How well does the Rayleigh model describe the E-vector distribution of skylight in clear and cloudy conditions?: A full-sky polarimetric study

We present the first high-resolution maps of Rayleigh behavior in clear and cloudy sky conditions measured by full-sky imaging polarimetry at the wavelengths of 650 nm (red), 550 ran (green), and 450 ran (blue) versus the solar elevation angle theta(s). Our maps display those celestial areas at which the deviation Deltaalpha = \alpha(meas) - alpha(Rayleigh)\ is below the threshold alpha(thres) = 5degrees, where alpha(meas) is the angle of polarization of skylight measured by full-sky imaging polarimetry, and alpha(Rayleigh) is the celestial angle of polarization calculated on the basis of the single-scattering Rayleigh model. From these maps we derived the proportion r of the full sky for which the single-scattering Rayleigh model describes well (with an accuracy of Deltaalpha = 5degrees) the E-vector alignment of skylight. Depending on theta(s), r is high for clear skies, especially for low solar elevations (40% < r < 70% for theta(s) less than or equal to 13degrees). Depending on the cloud cover and the solar illumination, r decreases more or less under cloudy conditions, but sometimes its value remains remarkably high, especially at low solar elevations (r(max) = 69% for theta(s) - 0degrees). The proportion r of the sky that follows the Rayleigh model is usually higher for shorter wavelengths under clear as well as cloudy sky conditions. This partly explains why the shorter wavelengths are generally preferred by animals navigating by means of the celestial polarization. We found that the celestial E-vector pattern generally follows the Rayleigh pattern well, which is a fundamental hypothesis in the studies of animal orientation and human navigation (e.g., in aircraft flying near the geomagnetic poles and using a polarization sky compass) with the use of the celestial a pattern. (C) 2004 Optical Society of America.