How the clear-sky angle of polarization pattern continues underneath clouds: full-sky measurements and implications for animal orientation

One of the biologically most important parameters of the cloudy sky is the proportion P of the celestial polarization pattern available for use in ani mal navigation. We evaluated this parameter by measuring the polarization p atterns of clear and cloudy skies using 180 degrees (full-sky) imaging pola rimetry in the red (650 nm), green (550 nm) and blue (450 nm) ranges of the spectrum under clear and partly cloudy conditions. The resulting data were compared with the corresponding celestial polarization patterns calculated using the single-scattering Rayleigh model. We show convincingly that the pattern of the angle of polarization (e-vectors) in a clear sky continues u nderneath clouds if regions of the clouds and parts of the airspace between the clouds and the earth surface (being shady at the position of the obser ver) are directly lit by the sun. The scattering and polarization of direct sunlight on the cloud particles and in the air columns underneath the clou ds result in the same e-vector pattern as that present in clear sky. This p henomenon can be exploited for animal navigation if the degree of polarizat ion is higher than the perceptual threshold of the visual system, because t he angle rather than the degree of polarization is the most important optic al cue used in the polarization compass. Hence, the clouds reduce the exten t of sky polarization pattern that is useful for animal orientation much le ss than has hitherto been assumed. We further demonstrate quantitatively th at the shorter the wavelength, the greater the proportion of celestial pola rization that can be used by animals under cloudy-sky conditions. As has al ready been suggested by others, this phenomenon may solve the ultraviolet p aradox of polarization vision in insects such as hymenopterans and dipteran s. The present study extends previous findings by using the technique of 18 0 degrees imaging polarimetry to measure and analyse celestial polarization patterns.