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It is essential to draw for 2-mb intervals in the trades and for 1-mb intervals close to the equator. In addition, the 1-mb interval should be used whenever the isobars are so widely spaced that even 2-mb isobars do not reveal the important features of the pressure field. It is thus readily seen how important it is to screen pressure reports carefully. A few bad pressures can distort the isobaric pattern in a grotesque way. All pressures from normally reliable stations should be drawn to-quite strictly and disregarded or modified only when excellent reasons exist. The 2-mb spacing is required because of the weak pressure gradient in the tropics.

The low-level streamlines, especially when at 200 feet, determine the character of the surface isobars to within about 5° of the equator. In the center of the equatorial zone, you must rely on the pressure reports alone. In fixing the course of the isobars, use the low-level streamlines. The streamlines are an invaluable aid in sorting out good and bad pressures, especially in the trades. However, streamline analysis is concerned ex-clusively with the wind field and does not convey information about the field of mass and its changes, which is essential for many prediction problems.


As an added plotting step on the surface chart, the 24-hour pressure changes that have occurred at all reporting stations should be computed and written just above their plotted sea-level-pressure values on the surface map. It will help if the plotter circles these changes so that they stand out among the mass of information on the surface chart. Over the ocean area, the procedure is to tabulate the pressure at all 5° latitude-longitude intersections and to compute and plot the pressure differences. Isallobars are then drawn at intervals of 0.5 to 1.0 mb, depending on the magnitude of the changes.

In an alternative method, the changes maybe obtained by using a light table and graphically subtracting the pressures of the current chart from the previous chart. This method, however, often proves less satisfactory because the isobars tend to run fairly parallel and intersect at very small angles, making interpretation difficult.

The main objective of 24-hour pressure-change charts is to track the isallobaric centers and note their changes in intensity. In addition, these charts furnish a check on the ocean isobaric analysis; if large regions of intense falls suddenly appear where none existed previously, you will do well to reexamine such areas. As was stated previously, the tropics is a region that experiences little change in its daily weather; therefore, any sudden intense pressure changes would signal a breakdown in its continuous nature. In general, isallobaric analysis yields good continuity, Usually, you will look for concentrated fall or rise centers that cover limited areas only. These are the areas of intensely disturbed weather. Extensive areas of fairly uniform pressure fails are considered harmless.

Occasionally falls of 1 to 2 mb over most, if not all, of the region of analysis will appear suddenly on one day, only to be cancelled out by corresponding rises within 24 to 48 hours. Such atmospheric pressure waves on a huge scale are not understood at present, but they do not appear to be of any significance. When such widespread falls occur, an area of isallobaric falls often appears as a center of still larger falls. On the other hand, an area of isallobaric rises may take the form of an area of small falls embedded in an area of larger falls.

Subsequent to the analysis, the isallobaric centers should be related to the low-level streamlines and the weather pattern; later they should be compared with the distribution of wind and temperature aloft. Fall centers containing extensive bad weather and associated with warming in the troposphere and anticyclonic circulation at 200 mb must be regarded as potentially dangerous. Rise centers associated with bad weather generally do not indicate immediate deepening, even though the intensity of the convection may be severe.

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