The NMC produces several upper-air anal-ysis charts twice daily, at 0000Z and 1200Z. All are done by the computers with limited analyst intervention. Output from the com-puter may be grouped into two general cat-egories: the North American charts, with plotted data, and the Northern Hemisphere charts, without plotted data. Figure 4-3-6 shows a typical North American upper-air analysis with plotted data. The North American charts are routinely available for the stan-dard levels 850, 700, 500, 300, and 200 millibars. The Northern Hemisphere charts are produced for the 500- and 300-millibar levels.

These upper-air analyses are sometimes referred to as constant pressure charts. The differences in air density/thickness are shown by isoheight lines, or lines of equal height of a constant pressure surface above mean sea level.

The different parameters shown on these charts are as follows:

Isoheight contours are shown by solid lines. Table 4-3-2 shows the approximate height of each chart, the base height of the isoheight lines, and the standard contour spacing of the isoheight lines. Each isoheight and height center are labeled in thousands, hundreds, and tens of meters.

Although troughs and ridges are shown by the isoheight contours, the placement of axis lines for these features is left to the user. Since the computer draws smooth isoheight contours, reanalysis of the contours may help locate the axis of troughs, especially short-wave troughs. This, of course, is not possible on the charts that do not display the plotted data. (Review AG2, Volume 1, Unit 8, Lessons 2 and 3, if necessary for relative information.)

Isotherms, or lines of equal temperature, are shown every 5C by dashed lines and labeled with the temperature. The computer tends to over-smooth the isotherms. Reanalysis of isotherms sometimes shows isotherm troughs much better than the computer analysis. This is important when attempting to identify short waves in the isoheight analysis. Isoheight troughs are usually associated with isotherm troughs, although the isotherm and isoheight trough axes may not necessarily coincide.

Plotted data on the North American charts is depicted on the charts according to the plotting models in figure 4-3-7. Station circles are filled in when the dew-point depression is equal to or less than 5C. Reanalysis of significant moisture at the standard levels should be done by criteria in table 4-3-3 to depict where significant moisture exists (enough moisture to form broken to over-cast cloud cover).  

Isotachs, or lines of equal wind speed, are shown as dashed lines at 20-knot intervals on both the 300- and 200-millibar charts, with areas of wind speeds 50 to 110 knots being shaded. Further analysis should be done to find the jet-stream axis

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