AbstractThis paper reports on microphysical and dynamical measurements made from an aircraft in the anvil of a winter cumulonimbus cloud which grew in maritime air near the United Kingdom. The anvil had a well‐organized structure, with ice content in the central region exceeding 1 gm−3. The ice appeared to be distributed uniformly over the size range of crystals, with half the total mass contained in particles larger than 1 mm. The total particle concentration may have exceeded 1501−1in places. A widespread supersaturation of typically 10% with respect to ice ensured that the ice crystals continued to grow by deposition. The microphysical and dynamical structures of the anvil are shown to be closely linked, and semi‐quantitative explanations are given for the evolution of particle size distributions in terms of deposition and gravitational s