Aircraft measurements of humidity in the low stratosphere over southern England 1972–1976

A frost point hygrometer installed in a Canberra aircraft of the Meteorological Research Flight has been used to make measurements of humidity in the high troposphere and low stratosphere. The results from 60 flights over southern England in the period 1972 to 1976 are reported. It is shown that above some level in the stratosphere the humidity mixing ratio (h.m.r.) becomes independent of height; the average h.m.r., by mass, in this region was (3.1±0.3)10−6. An annual oscillation in stratospheric h.m.r. of amplitude 0.4 × 10−6 and significant at the 5% level is found. The measurements are compared with an average h.m.r. of (1.9±0.2)10−6 derived from previous MRF measurements made in 1954 and 1955.