# Opportunistic Weather Sensing by Smart City Wireless Communication Networks
## 1 Introduction--Opportunistic Sensing of Weather  
Weather monitoring, especially for precipitation, is crucial for modern applications like forecasting, water management, and smart city development. It also plays a role in understanding global phenomena like climate change and renewable energy potential. Within an urban environment, accurate rainfall monitoring is especially important for water management such as flash in-city flood prevention, and to help urban planners and engineers use rainfall information both to design efficient city infrastructure and better manage and control the city during extreme weather events scenarios. Traditional weather monitoring tools, such as rain gauges and weather stations, are precise but offer limited spatial coverage, while remote sensing technologies like satellites and radars provide wider coverage but are less accurate near the ground [1, 2]. Moreover, the temporal resolution of remote sensing measurements is limited. Furthermore, installing and maintaining these systems is expensive and typically limited to developed countries, leaving regions such as Africa, Central Asia, and South America with poor or non-existing coverage. On the other hand, technological advancements use the atmosphere, and, in particular, the near-the-surface atmosphere, as a home for human-made electromagnetic radiation, for military and civilian uses. A few examples are radio transmission, cellular communication, satellite TV, and navigation systems. Nowadays, the electromagnetic spectrum is so crowded that in any given time and almost all around the globe, technology induced electromagnetic waves are present [3]. Naturally, the electromagnetic signals, traveling in the atmosphere in different frequencies, layers, and locations, carry information about the media they go through [4]. The idea of leveraging existing communication technologies for environmental monitoring, i.e., in an opportunistic use, is a relatively new and viable solution to improveenvironmental monitoring capabilities, as dedicated meteorological sensor network deployment involves high costs and expensive maintenance [5]. In [6], the Global Positioning System (GPS) was first used for water vapor sensing. Later, in 2006, it was first shown how commercial cellular backhaul networks can be used for rain monitoring [7; 8; 9]. This idea is based on utilizing the already deployed existing commercial wireless communication networks, and, in particular, commercial microwave and mmWave links (CMLs)--usually operating at frequencies above 6 GHz--for precipitation monitoring. The number of CMLs worldwide is immense (in order of millions of CMLs), and thus, adding this approach to the arsenal of weather monitoring can vastly help in reducing the need to deploy and maintain dedicated meteorological sensors. Specifically, in locations where dedicated meteorological equipment is lacking (e.g., Africa), cellular coverage indeed exists [10; 11; 12], further helping close the gap in the world-wide coverage of weather sensing. In recent years, the concept of integrated sensing and communication (ISAC) started to gain interest as the next generation leading technology [13]. It was recently presented how high-frequency E-band-based CMLs can be used to monitor rain in urban environments [14; 15] where traditional weather monitoring techniques often fail. The potential for weather opportunistic monitoring is further enhanced as future networks--and especially in urban environments--will become more diverse and include many line-of-sight microwave and mmWave links, all part of an xhaul mesh (combining the \"classic\" backhaul and fronthaul of wireless communication networks) that becomes a necessity for the modern city [16]. The ISAC (also known as joint communication and sensing--JCAS) paradigm aims at future technologies designed to optimally use the spectrum for both communication and sensing applications.