Rainfall/ˈreɪnfɔːl/ Definition 1 of 2Pro Tip 1/3Measurement Unit Rainfall is often measured in millimeters or inches to quantify precipitation. The region received 40mm of rainfall over the weekend. Pro Tip 2/3Time Period Specify the time when mentioning rainfall to clarify when the rain occurred. July's average rainfall is higher than in January. Pro Tip 3/3Climatic Indicator Rainfall is used to describe climate patterns and can indicate a wet or dry region. The savanna is characterized by seasonal rainfall.