Table 48 (continued) 205 



DETERMINATION OF HEIGHT BY THE BAROMETER AND REDUCTION OF 

 PRESSURE TO FIXED LEVELS 



Plateau correction. — In the United States, Canada, and Alaska, a so-called "plateau 

 correction" 8 P is applied to the reduced sea-level pressure obtained with the aid of 

 equation (6). The plateau correction in millibars is given by 



P c == 0.000210(f m - 7 m )* 2 (9) 



where t m h the mean dry-bulb temperature (°C.) of the air column between $ 2 and sea 

 level, and t m is the normal annual value of t m . For practical purposes (t m — t m ) may be 

 replaced by (t' mv — t' mv ). The purpose of the plateau correction is to cause the deviations 

 from the annual mean of the pressure reduced to sea level to have about the same value 

 for high-altitude stations as for surrounding stations near sea level. 



It is of greatest importance, for consistency of results, that all meteorological stations 

 in continental United States, Alaska, and any other contiguous regions where the plateau 

 correction is generally used, should conform to this practice. That is, harmony with 

 official sea-level pressures will not be secured in this area unless the plateau correction 

 is applied. 



Investigation has shown that in the case of a station of markedly greater (or lower) 

 elevation than the surrounding stations, the value of <!»» to be used in computing the 

 plateau correction is the average of the elevations of the surrounding stations. This 

 applies to a station located on an isolated mountain rising considerably above the general 

 level of the stations within several hundred miles radius. 



To compute reduced pressure given $i, <I>2, p?, and t mv . — 



1. Determine A* the geopotential difference between the level * a for which the 



pressure is given and the fixed level $i to which pressures are to be reduced. 

 (Note. — If the fixed level is sea level, A<f> is the geopotential of the station <l>j 

 and can be determined from Table 50. For other fixed levels geometric height 

 can be converted to geopotential by Table 50 and the difference A* computed.) 



2. Determine the mean adjusted virtual temperature tmv in °C. as outlined above. 



3. Determine A$T /T m v from Table 48 D as a function of A* and of tmv. (Note. — 



Since this function is linear in A3> the value of A$To/T mv for any A<i> can be 

 found by a linear combination of values in Table 48 D for the given value of 



tm,.) 



4. Determine A4>, 2 corresponding to the pressure p 2 from Table 48 B. 



5. Find the algebraic difference (A*, 2 — A$To/T mv ). According to equation (6) 



this difference is A«£>,i. 



6. Enter Table 48 B with the value of A<f>,i found in step 5 as the tabular value. 



The corresponding argument is the reduced pressure pi. 



7. Apply the plateau correction if pertinent. 



English units. — For computing heights or reducing pressure when data are given in 

 English or other systems of units, first convert to millibars, meters, and centigrade 

 temperatures by means of conversion tables in Section I. 



8 Ferrel, Wm., Annual Report of the Chief Signal Officer of the Army, 1886, app. 23, Washington. 

 See also reference 6. 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



