Abstract599 morning weather reconnaissance flights from the Royal Aircraft Establishment, Farnborough, over a period of approximately four years have been examined to determine the frequency of occurrence and thickness of layer cloud over Southern England. The chance of encountering 4/8 or more layer cloud in any kilometre height range decreased with height from about 1 in 2 near the surface to about 1 in 50 at a height of 12 km. Cloud layers when encountered were more likely to exceed one or two kilometres depth in the upper troposphere than in the lower but the frequency of encountering a layer more than 1/12 km deep was about 7 to 8 per cent at all heights. About 1 in 9 of all flights encountered cloud above the melting level more than 3 km thick. An examination of clouds in the temperature band 0 to – 15°C suggested that conditions suitable for severe clear ice formation on some aircraft might be encountered on about one per cent of flights. The same examination suggests that seeding of layer clouds by silver‐iodide smokes should produce some detectable modification of the cloud sheet on about 8 days a