Chapter 5 Measurement of Atmospheric Pressure

ln P 0P1Rz gdz0 T Vwhere:T v is the virtual temperature of the vertical air column, in K.R is the gas constant of dry air, in Jkg -1 K -1 .Z is the height from mean sea level to the barometer, in meters.Assuming that "g" is constant and is equal to the gravity acceleration at the observing point.The mean of virtual temperature is given by:TVmZz dzT0VIt results:P0 PexpgZRT VmTherefore, the reduction to mean sea level value P is given by: gZ P P0 P Pexp 1 RT VmNow T Vm is expressed as: T Vm =273.15+t m + m (K), where t m is the average temperature of the aircolumn, m is the effect of air humidity. Assuming the lapse rate of air temperature to be 0.5℃/100m results:tm t 0.005Z2where:t is the air temperature at the observing point.The value of m is statistically determined as a functionof the average air temperature. The relationshipbetween t m and m is graphically shown in Figure 5.9.This is statistically derived from surface observationdata obtained at eight meteorological observatories in mt mFigure 5.9 Relationship between m and t mJapan. This relationship is almost the same as that in the lower atmosphere under average9