The heat balance of a riming spherical target, representing a graupel pellet or small hailstone in a thunderstorm, has been studied in the laboratory in Manchester as a function of cloud droplet size. With droplets of mean volume diameter between 18 and 33 μm, a riming target of 5 mm diameter, temperatures of -6¿ and -15 ¿C, and velocities between 4 and 8 m s-1, the effect of the heat rise of the graupel was measured for a range of cloud effective liquid water contents (EW). For a constant rate of rime accretion, the heating effect of the droplets was found to increase with droplet size, the effect being more pronounced at lower temperatures and higher velocities. The value of &khgr;, a numerical term in the ventilation coefficient, was also found to be dependent on the droplet size, which implies a new dependence of the Nusselt number, a measure of ventilation, on droplet size. The results were viewed as being indicative of a droplet collision efficiency effect with smaller droplets producing a rougher, better ventilated surface which is cooler than a rime surface produced with larger droplets. The results show that the required value of EW to achieve wet growth increases with smaller droplets, and they indicate that the collisional charging of riming graupel pellets in thunderstorms, which is influenced by the surface temperature of the rime and its effect on vapor diffusion, will be more negative with larger droplets for which the rime structure is less readily ventilated. ¿ 1999 American Geophysical Union