A new version of the linear critical-angle refractometer for liquids is described: a 1728 element linear CCD array is used to scan the intensity profile of the output image, within which the position of a cut-off edge is a linear function of refractive index. The precision in locating this cut-off edge is greatly increased, especially for milky liquids, by differentiating the intensity profile numerically. Further information about complex particulate liquids is found to be given by the shape of the differentiated profile, and alternative internal and external illumination modes are provided which may give more information. The present instrument covers the refractive index range 1.3000 to 1.4700, with an accuracy of +or-0.0001 for transparent liquids. The light source is an LED at 635 nm. The intensity profile, the differentiated intensity profile, and the refractive index value are all computed, displayed and printed by a desk-top computer.

Measurements have been made of the liquid water content and droplet-size distributions within clouds enveloping the summit of Great Dun Fell, Cumbria. These covered extensive periods and a variety of meteorological situations.I t was found that in highly stable atmospheric conditions cap clouds contain narrow drop-size distributions and near adiabatic liquid water content. In less stable conditions mixing with the cloud environment was found to have a substantial effect on the microphysical properties of the cloud.In the cases studied the admixture of undersaturated air was found to produce a substantially subadiabatic liquid water content and the spectral shape experienced broadening in a way more consistent with the extreme inhomogeneous model of mixing described in Paper I* than with the classical treatment. In particular, it was found that a small number of large drops experience a greatly enhanced growth rate. I n a particular study, it was observed that the interaction of the cap cloud with a pre-existing stratocumulus deck produced a broad drop-size distribution and an enhanced liquid water content.

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A field study of the microphysical properties of a radiation fog was conducted in Meppen, West Germany, on 17 February 1978. It is described and interpreted in terms of the prevailing meteorological conditions. Pronounced periodic fluctuations in liquid water content (intervals ∼70s) were observed at times in the radiation fog. It is suggested that these were a result of convective motions in the form of Bénard cells. the drop size distributions were broad, containing drops of radii up to 25 μm. Calculations indicate that these larger drops cannot be produced by radiative cooling from the fog top. It is argued that these large drops may have been produced either by large supersaturation fluctuations near the fog top, resulting from downwards entrainment of warmer moister air, or by convective motions causing a fraction of the larger drops to make several excursions to the radiative cooling region near the fog top. the radiation fog was observed to clear when cloud was advected over the site. It is suggested that this was due mainly to eddy diffusion and gravitational sedimentation of drops to ground, after radiative cooling from the fog top had been severely curtailed by the cloud layer.

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