Does heavy rainfall, snow or hail affect ultrasonic wind measurement?

No, airborne ultrasonic pulses are not disturbed by heavy rain, snow, or hail located in the sample volume of the ultrasonic anemometer. Despite the additional water content in the sample volume compared to dry air, the relative change from an ultrasonic point of view is very small. In addition, the sample volume changes (content or temperature) are compensated by governing equations.

Even though large raindrop and hail impacts can cause detectable pulses, they are statistically very rare compared to the pulse repetition rate. Thus, they are easy to remove from time-of-flight histograms with further data processing.

Small rain drops, condensation or dust deposited on the transducer surfaces have no effect, because their relative layer thickness compared to the distance between the transducers is negligible. However, if the layer of condensation turns to ice, it can block the ultrasound signals. Under these circumstances, the heating option is required.