AbstractRoutine observations made in transport aircraft in January and July, 1955–57, are analysed to obtain the frequency of ice accretion in relation to air temperature and of turbulence in relation to wind speed and cloud. Comparative freedom from icing in the range 0° to – 3°C is attributed to the effects of kinetic heating. In January the frequency attains a maximum of 43 per cent of occasions in cloud at – 4° to – 7°C; in July 20 per cent at – 4° to – 11°C. There is a secondary maximum in both months near the lowest temperatures at which icing is reported, which is attributed to flight through the upper parts of convection cloud. The frequency of icing in July is generally much less than at the same temperatures in January.Turbulence is reported on about 5 per cent of all occasions in January and on 2 per cent in July; in cloud the frequencies are 18 and 12 per cent respectively. The frequency of turbulence tends to increase wi