Manually prepared surface-weather charts are quite different from the computer-generated surface-weather charts. At the very least, manually produced surface charts will include weather station plots, isobars, and fronts. A more in-depth chart may contain a neph-analysis (cloud analysis), air mass designa-tors, and a weather depiction analysis. At their best, manually produced surface-weather charts are highly colorful and extremely informative.

Computer-generated surface charts do not contain nearly as much information as the manually produced charts. They do provide an isobaric analysis; they may or may not include weather station plots or low-level-wind plots; and they do not contain fronts. Fronts must be hand-drawn on these charts.

Surface-Weather Analysis Charts  

Synoptic charts of surface pressure, wind, and frontal analysis are produced every 6 hours, beginning at 0000Z. An example of an FNOC surface-pressure preliminary analysis (SFC PRELIM) is shown in figure 4-2-1. This chart covers about an eighth of the globe and provides an initial look at the pressure systems. Highly smoothed isobars are drawn every 4 millibars (hectopascales), High- and low-pressure centers are indicated by H and L, with the exact center

indicated by a "+" mark. The central pressure is printed to the right of the "+" mark in tens, units, and tenths of millibars.

Shortly after the SFC PRELIM is produced, the computers complete the more detailed surface is typical of charts produced for users in the tropics. Note the smaller scale; this chart covers only about one-fifteenth of the globe. The isobars are drawn every 4 millibars. They show a much more detailed representation of the pressure pattern than is shown in the SFC PRELIM analysis, although smoothing may eliminate minor, yet significant pressure deviations. Winds are represented on this chart by standard wind plots on a five-degree grid. They do not represent actual reported winds, but computer-averaged winds over a five-degree-square area. Neither of the FNOC surface analyses depicts fronts or troughs.

The frontal depiction analysis (GG-Theta) is shown in figure 4-2-3. This example is an analysis from central Asia to central North America. The frontal positions are depicted with contours of percent probability of the frontal location. It is up to the user to determine the actual frontal placement, as well as the type of discontinuity warm front, cold front, occlusion, or pressure trough.

This analysis does not always provide the user with clear-cut frontal boundaries. Normally, fronts are located where the isolines are packed tightly around an elongated central core. The isolines parallel all fronts except occluded fronts. The isolines lie more or less perpendicular across occlusions. Usually, the higher probabilities surround areas where reports show strong discon-tinuities, such as large temperature changes, large wind speed and direction changes, or marked pressure rises and falls. The higher probability areas tend to indicate the locations of the more dynamic fronts.  

Notice in the tropical portion of the example (fig. 4-2-3) that several areas are surrounded by only 10-percent-probabilit y contours. These areas indicate that the computer has found only minor discontinuities. A minor discontinuity maybe a pressure fall or a wind shear line indicating a tropical wave. In the mid-latitudes, the lower probability contours may indicate a pressure trough or a very weak, non-weather-producing frontal position.

While you may infer frontal intensity (weak, moderate, or strong) from this chart, you would need to compare this chart to previous charts to infer frontal type (cold, warm, occluded) and frontal character (undergoing frontogenesis, undergoing frontolysis, or having no change).

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