An asset operational health monitoring system, in which a plurality of sensors are arranged to determine values of asset operation parameters pertaining to an instance of asset operation. One or more processing unit receives data corresponding to said asset operation values and determine a plurality of asset locations at a corresponding plurality of points in time. The one or more processing unit is arranged to receive data indicative of the location of a region of adverse environmental conditions which may impact on the operation of the asset and to compare the determined locations of the asset with the location of said region so as to determine whether one or more of said asset locations fall within said region. An indicator of operational risk associated with the presence of said asset in said region is output which can drive asset operation or maintenance decision making. The system may be used for ash cloud impact monitoring for aircraft, a fleet of aircraft or aircraft engines.