The ASOS Freezing Rain (FZRA) Sensor is based on technology initially developed to detect icing on aircraft in flight. The sensing device consists of a small cylindrical probe that is electrically stimulated to vibrate at its resonant frequency. A feedback coil is used to measure the vibration frequency, which is proportional to the mass of the probe. Magnetostriction is a property of certain metals in which a change in the (axial) dimension of a body causes a change in magnetization. It is used in the ASOS sensor to drive the probe at a natural resonant frequency of 40kHz. The axial vibration is of such low amplitude that it cannot be seen or felt. The probe is oriented vertically to provide optimal uniform exposure to freezing precipitation regardless of wind direction. This position also prevents birds from alighting.

When ice freezes on the probe, the combined mass increases and the resonant vibration frequency decreases. There is a well defined relationship between the measured frequency and the ice accretion on the probe. The freezing rain instrument is sensitive enough to measure accumulation rates as low as 0.01 of an inch per hour. The freezing rain sensor continuously monitors the resonant frequency of the vibrating probe, obtains a sample once a second, and once each minute averages the results to update the probe's current resonant frequency. When excessive freezing rain accumulates, (i.e., equal to or greater than 0.08 inch) the sensor goes into a heating cycle to melt the freezing rain from the probe and return it to the base resonating frequency. This process normally takes two to three minutes. During this time, the sensor status is set to "de-ice" and the output is not updated.