Abstract:        A cross-layering alarm application is proposed for supporting fire fighting operations. It runs on scattered wireless sensor networks (WSN) composed by several isolated WSNs, where sensor nodes can be destroyed by fire. Mobile patrol nodes deploy the alarm monitoring application and collect the alarm records, containing the set of sensor measurements above the threshold values. The application was implemented in TinyOS 2.0, on Telos B motes. It uses a new Multimode Hybrid MAC, which can be controlled by the application. The application uses asynchronous mode when no alarms are active to optimise energy consumption; changes to full on mode (without sleeping) to minimise delay during fire handling situations; and uses the synchronous mode (with reserved bandwidth) during the transference of alarm records to the patrol node, balancing delay and energy saving. The alarm application organises sensor nodes into a clustered virtual overlay network and run a peer-to-peer searching service on top of it. This service is used to locate nodes outside the danger area, and to locate alarm records. The application performance was tested using TOSSIM simulations. Simulations results show the application capacity to capture a fire evolution.