CHARTS FOR THE REDUCTION OF HEIGHTS FROM ANEROID READINGS BASED ON A STANDARD ATMOSPHERE AND A STANDARD LAPSE-RATE

Abstract

Atmospheric pressure at a given point is measured by the weight of the vertical column of air on a uniform cross section above that point, which again depends on the density and chemical composition of the air and also on the temperature and the gravity value throughout the air column. The humidity is deduced from simultaneous readings of both wet and dry bulb thermometers and the air temperature is known from the dry bulb thermometer readings. Since the thermometer readings are obtainable only at the points of observation, some assumptions with regard to the temperature distribution in the air column are necessary for an accurate estimation of heights. The methods generally used for the reduction of aneroid readings are thus based on two assumptions—the isothermal and the lapse-rate. According to the isothermal assumption the temperature of the air column is considered uniform throughout. But this is contrary to observed facts; for normally up to a certain limiting height the telnperature of the air is observed to fall with increase of height at a more or less constant rate. The assumption of a standard lapse-rate does on the other hand conform more closely to nature and has therefore been now in general use. The relevant “Aneroid Tables Based on a Standard Atmosphere and a Standard Lapse-Rate” by H.M.S.O. provide an accurate and straightforward computation. But the main defect with these elaborate tables, as rightly indicated by Col. D. R. Crone in his article on “Heights by Aneroid Barometers” (E.S.R. ix, 69, 311–317),is that “these are not well suited for rapid entry and the method of correction for observed humidities with the wet and dry bulb is fiddling and not suited for mass reduction of readings”. Moreover, the tables are also defective in the sensethat they give rise to errors which become appreciable when dealing with higher altitudes.