Upper Air Charts

Date : 21 January 1999

The standard form of an upper air station plot consists of temperature, humidity, wind speed and direction, and geopotential height at a specific pressure :

Upper air analysis on constant pressure surfaces is most meteorologically useful, and height and temperature contours are typically included :

On the above 500 mb chart, geopotential height (black - decameters) and temperature (red - °C) contours are subjectively analyzed. The station plots were made using Digital Atmosphere. A closed Low was over the Midwest, associated with snows of more than a foot over much of the region, and nearly 2 feet near the SW Lake Michigan shore.

Balloon Soundings

Similar with surface observations, upper air observations are (obviously) required for such a chart. This is primarily accomplished with balloon soundings at specified locations. A radiosonde (radio-transmitted sonde) is released. As it rises and drifts with the wind, the plotted variables are measured. Readings are obtained for mandatory and significant pressure levels. Mandatory levels are 1000, 925, 850, 700, 500, 400, 300, 250, 200, 150, and 100 (and 50 & 10 ?) mb. Significant levels are those in between at which temperature lapse rate or wind change significantly (more than a specific threshold - the precise values of which I am unaware of). Thus, the temperature plot is quite accurate and smoothly varies. A sounding is considered successful if 400 mb is reached. When sufficient soundings are made over a region nearly simultaneously (nearly can be an hour or more different), an upper air chart is made for each mandatory pressure, all readings plotted at the point of release (though the balloon drifts many miles - a significant source of error for upper atmospheric charts, actually). This radiosonde data is (of course) the source for atmospheric soundings previously discussed.

Direction of Gradient Wind

You may recall in a previous discussion (not yet here) that the gradient wind at a constant height such as sea level blows parallel with isobars, with low pressure to the left/right of the wind vector in the Northern/Southern Hemisphere. This statement can be equally made for height contours at a constant pressure as depicted on upper air charts. This can be illustrated mathematically, rewriting the gradient wind equation previously shown :

1/r [P/r]z = s2/r + f s

replacing pressure gradient on a constant height surface with height gradient on a constant pressure surface :

g [z/r]P = s2/r + f s

Thus, the basic interpretation of the equation does not change - horizontal gradient winds blow parallel with both isobars and height contours, and are stronger where isobars or contours are most closely-spaced. Please notice that geopotential height contours as typically plotted slightly differ from this, but a similar transformation for geopotential height can also be made. You may notice that winds on the 500 mb chart above generally follow this rule, though actual winds vary quite significantly from gradient winds for some places such as Albuquerque, NM.

Thickness

In a previous feature (not yet here), I illustrated how virtual temperatures determine pressure differences between fixed altitudes. Another way of thinking of this is that virtual temperature also determines height difference between pressure surfaces (standard values for gravity & dry air composition used for diagram) :

Thus between 2 pressure levels, a deep layer is relatively warm, and a shallow layer relatively cold. The term thickness is used when referring to distance between 2 of such layers. Thickness has many uses, but because of this correspondence with temperature, the most common is use as a guide regarding wintertime precipitation type. Observations indicate that 1000-700 mb geopotential thickness of 2840 m and/or 1000-850 mb geopotential thickness of 1300 m are approximate thresholds between rain and snow.

Waves in the Upper Air Flow

More important than thickness though is that height contours reveal the upper air flow and waves in it. Though the chart for the continental U.S. and southern Canada above reveals only part of a wave - a strong low pressure trof - a hemispheric 500 mb chart (main site, geopotential height colored) reveals series of trofs and ridges. We shall see that these waves largely determine large scale weather features such as cyclones and anticyclones; and because temperature determines positions of the geopotential height contours as illustrated, global distribution of heating and cooling ultimately determine those.


Text and embedded images are copyright of Joseph Bartlo, though may be used with proper crediting.

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