Real-time rainfall monitoring has always presented great benefits but faced practical challenges. The last decade presented key advancements, the major part of which revolves around employing the effects of precipitation on wireless electromagnetic waves, and in particular, communication signals. Here we present developments in rainfall monitoring involving wireless communication networks. In particular, we discuss the use of existing physical measurements taken by commercial cellular networks as proxies for rainfall. Each microwave-link, representing the channel between two base-stations in a cellular network, can serve as a rain-sensor since the attenuation of the signal along the link is proportional to rain rates. We discuss the opportunities and challenges in using these Opportunistic Sensors (OS) and demonstrate how improvements in coverage, accuracy, and resolution of rainfall measurements can be achieved with no hardware costs. In particular, we discuss the challenge of relating the available links' geometry to the feasible precision and resolution of the reconstructed rainfall map.