The spectral radiances at wavelengths 7.8--11.5 &mgr;m of the sea surface at small angles below the horizon show a well defined minimum. The location of the minimum, its sharpness and its magnitude depend on wavelength, sea state, atmospheric turbidity, sea and air temperatures. Remote sensing of sea state appears possible by interpreting spectral radiances measured in a vertical scan of the sea near the horizon. A close proximity of the minimum to the horizon indicates a high sea state.