It could become an indispensable safety device for outdoor enthusiasts - a cellphone that warns you to take cover if lightning is heading your way. Lightning can pose a major risk to people taking part in outdoor pursuits, such as golfing. So Nokia proposes a warning system that uses the fact that a lightning bolt is an electric current that emits radio waves. Each bolt produces frequencies between 10 hertz and 5 gigahertz, and the distribution of these signals depends on how far away the lightning is. The multiple receivers in a phone, such as Bluetooth, FM, tri-band GSM, Wi-Fi and RFID, can be tuned to pick up these signals, says Nokia (US patent application 2007/0085525). Software will then interpret them, work out the distance to the lightning, and tell you if strikes are getting closer.