Use of pitot tube to compensate for pressure deficiency caused by wind on Mount Washington, New Hampshire

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Abstract
By virtue of the Bernoulli effect, the wind blowing over the summit of Mt. Washington, New Hampshire, causes the station barometric reading to be less than the pressure at the same level in the free air near the summit but away from the influence of the mountain. The object of this paper is to determine the amount of pressure fall that results from the Bernoulli effect and to show how it may be compensated for to a good degree of approximation by use of a pitot tube connected to the barometer.The pressure deficiency produced by the wind at Mt. Washington has been ascertained by correlating barometric readings on the mountain with comparable data reported by surrounding low‐level radiosonde stations. The latter data were derived by interpolation from synoptic charts giving pressures of the 5000‐foot plane or heights of the 850‐millibar constant‐pressure surface. A curve of pressure correction to overcome the specified deficiency in barometric reading has been prepared, giving the correction as a function of wind velocity. This curve showed that the flow of air over the mountain causes the observed barometric pressure on the summit to decrease relative to free‐air pressure nearly in proportion to V2, where V = wind velocity observed on the summit. This fact suggested to the author that the impact pressure of the wind (ρV2/2, where ρ = air density) available from a pitot tube vaned into the wind might be used to compensate automatically for the pressure deficiency produced by the wind, since both impact pressure and pressure deficiency apparently are functions of V2. A vaned pitot‐tube assembly was built and mounted on the Observatory tower freely exposed to the wind. Tubing was used to connect the pitot tube to a fixed‐cistern mercurial barometer and to a pressure‐tight box containing a standard microbarograph. The impact pressure of the wind thus imposed directly on both the barometer and the microbarograph is found to compensate approximately for the deficiency in pressure due to the wind blowing over Mt. Washington. The pressure observed at the station, with the pitot tube connected to the barometer, was compared to the local free‐air pressure derived by interpolation from the upper‐air synoptic charts. Tables of frequency of various discrepancies between these two pressures are given. These tables show that zero (0) discrepancies occur with greatest frequency, although a discrepancy as large as eight millibars has been observed out of 176 comparisons. It is believed that a major proportion of each large discrepancy results from errors in location of the isograms on the upper‐air charts that served as a source for the comparative data. Judging by the data available, the mean discrepancy in station pressure found when using the pitot tube with the barometer at Mt. Washington is of the order of one millibar.

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