This chart indicates the height of the 1000-mb level above and below sea level. When below sea level, it is indicated by negative height values. For reasons previously discussed, 1000-mb height values are not very accurate in mountainous areas. Use them with caution.

A 1000-mb chart is constructed from a surface chart by assuming that 7 1/2 mb equals 60 meters for temperatures between 30F and 70F; that 8 1/2 to 9 mb equals 60 meters for temperatures below 30F; and that 6 1/2 mb equals 60 meters for temperatures above 70F. These ratios give valid approximations of 1000-mb heights; how-ever, heights from radiosonde soundings should be used whenever available.

The principal use of 1000-mb chart is in constructing space differential (thickness) charts. The 1000-mb chart serves as the base level for the 1000-700- and 1000-500-mb charts.

The principal uses of the 850-mb chart are to locate surface frontal positions, to determine the representativeness of surface winds and temperatures, to determine the depth of moisture patterns in winter, and to serve as the surface chart in mountainous and plateau areas where the mean elevation is around 5,000 feet. 

Temperature and moisture analyses should be made in close conjunction with the surface chart whenever possible. A complete and careful isotherm analysis at this level in conjunction with the wind and height analysis will lead to the correct placement of fronts both at this level and the surface. A thumb rule to guide you in locating most fronts is to look for the 850-mb warm front roughly 2 1/2 to 3 degrees latitude ahead of the surface front and cold fronts 3/4 to 2 degrees latitude behind the surface front. An isotherm analysis also serves as a good indication of the 1000-700-mb thickness pattern. The isoheight interval is the same as that used by the National Weather Service, 30-meters.

The 700-mb chart is used mostly to determine the vertical extent and structure of fronts and pressure systems, or to play the role of the 850-mb chart over areas where the mean elevation is around 10,000 feet. It is also used to analyze moisture patterns in summer, when moist tongues extend to greater heights than in winter, because of convective activity. Other uses are in fore-casting (steering currents for certain shallow pressure systems are determined at this level) and differential analysis.

Short waves are a predominant feature of this chart. Because of their influence on cloudiness, frontal intensity, precipitation areas, etc., these wave features are carefully studied and tracked. The isoheight interval is the same as that for the 850-mb chart-30 meters.

The 500-mb chart is the most widely used of all upper-air charts. Primary features are the warm highs and cold lows, with their associated ridges and troughs. Long waves are identifiable at this level, but most short waves have lost their identity.

For various reasons, the 500-mb level comes closest to representing the mean state of the atmosphere at observation time. Since this level approximately divides the atmosphere with respect to mass, the 500-mb chart is often used, in conjunction with the 1000-mb chart, to provide a layer analysis of the lower half of the atmosphere.

This chart also provides winds at a common flight level for piston-engine aircraft, gives a fair approximation of the horizontal position of the jet stream when no 200- or 300-mb chart is available, and provides an important base upon which to construct higher-level analyses. The importance of this last point is based on the rapid decrease in available data above 500 mb. The 500-mb chart is also used extensively in forecasting the movement and development of sea level systems and fronts. Contour spacing is 60 meters.

The primary features of the 300-mb chart are the permanent and semipermanent highs and lows, certain dynamic lows, long waves, the polar jet stream in winter, and the tropopauses, especially the arctic and mid-latitude tropopauses in winter.

Its primary uses are in forecasting; determin-ing the characteristics of long waves; analyzing and forecasting jet streams; analyzing the tropo-pause in winter; determining vorticity distribution; and in the case of tropical lows that do not show a closed circulation at this level, steering currents. It is also an indispensable tool in planning jet aircraft operations.

The contour interval is normally 120 meters, but a 60-meter interval maybe used in areas where a finer degree of delineation is required. 

THE 200-MB CHART 

The operational use and contour interval of the 200-mb chart are the same as those of the 300-mb chart. In fact, the 200-mb chart is used as an adjunct to the 300-mb analysis. In summer, it plays the same role, with respect to the jet stream, that the 300-mb chart does in winter. In winter, its principal use is in estimating changes in the temperature advection pattern in the stratosphere.

The 150-, 100-, 50-, and 25-mb charts are normally prepared at major centers and are used primarily for research purposes. Data at these levels is so scanty that their construction is almost solely based on extrapolation of data from lower levels. As the operational ceilings of jet aircraft increase, it can be anticipated that practical uses of the 150- and 100-mb charts will also increase.

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