90 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



tion, issued by the Hydroirraphic Office, United States Navy Depart- 

 ment, Wasliing-ton. D. C. ; Wireless Weather Messages, M. O. 

 252, issued by the Meteorological Office, Air Ministry, London, 

 England; and Les Messages Synoptiques du Temps, issued by the 

 International IVIeteorological Organization, Utrechtsche weg 194, De 

 Bilt, Netherlands. 



PREPARATION OF WEATHER MAPS 



After the weather observations from various ships and land 

 stations are secured by radio, they are entered on a skeleton map or 

 a blackboard. 



If a blackboard is used, it should provide for at least two maps, 

 of the current and preceding days, or of the current and ])receding 

 observations if two or more maps are prepared daily. It is im- 

 portant that the map of the previous day or observation be retained 

 for comparison with the current map to determine the rate of move- 

 ment of weather conditions shown thereon. 



Outline charts or base maps specially designed for drawing 

 weather maps at sea are furnished by the United States Weather 

 Bureau and some other meteorological services to mariners Avho co- 

 operate in furnisliing weather observations by radio or mail. If 

 suitable base nuips are not available they may be improvised; a 

 sheet of transparent paper is placed over an appropriate small scale 

 map and the continental outlines are sketched in; the weather ob- 

 servations are then entered, the paper is removed and the map is 

 completed by drawing the lines of equal pressure — the isobars. 



A plan which requires less labor is as follows : A suitable map 

 is covered with tracing })aper fastened down with pins or thumb 

 tacks. The observations are entered and the map is completed. The 

 tracing paper is left in place with the meteorological data thereon 

 until time to draw another map. After removal, the tracing paper 

 may be filed aMay. By this method it is not iiecessary to sketch in 

 the continental outlines. If any particular map is consulted at a 

 later date it may be placed in position over the base map if the 

 reference points have been marked on the tracing. 



Some base maps designed for preparing weather charts have small 

 circles at the locations of selected weather stations from which ob- 

 servations are available. The circle is used to show the percentage 

 of the sky covered by clouds at the time of observation. For obser- 

 vations from ships, since they have no fixed reporting position, it is 

 not possible to have fixed circles ])rinted on the base chart. In en- 

 tering the state of the sky on the map it is, therefore, necessary 

 to draw a small circle at the geographical position from which each 

 ship report originated; this is also done for each land-station location 

 if such circles are not already provided on a printed base. 



An abridged list of symbols for entry of state of weather is shown 

 in figure 30. 



In ship reports containing only the universal groups, the amotmt 

 of cloudiness is not always specifically given. In such eases the loca- 

 tion of the observing station or ship may be shown by a dot instead 

 of a circle, as in examples "C" and "D" in figure 31. The supple- 

 mentary groups, when included in the reports, always contain the 

 amount of cloudiness. 



