A calculation has been made of the meridional circulation sufficient to transport heat between the radiational sources and sinks in the stratosphere (15-55 km) and mesosphere (55-80 km). Assuming that the principal heat sources are in the region of the equatorial tropopause and in the mesosphere over the summer pole, with the corresponding heat sinks near the polar tropopause and over the winter pole in the mesosphere, the main circulation patterns are found. Below 30 km there is rising air over the equator with outflow towards both poles and descent towards the tropopause in high latitudes in both hemispheres. At higher levels there is ascent over the summer pole and descent over the winter pole with a well defined flow towards the winter pole above 50 km and a more indeterminate flow pattern between 30 and 50 km. The order of magnitude of the speed of these circulations is m sec-' horizontally and cm sec-' vertically.The calculations were made for the solstices, equinoxes and mid-times between them and several particle trajectories were determined. The results are in agreement with many of the observed physical features of the stratosphere and mesosphere, particularly the distribution of tracer elements.

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SUMMARY125 drop samples in ten layers of low cloud of thickness 700-7,500 ft (210-2,300 m) have been obtained using the magnesium-oxide coated-slide technique. These observations are discussed in terms of the condensation-coalescence mechanism. Comparison has been made between these observations and a similar set, also obtained by the Meteorological Research Flight, Durbin (1959), for convective cloud, from which it is shown that the coalescence process is in greater evidence in the layer clouds. Values of water content deduced from the drop spectra are higher than those previously reported by other workers and correspond in magnitude to the full adiabatic values suitably modified by redistribution within the cloud.Some measurements are also discussed relating to the effect of the aircraft on the representativeness of the drop samples. The extension of the magnesium-oxide calibration to drops larger than 100 p diameter has also been examined experimentally.

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Aircraft observations of droplets greater than 100 μ diameter in 20 low layer clouds indicate that these drops can occur in concentrations of approximately 0·1 per litre to 10 per litre in clouds of 1,000 ft (300 m) to 7,000 ft (2,100 m) thickness. These observations are presented together with data relating the thickness and height of a further 120 layers of low cloud to their ability to produce precipitation at the ground. A diagram is shown which, it is suggested, might be of use as an objective aid for forecasting the occurrence of precipitation from low‐layer cloud. The results are discussed qualitatively in terms of the condensation‐coalescence mechanism of droplet growth and demonstrate the efficiency of layer clouds as producers of large drops (and consequently precipitation). There is evidence that, for the range of cloud depths considered, coalescence is the predominant mechanism for drop growth for temperatures down to – 11°C to – 12°C.