This International Standard describes the characteristics of Thunderstorm Warning Systems (TWS) in order to implement lightning hazard preventive measures. Single sensors and/or a network of sensors (e.g. Lightning Location System) can be used as a TWS. This standard provides requirements for sensors and networks collecting accurate data of the relevant parameters, giving real-time information on lightning and atmospheric electric activity. It describes the application of the data collected by these sensors and networks in the form of warnings and historical data. This standard includes: " a general description of available techniques for TWS; " guidelines for alarming methods; " informative examples of possible preventive actions. The following aspects are outside the scope of this standard: a) Lightning Protection Systems: such systems are covered by the IEC 62305 series; b) Other thunderstorm related phenomena such as rain, hail, wind; c) Satellite and radar based thunderstorm detection techniques; d) Portable devices (a device where the sensor is not fixed) NOTE Calibration and testing of portable devices may not be sufficient to provide efficient warning.