INSTKUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 93 



servation, a defective barometer, or to an error in transmission by 

 radio. 



In a convenient place near to tlie lowest barometer reading on 

 the map, the word low is written; near the jiighest barometer 

 reading- the word high is written. If the lowest and highest 

 barometer readings fall well within the field of observations, the 

 words Low^ and high will mark the locations of the centers of the 

 principal cyclone and anticyclone, respectively. In many cases, 

 however, the actual center of the principal cyclone will lie outside 

 of the field of observation. In such cases the word low merely 

 marks the place of lowest barometric pressure on the map and not 

 the actual center of the cyclonic system. 



Frequently there are a numbers of separate and diptinct regions 

 of low and high pressure, hence the words low and high may 

 each apj^ear in several places on the map. 



The weather map of February 27, 1933, wdth the isobars drawn 

 from data contained in the Major Marine Bulletin broadcast at 10 

 p. m., 75th meridian time (0300 G. C. T., Feb. 28, 1933), is shoAvn in 

 figure 33. 



A deep low is centered near Sable Island (fig. 33) with lowest 

 barometer reading 28.74 inches. The wind circulation and barometer 

 readings in the northeastern portion of the map indicate the presence 

 of another low^ center to the eastward. A thiixl low center appears 

 in the northwestern Gulf of Mexico, a short distance southeast of 

 Galveston, while a fourth is evident from wind circulation and 

 slightly lower pressure in the extreme northwestern portion of the 

 ]nap. The principal high overlies the lake region and central 

 valleys. 



If the barometer readings and wind arrows clearly indicate that 

 the center of the low or high is within the field of observations, a 

 circle is drawn to enclose the central region of lowest or highest 

 pressure, as has been done in the case of the low centered at Sable 

 Island in figure 33. If it appears that the region of highest or low- 

 est pressure lies beyond the boundaries of the map or outside the 

 field of observations, the isobars are left open as in the case of the 

 LOW in the northeastern corner of the map in figure 33. 



After the isobars are drawn to enclose, or partially enclose, all 

 of the separate and distinct regions where pressure is higher or lower 

 than in surrounding areas on the chart, lines for intermediate pres- 

 sures are drawn for each one-tenth inch. If the highest pressure is 

 30.46 inches, an isobar is drawn for 30.40 inches and so marked 

 with the word high at the center. If the lowest pressure is 

 29.23 inches, an isobar for 29.30 inches is drawn wdth the word low 

 at the_ center. All intermediate lines, 29.40, 29.50, 29.60, etc., up to 

 30.30 inches, are then drawn in. At the ends of the line, or in a 

 break in the line, the pressure is indicated (fig. 34). 



In drawing the lines it should be kept in mind that pressure must 

 be relatively high on one and the same side of a line and relatively 

 low on the other side throughout its course. This requirement is 

 sometimes troublesome for the beginner to satisfy when the isobaric 

 map is "flat", i. e., when there are only slight pressure differences 

 over considerable areas. Practice in preparing weather charts with 



