Although temperature fluctuations dominate the variance of optical refractive index fluctuations it has been shown recently that humidity fluctuations can also be important (e.g., Friehe etal., 1975). This paper reports on simultaneous measurements of temperature, humidity and pressure so that the relative importance of all three can be investigated. For the dry site where the measurements were made, the humidity contribution was less than other investigators had found. The major contribution of the pressure fluctuations was through their covariance with temperature, but this term was found to be between 0.03 and 0.4% of the total variance. The results thus confirmed that pressure fluctuations can be neglected in most circumstances. Both the temperature and humidity spectra displayed -S/3 power laws at small scales while the temperature-humidity cospectrum decreased more rapidly than a -5/3 power law. The temperature-pressure cospectrum decreased even more rapidly than the temperature-humidity cospectrum. The temperature-pressure correlation coefficient was found to be about -0.1. The humiditypressure correlation was typically between *O.OS and the cospectrum poorly defined. * Contribution of the Bedford Institute of Oceanography.