44 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



Recommendations regarding the procedure for reporting pre- 

 cix^itation that has recently begun are as follows: If the precipita- 

 tion is not clearly of the "shower" type, the observer should report 

 it by one of the numbers of the appropriate decades, according as 

 it is "drizzle", "rain", or "snow." In determining whether the pre- 

 cipitation is of the "continuous" or "intermittent" type, the observer 

 should be guided by the character of the precipitation during the 

 period since it began and by his experience of the weather of the 

 last three hours. 



(j) Use of the numbers 20, 30, 40, 50, 60, 70, 80, 90 must be 

 avoided if possible. The descriptions accompanying these numbers 

 are not specific enough to be of any great value. The numbers 

 have been introduced in the table solely to meet certain exceptional 

 cases, such as when the observer is unable, because of darkness, to 

 determine which number in the complete scale should be used. It 

 is desirable that their use be strictly confined to unusual circum- 

 stances of this sort. 



(k) When the choice of a number for the radio message is defi- 

 nitely limited to the selection of either 02 or 03, it is important to 

 make a careful distinction between a sky that is 10/10 covered with 

 clouds (overcast) and one more than 9/10 covered but less than 

 overcast. Therefore, 03 (overcast) should be reported only when 

 an unhnoken cloud sheet covers the sky; that is, a cloud layer with- 

 out any openings. 



(1) Observers should not send the number 14 (squally weather) 

 for "present weather" in the radio message.^ "Squally weather" 

 has too general a meaning; it does not give a sufficiently accurate 

 description of the state of the weather. Close examination of vessel 

 weather reports indicates that observers sometimes describe the 

 state of weather as "squally" if the wind is unusually gusty (as 

 j's frequently the case when it is blowing hard). At other times 

 "squally" is used to describe the occurrence of isolated, definite 

 squalls or heavy showers associated with cumulonimbus (Cunb) 

 clouds, which pass over the ship at most once or twice an hour. 



The two phenomena are quite different and should not be con- 

 fused. A distinction between them is incorporated in the Interna- 

 tional Code for recording wind direction (DD). The phenomenon 

 of unusual gustiness is indicated by the addition of 33 to the appro- 

 priate number for the true wind direction. That of isolated squalls 

 is indicated by the addition of 67 to the appropriate number for 

 the true wind direction, if the squalls have occurred at the time of 

 observation or during the hour preceding it. To illustrate, for a 

 west-southwest wind the observer will use the number 22 in the wind 

 direction code table, but if the wdnd is unusually gusty he will add 

 33; consequently he should use the number 55 for the wind direc- 

 tion. Another example : For a north-northwest wind the observer 

 will use the number 30 in the table, but if a squall is occurring at 

 the time of observation, or has occurred during the preceding hour, 

 he will add 67; consequently, he should use the number 97 for the 

 wind direction. 



^ With this exception : In the tropics during the hurricane season tlie number 14 may 

 be used when the weather appears to be abnormally squally. Such a condition may indi- 

 cate that a tropical storm is forming. 



