Boundary-layer measurements were carried out in the flow over six preserved samples of frost grown on aluminum plates exposed to the clear night sky. The objective of the work was to determine effective height and spacing characteristics of the frost. This was achieved by examining the effects of the frost roughness on the logarithmic portion of the velocity profiles near the frosted surfaces. It was shown that suitable height and spacing parameters could be determined from the geometry of the higher formations of the frost samples. The ratio of equivalent sand roughness height to average frost formation height varies widely between frost samples.