An isallobar is a line of equal change in atmospheric pressure during a specified time interval. In other words, isallobars are lines depicting equal pressure tendencies. There are two types of isallobars: anallobars and katallobars. Anallobars are lines depicting rising or positive pressure tendencies and are drawn in dashed blue lines. Katallobars are lines depicting falling or negative pressure tendencies and are drawn in dashed red. The zero pressure tendency isal-lobar is drawn in dashed purple. Isallobars are usually drawn for an interval of 1-mb change.

Figure 7-4-5.Isallobars.

For example, all stations having a pressure tendency plotted as minus 10 would be connected with a katallobar representing a 1-mb change. (See fig. 7-4-5.)

Isallobars reliably indicate the direction pres-sure systems have been moving and conse-quently are useful in forecasting their future movements. Lows normally move in the direc-tion of maximum pressure falls and highs in the direction of maximum pressure rises. The speed of both systems is directly proportional to the isallobaric gradient.

Intensity changes of low- and high-pressure centers can also be determined by using isallo-bars. Pressure tendency changes are due to both movement and pressure changes within the centers. If movement of the center was the only factor producing the pressure falls and rises in advance of pressure systems, there would always be pressure falls ahead of a low and rises ahead of a high. However, we know through experience that this is not true, because the intensity changes within the centers also determine the overall amount of pressure tendency changes. Therefore, the fol-lowing rules apply:

1. When the zero isallobar lies to the rear of the low-pressure center or trough line, the system is deepening. (See view A in fig. 7-4-6.) The reverse is also true. If the zero isallobar is ahead of the low or trough line, the system is filling.

2. When the zero isallobar lies to the rear of a high-pressure center or ridge line, the system is building. (See view B in fig. 7-4-6.) The reverse is also true. If the zero isallobar is ahead of the high or ridge line, the system is weakening.

The orientation of the zero isallobar can also be used to determine whether the pressure gradient is increasing or decreasing around a low or high center. If the zero isallobar is curved to the right around a low or high center, the gradient will increase. (See views C and D in fig. 7-4-6.) The isallobars are oriented in the manner shown in figure 7-4-6, views C and D, because the pressure is falling (or rising) faster in the center of the system than around the outer edges. The reverse is also true. When the zero isallobar is curved to the left, the gradient will decrease.

Figure 7-4-6.Orientation of isallobars to determine intensity and gradient changes.

In using any of the rules based on the pressure tendency, you must remember that the pressure tendency represents past conditions and is not necessarily what will continue to occur. Consequently in using tendencies as indications of deepening, filling, movement, and so forth, you must study the past trend of the tendencies.

NOTE: Before proceeding to the final lesson ("Southern Hemisphere Analysis"), obtain the latest practical training publications and appendices on surface chart analysis.

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