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

István Pomozi¹, Gábor Horváth^1,* and Rüdiger Wehner²

¹ Department of Biological Physics, Eötvös University, H-1117 Budapest, Pázmány sétány 1, Hungary,
² Institut für Zoologie, Universität Zürich, CH-8057 Zürich, Winterthurerstrasse 190, Switzerland

View larger version (70K): [in a new window]   Fig. 1. (A-C) Spatial distribution of brightness and colour, degree of polarization and angle of polarization (measured from the local meridian passing through the observed celestial point) over the entire clear sky measured by full-sky imaging polarimetry in the blue (450nm) spectral range for different hourly positions of the sun [from sunrise (06:00h=UTC+1, where UTC is universal time code, row 1) to noon (12:00h=UTC+1, row 7)] on 26 August 1999 in the Tunisian Chott el Djerid. (D,E) Patterns of the degree and angle of polarization of skylight calculated using the single-scattering Rayleigh model for the same positions of the sun as those in A-C. (F-H) Brightness and colour, degree of polarization and angle of polarization patterns of cloudy skies at different places in Tunisia between 27 August 1999 and 4 September 1999 measured by full-sky imaging polarimetry in the blue (450nm) spectral range for the same solar azimuth and for approximately the same solar zenith angles as those in A–C. The red or black regions of the sky in columns B, G or C, H, respectively, are overexposed. The position of the sun is indicated by a black or white dot. The radial bar in the pictures is a wire holding a small disk to screen out the sun.

View larger version (71K): [in a new window]   Fig. 2. As Fig.1 from 13:00h (=UTC+1, row 1) to 19:00 (=UTC+1, row 7, sunset).

View larger version (47K): [in a new window]   Fig. 3. Regions of the clear (A) and cloudy (B) sky shown in Fig.1A,1 and F,1, respectively, with polarization patterns useful for or inappropriate to reliable cricket navigation calculated on the basis of the celestial polarization patterns measured by full-sky imaging polarimetry in the blue (450nm) spectral range. Blue (useful for navigation): regions of the clear sky where the degree of polarization > 5%. Yellow (inappropriate to navigation): regions of the clear sky where 5%. Green (useful for navigation): regions of the clouds where > 5% and |clear sky–clouds| 6.5°, where is the angle of polarization. Red (inappropriate to navigation): regions of the clouds where 5% and/or |clear sky–clouds| > 6.5°. Black: region of the sky where the photoemulsion was overexposed. The numerical values of , clear sky and clouds originate from quantitative full-sky measurements.