On the Measurement from an Aircraft of Buoyancy and Vertical Air Velocity in Cloud

Abstract

It is shown that the wet bulb temperature is a close approximation to cloud temperature unless the cloud drops are all large and there are violent up or downdrafts. For a small thermometer element the correction for kinetic heating of the wet bulb is unaffected by the presence of liquid droplets. Buoyancy in cloud may thus be determined from wet bulb temperature measurements and from a knowledge of liquid water content. Vertical air velocity may be determined by summing two components: the aircraft velocity found by integrating electronically its vertical accelerations, and the vertical air velocity relative to the aircraft found from measured values of the aircraft forward speed and the angle of inclination of the approaching air to the true horizontal. Corrections are necessary based on variations in pressure altitude, on rate of change of aircraft pitch and on other parameters which are discussed. The elements of a system are described which has a response time of about 0.1 sec and accuracy about O.1C in virtual temperature and 0.2 m sec⁻¹ in gust vertical velocity. The results of some preliminary measurements are given.