An important parameter for calculation of the impact of cirrus clouds on climate is the asymmetry parameter. The asymmetry parameter captures the degree to which light is scattered by individual ice crystals towards the ground rather than reflected upwards to space. The measurements presented in this study show that, for a common type of ice crystal called bullet rosettes, cirrus clouds may be far more efficient at reflecting sunlight than is typically assumed in the radiative transfer models used to represent the role of clouds in climate.