Autoconversion Rate is the rate at which water mass in the form of cloud droplets forms raindrops. An Accretion Rate can be considered independently, consisting of the rate at which water mass accretes to existing raindrops. A distinction between the two can be made by specifying a size (radius) above which a droplet is to be considered an existing raindrop.
These are used in modeling Atmospheres. Depending upon the model's complexity, an estimate average rate may be provided, or a physical formula can be used to calculate it under varying circumstances.