The variance and power spectrum of atmospheric optical refractive-index fluctuations are shown to be composed of three terms: the variance and power spectra of the temperature and humidity fluctuations and the correlation and cospectrum of the temperature and humidity fluctuations, respectively. Humidity fluctuations are found to be significant because of the correlation term. The signs of the temperature–humidity correlation and cospectrum can be positive or negative, and therefore can add to or subtract from refractive-index fluctuations caused by only temperature fluctuations. The results of two atmospheric boundary-layer experiments are reported, which show the large effect of the temperature–humidity correlation term. For cold air blowing over warm ocean water, the correlation term was positive and accounted for 17% of the total refractive-index variance. For dry hot desert air blowing over the cold Salton Sea, the correlation was −268% of the total, effectively cancelling the contribution due to temperature variance.
© 1975 Optical Society of America
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