Fig. 2. Colour photographs (without polarizers) of the mirror image of the clear sky reflected from the grey water-dummy (glass pane underlaid by matt light grey cloth), patterns of the degree (d) and angle (; measured from the local mirror meridian) of linear polarization of reflected skylight, and the area detected polarotactically as water versus the solar elevation (_s) and time (local solar time = UTC + 2 h). The polarization patterns are measured by 180° field-of-view imaging polarimetry in the blue part of the spectrum. Chequered areas show those regions that are inappropriate for comparative analyses due to unwanted overexposure, shadows and mirror images of the polarimeter, its holder and remote cord. In column 4, regions are shaded by black where d>d_tr=5% and 85°95°. An imaginary polarotactic water insect is assumed to consider a surface as water if these two conditions are satisfied for the partially linearly polarized reflected light. In column 4, the regions where these criteria are not satisfied remain blank. The positions of the mirror image of the sun are shown by dots, and the Brewster angle (56° from the nadir for glass with an index of refraction of 1.5) is represented by an inner circle within the circular patterns. Because of disturbance by early morning dewfall, reflection-polarization patterns at low solar elevations are presented here only for the sunset and dusk period.