INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS gg 



sumably therefore all auroras, are due to electrical discharges ; and 

 the further fact that they vary in frequency ^yith the sun-spot period 

 indicates that this current either comes from or is induced by the sun. 



PART VII. PREPARATION OF WEATHER MAPS ON 



SHIPBOARD 



If the shipmaster desires to have weather maps prepared at sea 

 he must secure observations by radio and, since they are in code, he 

 must have them deciphered before making entries on the map. The 

 observations may be secured by either of two methods. He may 

 exchange weather reports with other ships in the region to be 

 mapped. However, this practice, if followed generally, would result 

 in much confusion by crowding radio communication channels and 

 involving a great deal of unnecessary labor for all concerned. It 

 is the only method possible in regions that are not covered by regular 

 broadcasts of weather data by a national meteorological service ; it 

 is very helpful when the ship is in the vicinity of a severe storm, 

 even when regular broadcasts are available, since special reports col- 

 lected in this manner serve to supplement the broadcast data and also 

 provide information during intervals between the broadcasts. 



The practice of copying the daily broadcasts of data arranged 

 especially for preparation of weather maps is recommended, when- 

 ever feasible. One transmission by radio then serves all ships in the 

 area with selected reports favorably distributed over the region. 



CODE USED 



The International Figure Code is now used quite generally in the 

 weather broadcasts, hence the problem of deciphering the observa- 

 tions has been greatly simplified. The reports are considerably ab- 

 breviated by use of code, hence much less time is consumed in trans- 

 mission than in individual exchanges in plain language. 



RADIO BROADCASTS 



The principal radio bulletins broadcast by the Weather Bureau 

 for the benefit of ships at sea are known as Major Marine Bulle- 

 tins. Separate bulletins are broadcast for Pacific and Atlantic 

 waters, the latter including the Gulf of Mexico and Caribbean Sea. 

 Both of these bulletins contain forecasts, warnings, and pressure 

 synopses, which are followed by land-station reports and ships' ob- 

 servations suitable for preparing weather maps. 



Many other local and special broadcasts are made by the Weather 

 Bureau for the benefit of mariners. A description of the bulletins 

 and local broadcasts, with schedules and explanations of the codes 

 used, is contained in Circular No. 1 — Radio, issued by the Weather 

 Bureau. The bulletins are subject to change from time to time, and 

 the revised editions are distributed to mariners and others. 



Schedules and frequencies of broadcasts containing forecasts, 

 warnings, and meteorological data from the weather services of other 

 countries and, briefly, the contents of the broadcasts, are given in 

 one or more of the following publications : Radio Aids to Naviga- 



