We analyse observations of the size distribution of raindrops from a variety of cloud types and precipitation rates. The measurements show a transition from large raindrops observed in convective showers and frontal rainbands to higher concentrations of much smaller drizzle drops observed in stratocumulus clouds. The observations are used to develop an improved parametrization of the raindrop size distribution that better captures the observed transition for use in single-moment microphysics schemes. Sensitivity tests are performed in both climate and numerical weather prediction versions of the Met Office Unified Model to demonstrate that the new parametrization leads to improvements in various precipitation and cloud-related metrics. Copyright © 2012 British Crown copyright, the Met Office. Published by John Wiley & Sons Ltd.