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  • 1. Hegedues, Ramon
    et al.
    Akesson, Susanne
    Horvath, Gabor
    Polarization patterns of thick clouds: overcast skies have distribution of the angle of polarization similar to that of clear skies2007In: Optical Society of America. Journal A: Optics, Image Science, and Vision, ISSN 1084-7529, E-ISSN 1520-8532, Vol. 24, no 8, p. 2347-2356Article in journal (Refereed)
    Abstract [en]

    The distribution of polarization in the overcast sky has been practically unknown. Earlier the polarization of light from heavily overcast skies (when the Sun’s disc was invisible) has been measured only sporadically in some celestial points by point-source polarimetry. What kind of patterns of the degree p and angle a of linear polarization of light could develop after transmission through a thick layer of ice or water clouds? To answer this question, we measured the p and a patterns of numerous totally overcast skies on the Arctic Ocean and in Hungary by full-sky imaging polarimetry. We present here our finding that depending on the optical thickness of the cloud layer, the pattern of a of light transmitted through the ice or water clouds of totally overcast skies is qualitatively the same as the a pattern of the clear sky. Under overcast conditions the value of a is determined predominantly by scattering on cloud particles themselves. Nevertheless, the degrees of linear polarization of light from overcast skies were rather low (p <= 16 %). Our results obtained under overcast conditions complete the earlier findings that the a pattern of the clear sky also appears in partly cloudy, foggy, and smoky skies. Our results show that the celestial distribution of the direction of polarization is a very robust pattern being qualitatively always the same under all possible sky conditions. This is of great importance for the orientation of polarization-sensitive animals based on sky polarization under conditions when the Sun is not visible. (c) 2007 Optical Society of America.

  • 2. Hegedus, R.
    et al.
    Åkesson, S.
    Horvath, G.
    Polarization of "water-skies" above arctic open waters: how polynyas in the ice-cover can be visually detected from a distance2007In: Optical Society of America. Journal A: Optics, Image Science, and Vision, ISSN 1084-7529, E-ISSN 1520-8532, Vol. 24Article in journal (Refereed)
    Abstract [en]

    The foggy sky above a white ice-cover and a dark water surface (permanent polynya or temporary lead) is white and dark gray, phenomena called the "ice-sky" and the "water-sky," respectively. Captains of icebreaker ships used to search for not-directly-visible open waters remotely on the basis of the water sky. Animals depending on open waters in the Arctic region may also detect not-directly-visible waters from a distance by means of the water sky. Since the polarization of ice-skies and water-skies has not, to our knowledge, been studied before, we measured the polarization patterns of water-skies above polynyas in the arctic ice-cover during the Beringia 2005 Swedish polar research expedition to the North Pole region. We show that there are statistically significant differences in the angle of polarization between the water-sky and the ice-sky. This polarization phenomenon could help biological and man-made sensors to detect open waters not directly visible from a distance. However, the threshold of polarization-based detection would be rather low, because the degree of linear polarization of light radiated by water-skies and ice-skies is not higher than 10%. (c) 2006 Optical Society of America.

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