Atmospheric refractometry at high relative humidities

Abstract

Precise measurement of changes in air refractivity is provided by the high-frequency (h.f.) refractometer at relative humidities below 50%. Above this limit, the indicated refractivity may exceed the true refractivity by an amount that increases with humidity. The experimental techniques of gravimetry, ellipsometry, and photomicrography are combined with refractometer observations in a controlled environment to study the source of this anomaly. It is found that the anomaly is due to the added polarization of a layer that forms progressively upon the sensor surfaces, that the layer growth is insensitive to the h.f. electric field, and that the layer density lies between that of moist air and of bulk water while its depth increases to some 1500 Å near saturation humidity. At humidities above 90%, microscopic water droplets also form on the sensor surfaces. These observations are attributed to vapor adsorption and condensation on the sensor, with the latter related to surface asperities; the relative contribution of these two components in the layer structure depends upon the method by which the environment has been humidified.