Fifty-one soundings with balloon-borne frost-point hygrometers provided measurements to a height of 94,000 ft of the vertical distribution of water vapor over Trinidad, West Indies, Washington, D.C., and Thule, Greenland, during 1964 and 1965, the International Years of the Quiet Sun. The stratospheric measurements were obtained during the balloon descent with sample collection ahead of the instrument train to avoid extraneous moisture from the flight system.

The observed mixing ratios of the lower stratosphere to a height of 73,000 ft are for nearly all cases within the range of 1.2 × 10−6 – 3.3 × 10−6. Above 73,000 ft the range broadens to include higher mixing ratios, but the majority of cases fall within the same narrow range as for the lower stratosphere. The vertical distribution of mixing ratios for most purposes is better represented by the medians of the distributions by level than by the arithmetic means. The median vertical distribution of stratospheric mixing ratio to a height of 94,000 ft for Washington, D.C., and for Trinidad is within the mixing ratio range of 2 × 10−6 –3 × 10−6.

A seasonal variation of mixing ratio for the low stratosphere is evident for both Washington, D.C., and Trinidad, with lowest mixing ratios in late winter and spring and highest in late summer and fall. The magnitude of the seasonal change decreases with height so that a seasonal reversal of the vertical gradient of mixing ratio for the lower stratosphere is also observed. A systematic latitudinal gradient of mixing ratio in the stratosphere is not discernible in the data.

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