34 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



When the position is doubtful, a note should be made to that effect. 

 Greenwich civil time (GG). In this column the Greenwich time 

 of the observation should be recorded to the nearest whole hour. 

 Two figures sliould be entered, according to the scale 00 to 24. 

 When observations are reported by mail only they are entered once 

 daily at 12 hours G. C. T. Observations to be sent by radio are 

 entered twice daily at 00 and 12 hours, G. C. T. Intermediate ob- 

 servations for radio messages are sent in certain seasons from 

 ships speciallv designated for that purpose, the hours being 06 and 

 18, G. C. T. 



WIND 



Wind direction {DD). — The direction of the wind to be recorded 

 is the ti^e direction, not the magnetic. Its direction as given by 

 the compass should, therefore, be corrected for the magnetic varia- 

 tion and for the deviation, if this is large, as is sometimes the case. 



Tlie direction of the wind is best ascertained by making use of 

 the fact that the crest lines of the smallest ripples on the sea surface 

 are perpendicular to the direction of the wind. These ripples, as 

 every seaman knows, ai'e very sensitive to sudden changes in the 

 character of the wind. Accentuation of them by a localized increase 

 in wind velocity produces an apparent darkening of the sea surface, 

 which serves to show the rate of travel of individual gusts or puffs. 

 With wind forces of 6 (Beaufort) or more the wind direction may 

 also be correctly estimated from the direction of the streaks of foam 

 which are then formed. At night, however, the observer may not 

 be able to distinguish the lipples on the surface, or, if it is raining 

 very hard, the pattern of the wind ripples may be obliterated. In 

 these circumstances use may be made of the ''"apparent"' wind (see 

 p. 37) properly corrected for the movement of the ship. 



The direction of the wind may be entered in the appropriate col- 

 umn on Form 1210A either directly in terms of compass points or in 

 code, according to the scale 01 — 32, in wdiich 08 = E, 16 = S, etc. 

 Howe\"er, inasnuich as the wind direction must be coded in figures 

 whenever an observation is transmitted by radio, it is customary 

 for most observers to enter the code number, and this procedure is 

 preferred by the Weather Bureau. Therefore, observers who have 

 been accustomed to recording the wind direction directly in terms of 

 compass points are urged to make a practice of using the numerical 

 scale instead. This scale is given in the code table for DD shown on 

 the following page. 



Occasionally the observer is confronted by a peculiar problem 

 when making an observation. If a squall happens to pass over the 

 ship just at the time of observation the direction of the wind may 

 suddenly change by an appreciable amount. The observer may not 

 be able to tell whether the change represents a definite and wide- 

 spread shift or simply a local disturbance, after the passage of which 

 the wind will revert to the direction it had maintained prior to 

 the squall. If he believes that it represents a local and temporary 

 condition, he may think it proper to record the wind direction which 

 had prevailed before the squall. The correct procedure in such a 

 case is as follows: Record the wind direction that happens to he 

 prevailing at the moment of observation^ even if it may appear to 



