How we measure snow depth
A snow depth sensor is an electrically-powered active device with on-board data-processing that measures the distance between the sensor and the ground surface beneath it.
An ultrasonic transducer transmits pulses and listens for the returning echoes from the surface.
The distance measurement is based on the time delay between transmission of the pulse and the time of the return of the echo. An independent temperature measurement is required in order to compensate for the temperature-dependent variation of the speed of sound in air.
The sensor output is normalised to zero under conditions of no lying snow. The measurement is made over an artificial grass surface having approximately the same thermal properties of a grass surface but which avoids spurious readings caused by the slow growth of grass. At manned stations without automated sensors, snow depth is measured periodically by the observer using a measuring rod at a number of representative locations.