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Representativeness of Data

Even when all the probable errors in the report are accounted for, some elements of the report may still be inconsistent with corresponding elements of other reports nearby or within the same air mass. Such elements are said to be unrepresentative. Any meteorological element subject to purely local influences, such as heating or cooling, terrain, water sources, local con-vergence, and the like, is likely to be unrepresent-ative. Because ocean surfaces are more uniform horizontally than are land surfaces, you might expect ship reports to be more representative than continental reports. This is generally true, except in the vicinity of well-defined currents, eddies, and a few other areas weíll discuss later. Similarly, when you are comparing levels of the atmosphere, properties of air in the friction layer (lower than 2,000 feet) are more subject to local influences than are the corresponding properties aloft. The unrepresentativeness of elements in this layer is the principal reason you must look at more than just the surface chart when performing analysis. No surface analysis can be considered complete until it has been shown to be geometrically and meteorologically consistent with corresponding upper air analyses. Since surface or sea level data are observed at the base of the friction layer, where local influences are most effective, it is possible to find conditions under which each ele-ment of the surface report is nonrepresentative. The following section discusses the represent-ativeness of various meteorological elements.

SEA LEVEL PRESSURE.ó Sea level pressures reported by stations at or near sea level are the most representative of all weather elements, although (especially in the case of ship reports) they are subject to error. Sea level pressures reported from mountain stations which extend well above the average sur-rounding terrain are not representative and must be disregarded when drawing the sea level pressure pattern. The reason for this lies in the fact that the average elevation of the surrounding stations is used in the pressure reduction computations and not the actual elevation of the station in question. As all stations above sea level must reduce their station pressures to sea level, it is important that you have a basic understanding of why and how this is done. First, the why. The greatest pressure changes occur as we move up and down in the atmosphere. For example, Leadville, Colorado is 10,158 feet above sea level. At that height, the pressure would be approximately 700 millibars. If Leadville could be lowered to sea level, the pressure would be approximately 1013.2 millibars. Thatís a difference of over 300 millibars due to elevation alone. Because weather reporting stations are located at various elevations throughout the world, if these pressure dif-ferences were not eliminated, it would be almost impossible to make worldwide comparisons of pressure. Therefore, scientists agreed on sea level as the standard or fixed level.

To a much lesser extent, but just as impor-tant, are the effects of temperature, density, and moisture content on pressure. These properties, along with a stationís elevation, must be taken into account in reducing station pressure to sea level. The reduction process uses the hypsometric equation. The factors of the equation are station pressure, station elevation, and the mean virtual temperature of an assumed air column extending vertically downward into the earth to sea level. Because no such column exists or is possible, it is dubbed "fictitious," and the mean virtual temperature must therefore be derived from a combination of observed, assumed, and climatic properties; they are temperature, lapse rate, and humidity. These properties must closely resem-ble those occurring in the actual atmosphere in the vicinity of the station but at lower elevations. If they donít, the sea level pressure obtained will be in error and will not fit the sea level pressure pattern.

The representativeness of any sea level pressure obtained in this manner is dependent on the representativeness of the fictitious air column used in computing it. The chances of correctly estimating the properties of the column decrease as the column increases in length. In other words, the higher the station above sea level, the greater the chance for error. Overestimates of the mean virtual temperature of the column result in sea level pressures that are unrepresentative on the low side and vice versa. Because of this, the intensity of thermal highs and lows in mountain and plateau regions is, to some extent, fictitious on surface charts. It is also the reason a level other than sea level is often used when constructing charts in these areas.

Table 7-1-1 shows probable errors in pressure observations that can be used as a guide in analysis of sea level pressure.

Table 7-1-1.óProbable Errors in Pressure Observations

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