1. The fraction of all water drops in the path of a falling larger drop that make contact with the larger drop. Calculations using approximate drag forces between large and small cloud drops predict low collision efficiencies (<10%) when the large drop is less than 40 μm in diameter but collision efficiencies approaching 100% when the larger drop is greater than 80-μm diameter. Thus theory indicates that the collision–coalescence process for precipitation formation requires at least a small fraction of cloud drops to be larger than about 40-μm diameter. Because collision between drops does not necessarily result in coalescence, it is important to distinguish between collision efficiency and coalescence efficiency. Their product, termed the collection efficiency, gives the fraction of drops that collide and coalesce. See warm rain process. 2. The fraction of drops or aerosol particles colliding with precipitation particles. See accretion, washout.