INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 95 



Avell-defined pressure areas will be of assistance in dealing witli 



maps of the "flat" type. 



When the isobaric line is draAvn, say, for 29.90 inches, there may 



be no observations of pressure exactly 29.90 inches. There may be 



some readings of 29.86, 29.88. 29.92, etc. The line for 29.90 is always 



drawn between readings which 

 are below and above 29.90, and 

 never between two readings in 

 the .90's or two in the .80's 

 (see fig. U). The 29.90 line 

 will be drawn nearer to a loca- 

 tion with pressure 29.92 inches 

 than another with 29.84 inches, 



Figure 34. — The 29.90 line is shown properly 

 drawn among readings in the .80's and the 

 .90's. The dot (or station circle) showing 

 the location of the ship or land station is the 

 point of reference in drawing the line. The 

 line should not be drawn with reference to 

 the figures for barometric pressure, since 

 they are not placed at the exact location of 

 the observing station or ship, but at a con- 

 venient point nearby, usually to the right. 



Figure 35. — The ob- 

 server standing with 

 his face to the wind 

 (NE) at the posi- 

 tion shown by the 

 dot "A" will have 

 the LOW and 

 HIGH centers in 

 the general direc- 

 tions indicated by 

 the small arrows. 



with distances in proportion to the ditfei'ences between the readings 

 entered and 29.90 inches. In some instances, with well-developed 

 LOWS, there may be diiferences of 0.20 to 0.30 inch or even more 

 between adjacent observations. In such cases two or more isobars 

 must be drawn between the two observation points. 



It is important to keep in mind that the wind directions entered 

 on the chart are closely related to the distribution of pressure. The 

 flow of air is, roughly, inclined at sea 20° to 30° to the isobars. 

 The law of Buys Ballot, to the effect that the observer, standing 

 with his face to the wind in the Northern Hemisphere, will have 

 the center of low pressure on his right, is helpful when observa- 

 tions are scattered. For example (fig. 35), if the wind is from the 

 northeast, the observer, with his face to the northeast will have the 

 low pressure to his right, or to the southeast, and the high pressure 

 to his left, or to the northwest, roughly. 



After drawing a few maps, the beginner will become familiar 

 with the clockwise and counterclockwise wind systems of the high 

 and LOW, respectively (in the Northern Hemisphere), and with the 

 conditions which are associated with the development of secondary 

 centers. 



