A method of monitoring the accumulation of matter on an external surface of an aircraft involves applying varying electric fields to the aircraft surface, and a sensor monitor receiving a time-series of measurements of a temperature of the aircraft surface and a phase and a magnitude of a current induced in an electric field sensor unit from the varying electric fields. The electric field sensor unit is mounted proximate the aircraft surface. The time-series of the measurements varies in accordance with the instantaneous accumulation of matter on the aircraft surface. The sensor monitor generates an assessment of the accumulation of matter on the aircraft surface from a correlation of the time-series of the measurements with at least one predetermined matter accumulation profile. Each predetermined matter accumulation profile comprises a corresponding time-series of current phase, current magnitude and temperature measurements.