42 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



It is most important that the number 19 be sent when it applies. 

 When the ship is in tropical or subtropical waters of the Nortli 

 Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico, or the Pa- 

 cific Ocean off the coast of Mexico or Central America, and there is 

 evidence of the formation of a tropical disturbance, the number 19 

 from the "present weather" table should be sent in the radio mes- 

 sage. Even if the ship is not scheduled to send daily radio reports, 

 a special observation should be sent by i-adio to "Observer, Jackson- 

 ville" when on the Atlantic side and to "Observer, San Francisco" 

 when on the Pacific side. The Government will pay the toll charges 

 on the message. 



In selecting the proper number from the table for "Present 

 weather" the following will be of assistance : 



(a) A number from decade 2 (numbers 20-29) may be used in 

 combination with a number from some other decade for entry in 

 the observation form, but a number from 20 to 29 should be included 

 in the radio message only in the case of precipitation during the last 

 hour; never when precipitation is occurring at the time of observa- 

 tion. 



(b) Decade 3 (numbers 30-39) will apply only very rarely, since 

 the numbers contained in it refer to phenomena that occur pri- 

 marily over land. 



(c) Decade 4 (numbers 40-49) should be recorded only in the 

 case of moderate, thick, or dense fog without actual precipitation. 

 The horizontal visibility must be less than 1 kilometer (1,100 yds). 

 Fog, together with actual precipitation, should be reported by one 

 of the numbers 57, 67, or 77. 



(d) Decade 5 (numbers 50-59) should be recorded only in the 

 case of drizzle, i. e., fairly uniform precipitation (w4th or without 

 fog) consisting of tiny and extremely numerous droplets which ap- 

 pear almost to float in the air. Drizzle usually falls out of a con- 

 tinuous, dense and low layer of stratus cloud. It should always be 

 carefully distinguished from fine rain. Small rain drops falling 

 from broken shower-type clouds should never be classified as driz- 

 zle. Two criteria wliich may be helpful in distinguishing drizzle 

 from light rain are: (1) Drizzle droplets falling on absolutely calm 

 water do not ]3roduce noticeable ri]5ples when they strike the sur- 

 face; (2) drizzle is characteristically accompanied by a horizontal 

 visibility of less than 2 nautical miles. 



If precipitation is not occuring at the time of observation but there 

 has been drizzle in tlie preceding hour, one of the numbers in decade 

 2 should be used. If ordinary rain falls simultaneously with drizzle 

 at the time of observation, one of the numbers 58 or 59 should be 

 used. If fog occurs simultaneously with drizzle, the number 57 

 shoidd be used. 



(e) Decade 6 (numbers GO-69) should be used only in the case of 

 fairly continuous precipitation of ordinary rain drops from a con- 

 tinuous cloud sheet. The sky in this case is, as a rule, covered with 

 a layer of rain clouds forming from an altostratus system, or with 

 a uniformly gray but relatively high canopy of clouds, generally 

 with formless masses of cloud below (wliich may even be present in 

 such quantity that the up])er clouds are completely hidden). 



