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.

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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A description is given of experiments and calculations performed to investigate the effect of the entrainment of undersaturated air upon the microphysical structure of cap clouds at Great Dun Fell, Cumbria. Three case studies are presented: in I the properties of the cloud were essentially adiabatic; I1 provided evidence for mixing intermediate between the homogeneous and extreme inhomogeneous types ; in 111 the mixing was extremely inhomogeneous. Consistent with this picture, the time constant for turbulent erosion of the entrained parcels of air is significantly greater in 11 than in 111. Spectral changes observed in I1 and I11 agree well with predictions from a diffusive model of mixing (Baker and Latham 1982).High frequency acoustic sounders. Atmos. Envir., 13, 347- 350. Microphysics of Clouds and Precipitation, D. Reidel (pub.). The measurement of cloud droplet spectra. Znt. Con/. Cloud Phys., Boulder, Colorado, 576-580. A field study of nocturnal stratocumulus: 111. High resolution radiative and microphysical observations. Quart. J. R. Met. SOC., 108, 145-166. An asymptotic theory of incompressible boundary-layer flow over a small hump. J. Fluid Mech., 101, 647-670.