INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 4^ 



Situations may occur, however, in Avliich it is especially difficult to 

 decide what is the proper number for the radio message. If the 

 observer has entered more than one number for present weather and, 

 after referring to the instructions, is still in doubt as to which one 

 to send in the radio message, he should send the highest number 

 entered.^ For example, Avith entries of 02, 10, 25 (cloudy, precipita- 

 tion within sight, rain showers in last hour) he should send the num- 

 ber 2'5 by radio. 



In the Dailj^ Journal the weather may be described briefly in 

 plain language, abbreviations, Beaufort notation, or by numbers 

 preceded by "weather" or "wea." For example, "wea 03, 10, 14, 19" 

 would indicate "overcast, precipitation within sight, squally weather, 

 signs of a tropical storm". 



Explanatory remarks on the code tahle for ^'■Present 'weather'''' and 

 instructions as to its use. — Since the publication of the previous 

 edition of this manual, the application of new methods of analyzing- 

 weather reports for purposes of forecasting and making meteoro- 

 logical stuclies has served to emphasize the great value of accurate 

 "present weather" observations from ships. This applies particu- 

 larly to the correct identification of the various precipitation forms. 

 Provision has been made in the International Code for a large num- 

 ber of "present w^eather" descriptions, as is obvious from a glance at 

 the code table. 



The table is divided into 10 "decades", i. e., into 10 groups, each 

 of which contains 10 numbers. These decades, which correspond to 

 10 main types of weather, are numbered to 9. The first digit of 

 any number in the table indicates the decade to which that particular 

 number belongs. 



After the observer has selected the decade which best applies to 

 the prevailing weather conditions, he should pick out from that 

 particular decade the number which most correctly describes the 

 weather at the time of observation or during the hour preceding. 

 Neither when selecting the decade nor in the determination of the 

 complete number must account be taken of Aveather phenomena 

 which occurred more than 1 hour prior to the time of observation 

 (except in cases ww = 15 or 16). Occasionally it will be appropriate 

 to select numbers from two or more different decades for entry on 

 the observation form, as illustrated above. However, in selecting the 

 proper decade and number for transmission in the radio message, the 

 observer should remember that correct and explicit information re- 

 garding types of precipitation is of the most value, both to the Gov- 

 ernment forecaster and to the mariner who is constructing his own 

 weather chart. An accurate report of hydrometeors (i. e., precipita- 

 tion forms) not only tells what kind of air mass is present but also 

 what kind of thermodynamic process is taking place aloft, the 

 knowledge of both of which is of great assistance in drawing an in- 

 ference as to the weather of the future. Therefore, as far as the 

 radio message is concerned, numbers 00-18 should he used only when 

 no precipitation has occurred during the preceding hour, and also no 

 ■precipitation at the time of observation. 



^ Exceptional cases in which tliis rule does not hold will be noted later. 



