Experimental detection of bleaching of fresh ice in the optical range near 0$^\circ$C

The results of laboratory measurements of the transparency of fresh ice blocks in the optical range in the temperature range from -15$^\circ$C to 0$^\circ$C are presented. The studies were performed at two wavelengths in the visible range (at a wavelength of 535 nm) and the ultraviolet range (at a wavelength of 370 nm). We studied the ice of a natural fresh water body with a characteristic predominant spatial orientation of the main optical axis of the crystals. It has been established that when approaching a temperature of 0$^\circ$, the sample becomes bleaching (decrease in attenuation). The effect occurs in the range -0.5–0$^\circ$C. It is determined by the occurrence of plastic deformation due to thermal stresses caused by the initial stage of the ice-water phase transition. The bleaching in experiments with ice blocks $\sim$10 cm thick was 3–25%. The results obtained are of interest for solving remote sensing problems, since melting ice is a widespread object due to its significant heat of phase transition. Previously, the effect of bleaching of fresh ice was also discovered in the microwave range when the temperature of the sample approached the phase transition point.