A radar designed to measure microwave backscattering, quantified as the scattering coefficient or normalized radar cross section 0, from natural media. Exposed glacier ice in the ablation zone lacks a distinctive mass-balance-related signature at microwave wavelengths. In the percolation zone, subsurface ice lenses are strong scatterers, but there is a sharp reduction in backscattering when meltwater appears at the surface. When wet, the surface becomes a more nearly specular (forward) reflector and appears radar-dark instead of radar-bright. In the dry-snow zone radar returns are unaffected by liquid water, which is absent, and the scattering coefficient contains information on snow grain size and possibly on the accumulation rate. Scatterometers have relatively poor spatial resolution (several to some tens of kilometres), which can be improved by temporal averaging, but they compensate by offering wide and frequent coverage. Seawinds, on the polar-orbiting quikscat satellite (1999-2009) has been a productive scatterometer. Intended for the measurement of ocean-surface wind speeds, it has also proved valuable for measuring the extent and duration of melting on ice caps and ice sheets. See also brightness temperature.