 

 trace. 



Precipitation measurements are made from 

 samples caught in gages, or from samples 

 taken from representative areas when the catch 

 of solid forms in the gage is not representa- 

 tive. 



Precipitation observations are taken at the 

 time of each regular 6-hour observation and 

 at the time of the midnight observation. 



The tube of the 4-inch rain gage, when full, 

 holds 1 inch of rainfall. In the event that more 

 than 1 inch falls between the above-mentioned 

 observations, the excess drains into the over- 

 flow container. At the next observation the 

 amount collected in the measuring tube and 

 overflow container is measured. 



The rain gage unit of the AN/GMQ-29( ) 

 and AN/GMQ-14( ) records each 0.01 inch of 

 liquid water precipitation that falls at the sta- 

 tion. 



The depth measurement of the water equiva- 

 lent of the solid forms of precipitation, such 

 as snow, ice pellets, hail, and freezing rain, 

 is more difficult to obtain. The rain gage can 

 be used, provided the collector ring and meas- 

 uring tube are removed. The solid forma are 

 collected in the overflow 'jontainer. At the time 

 of the observation a measured amount of warm 

 water can be added to the contents of the con- 

 tainer to melt the solid forms. Once melted, 

 the measurement can be made as for rainfall. 

 The added amount of water, of course, must 

 be deducted in computing the total water equiva- 

 lent. In measuring snow, another method is to 

 obtain a definite volume of the snow from a 

 representative area by removing the overflow 

 container and using it like a doughnut cutter 

 to remove a snow section. This can be melted 

 and measured as described above. 



When the water equivalent of snow cannot 

 be accurately measured by melting, the observer 

 may use one-tenth of the average snow depth as 

 the water equivalent; that is, 10 inches of snow 

 correspond to 1 inch of melted snow. 



However, when use of a ratio method is 

 necessary and the mean ratio for the station 

 is known to differ from 1/10, or there is reason- 

 able evidence that an individual situation re- 

 quires a difference ratio, estimate the amount 

 on the basis of available evidence. For example, 

 it may be known that at a particular station 

 8 inches of snow usually correspond to 1 inch 

 of melted snow; therefore a l/8 ratio is used. 

 Enter the ratio used and the reasons for its 

 use in block 90 of MF1-10. 



Other than determining the vertical depth 

 of the water equivalent of the solid formr>, the 

 observer must measure them — especially snow — 

 in their solid state. This is accomplished by 

 thrusting a measuring stick Into snow at sev- 

 eral places and obtaining an average of all the 



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