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Identify the procedures for drawing isobars on a plotted surface chart.


General drawing procedures

Wind speed and spacing of isobars

Geostrophic and gradient wind scales

Additional rules and considerations

Isobaric patterns

Summary of recommendations for drawing isobars


Isobars are lines of equal or constant pressure. On the surface chart, they are drawn connecting points of equal sea level pressure. When the isobaric analysis is completed, the pressure pattern with its highs, lows, troughs, and ridges will have been depicted.

Extreme care is essential in isobaric analysis. It is in this area, more so than any other, that sta-tion reports get disregarded. Before you throw out a report for not fitting into your idea of the pressure pattern, take the time to check the report and the history of the area. Many disregarded reports are later found to be correct.

Another reason for ensuring an accurate depiction of the pressure pattern is its effect on upper air analyses. In areas of sparse or no upper level data, the surface analysis becomes the basis from which to draw the upper level patterns. Keep in mind your analysis should do the following:

1. Be compatible with current upper level data and continuity (past history). Pressure patterns, stages of system development, and pressure gra-dients should show logical development from previous charts.

2. Show continuity based on accepted models and experience, when there’s an absence of real-time data to support your intensifying or weakening pressure systems.

3. Show the proper isobaric angle at fronts, especially to avoid violation of data in attempt-ing to conform to preconceived models.

4. Take into account local and geographical effects, especially in mountainous regions and along coastlines.

Learning Objective: Identify the pro-cedures for drawing isobars on a plotted surface chart.

Isobaric analysis can be mind-boggling and time-consuming; however, as you gain experience and skill, the pressure patterns will become ap-parent before you even begin to draw the in-dividual isobars. Knowing where to begin and how to proceed is the goal of this section, but before we get to that, you’ll have to be aware of some general information about isobaric analysis. Isobars are drawn for every 4 millibars mb except in the area on your chart between 25°N and 25°S. There, they’re drawn at 2-mb intervals. These intervals use 1000 mb as the base value. For example, isobars lower than 1000 mb will be drawn for standard values of 996, 992, 988, etc. Isobars greater than 1000 mb will be 1004, 1008, 1012, etc. Of course, if you’re analyzing the area between 25°N and 25°S, the standard interval will be every 2 mb. The 2-mb interval may also be used anywhere outside of this tropical region, if you feel it is necessary to better define the pressure pattern.

Isobars are sketched in lightly, using a 2H pen-cil. Their configuration is one of closed loops or of curved lines that extend to the edge of the chart. When you first sketch isobars, they will most likely appear ragged. See figure 7-2-1, view A. Be-cause large-scale air movements are relatively smooth, your analysis should reflect this. Figure 7-2-1, view B, shows isobars correctly drawn to take the shape of smooth lines. At some point along the isobars, they must be labeled. Closed loop isobars are normally labeled at the top or bottom of each loop. (See fig. 7-2-1.) Those iso-bars not closed must be labeled at each end.

Isobars are normally labeled using the tens and units digits of the standard values. A 1008 isobar would be labeled 08; a 996 isobar, 96, etc. As you sketch in the isobars, REMEMBER: ISOBARS REPRESENTING DIFFERENT PRESSURE VALUES NEVER TOUCH OR CROSS. If iso-bars were to touch or cross it would indicate two different pressures at the same place and time, which is impossible. 

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