92 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 





66 y 



29.90 



76 





\ 



7<^ 

 30./2 



£ 



y? 



S2 

 J0.02 



FicaiRK 32. — Examples of entry of data on weather maps. 

 A. Wind northeast, force 3, partly cloudy, air tempera- 

 ture 70. pressure 29.92. B. Wind southeast, force 4, rain- 

 ing, air temperature 68, pressure 29.90. (". Wind west, 

 force 2, clear, air temperature 78, water temperature 76, 

 pressure 30.04. D. Wind north, force 5, snowing, air tem- 

 perature 28, pressure 29.86. J]. Wind northwest, force 8, 

 clear, air temperature 76, water temperature 68, pressure 

 30.12. F. Wind southwest, force 4, overcast, thunder- 

 storm, air temperature 82, pressure 30.02. 



barometric pressure is given in millibars with the first figure (or 

 figures) omitted. Thus, 998 millbars is coded as 98; 1,021 millibars 

 is given as 21. 



Temperatures in reports from land stations and from ships co- 

 operating with the Weather Bureau are given in Fahrenheit degrees. 

 The supplemental groups of the code also contain a figure for the 



difference between 

 air and water tem- 

 peratures, from 

 which the water 

 temperature may be 

 com])uted a[)proxi- 

 mately. 



If both air tem- 

 perature and pres- 

 sure are entered, the 

 figures for tempera- 

 ture are placed 

 above those for 

 pressure. If both air 

 and water tempera- 

 tures are entered, 

 the first figure is air 

 temperature, the 

 middle figure water 

 tem])erature, and 

 the last figure baro- 

 metric pressure. In entering biuometric ])ressure the inches may be 

 omitted and two figures used for liundredths of an inch. 



Examples of complete entries, including temperature and pres- 

 sure, are given in figure 32. 



THE ISOBARS 



All available weather re])orts from regions of interest are entered 

 on the chart in .standard form in the manner indicated in the fore- 

 going and the map is then ready for drawing of the isobars, or lines 

 joining points of equal pressure. 



Atmospheric pressure reduced to sea level, as reported in ships' 

 Aveather observations and in the radio broadcasts, is used. It is 

 customary to draw isobars for each one-tenth inch of ])ressiire. If 

 millibars are used, lines are drawn at intervals of 4 millibars. 



AVlien numei'ous observations ai'e secured and entered on the chart, 

 the isobars may be drawn satisfactorily by adhering rather closely 

 to the pressures entered. In preparing weather charts at sea. how- 

 ever, the observations are often widely scattered, which makes it 

 necessary to study carefully the wind circulation, as shown by the 

 wind arrows, before drawing the isobars. 



In some areas of considerable extent there may be no observations 

 at all. The wind directions at surrounding stations must then serve 

 as a guide in extending the isobars into or through such regions. 

 A study of the wind circulation is useful also in the event that an 

 incorrect pressure reading appears on the map, owing to faulty ob- 



