Chapter 11 — WATCH ROUTINES (CONTINUED) 



analyzing the surface weather chart is to study 

 the previous charts and draw past fronts and 

 pressure systems (with a yellow pencil) on the 

 chart being analyzed. The past history of the 

 weather is very important and very significant. 

 In most weather offices it is the practice to 

 enter the past history on a chart BEFORE 

 it is even plotted. Prior to tracing past history, 

 any corrections to previous analyses that may 

 be necessary due to late or additional reports 

 should be made. In any event, the study of the 

 past history of the weather is the FIRST STEP 

 in map analysis, whether the chart is the sur- 

 face chart, a constant pressure chart, or a 

 supplementary chart. 



The second step leading to the complete 

 analysis consists of visually scanning the chart 

 and noting the plotted information, areas of 

 high and low pressure and the general windflow. 

 Also check for obviously erroneous reports. 

 When a report is discovered to be doubtful, 

 first check for accuracy before discarding it. 

 The wind group may have been garbled, but 

 the remaining entries may be correct. A mis- 

 plot of ship position is the most frequently made 

 plotting error. A check against the original 

 r sport should relocate the report and make 

 it perfectly useable. 



Isobaric Analysis 



The third step consists of lightly sketch- 

 ing lines in pencil to connect points of equal 

 pressure. These lines, called isobars, outline 

 areas of high and low pressures. 



The drawing of an isobar is merely the 

 drawing of a line which follows the general 

 windflow and connects points having equal nu- 

 merical pressure values. The numerical values 

 referred to when drawing isobars are the pres- 

 sure values plotted near each station circle. 



Isobars are drawn for 4-mb intervals from 

 25° latitude poleward and for 2-mb intervals 

 from the Equator to 25° latitude using 1,000 

 millibars as the base value. To determine the 

 exact values for which isobars are drawn be- 

 tween the Equator and 25° N and S, vse 1,000 

 millibars as a base value and proceed toward 

 both increasing and decreasing values by in- 

 crements of 2 millibars. 



To determine the exact values for which 

 isobars are drawn in the area between the poles 



and 25° N and S, use 1,000 millibars as a base 

 value and proceed toward both increasing and 

 decreasing values by increments of 4 millibars. 



Isobars should be drawn so that they agree 

 not only with the pressure, but also with the 

 wind. The distance between isobars should agree 

 with the reported wind speeds. The stronger 

 the wind, the closer the isobars are to one 

 another. It must also be kept in mind that 

 winds blow across isobars at a slight angla, 

 inward toward a center of low pressure and 

 outward from a center of high pressure. If 

 the terrain is smooth, this angle is small; 

 however, the rougher the terrain becomes, the 

 greater is the angle. Over ocean areas, winds 

 blow across the isobars at an angle of 10° 

 to 20°; over very rough land, the angle may 

 be as much as 40°. 



In representing large-scale movement of the 

 air, simple isobars are more probable than 

 complicated isobars, (See fig, 11-6.) The il- 

 lustration at the left side of figure 11-6 shows 

 how the beginner draws the isobars to fit the 

 barometer readings precisely. The illustration 

 on the right side of figure 11-6 shows the iso- 

 bars correctly drawn to take the shape of smooth 

 lines. An irregular appearance is frequently 

 due to minor errors in observations. Hence, 

 irregularities that do not show any systematic 

 arrangement are likely to be the reflection 

 of errors. If a reported pressure seems in- 

 correct, compare it with the previous report 

 from the same station to determine whether the 

 pressure change is probable. 



Isobars must always appear as simple curved 

 lines or as closed lines. Isobars may begin 

 and end in the following manner: 



1. Originate on one edge of the chart, trace 

 a path connecting points of equal pressure val- 

 ues, and terminate on any edge. 



2. Begin anywhere on the chart, trace a path 

 connecting points of equal pressure values, and 

 join ends to form a closed curve. 



ISOBARS REPRESENTING DIFFERENT 



PRESSURE VALUES NEVER TOUCH OR CROSS. 

 Touching or crossing signifies two different 

 pressures at the same time and place, which 

 is impossible. 



In drawing isobars over large areas, start 

 in a region where they are easy to draw. Draw- 

 ing isobars over land areas where observa- 

 tions are numerous is less difficult than over 

 ocean areas where reports are limited. The 

 best method is to draw isobars over land regions 



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