All-sky imaging: a simple, versatile system for atmospheric research

Abstract

A simple and inexpensive fully automated all-sky imaging system based on a commercial digital camera with a fish-eye lens and a rotating polarizer is presented. The system is characterized and two examples of applications in atmospheric physics are given: polarization maps and cloud detection. All-sky polarization maps are obtained by acquiring images at different polarizer angles and computing Stokes vectors. The polarization in the principal plane, a vertical cut through the sky containing the Sun, is compared to measurements of a well-characterized spectroradiometer with polarized radiance optics to validate the method. The images are further used for automated cloud detection using a simple color-ratio algorithm. The resulting cloud cover is validated against synoptic cloud observations. A Sun coverage parameter is introduced that shows, in combination with the total cloud cover, useful correlation with UV irradiance.

Calibration of an all-sky camera for obtaining sky radiance at three wavelengths

“…López-Alvarez et al (2008) derived sky radiance spectra from sky images using a linear pseudo-inverse method, and Olmo et al (2008a) used these spectra, and an iterative method based on radiative transfer, to obtain AOD at 550 nm. Other authors used a sky imager with filters to measure the relative sky radiance polarization (Horváth et al, 2002;Kreuter et al, 2009). Some works were performed using sky imagers carefully calibrated in the laboratory to obtain spectral radiance values Zibordi and Voss, 1989;Voss and Liu, 1997;Cazorla et al, 2009).…”

Remote sensing of lunar aureole with a sky camera: Adding information in the nocturnal retrieval of aerosol properties with GRASP code

“…It should be noted 95 also that sky cameras usually present a low signal to noise ratio. Cloud detection is the 96 most spread use of sky cameras [Long et al, 2006; Calbó and Sabburg, 2008; Cazorla et 97 al., 2008a; Ghonima et al, 2012;Kazantzidis et al, 2012; Mandat et al, 2014, Alonso 98 et al, 2014 though they have been used with other purposes [Horváth et al, 2002; 99 Cazorla et al, 2008b;Kreuter et al, 2009; Sigernes et al, 2014], including the 100 retrieval of sky radiances in daytime [Voss and Zibordi, 1989; López-Alvarez et al, 101 2008; Román et al, 2012; Toshing et al, 2013; Chauvin et al, 2015]. The CCD spectral response, given by the manufacturer, is shown in Fig.…”

Reflective all-sky thermal infrared cloud imager

“…All-sky imaging systems have used either reflective optics or fisheye lenses [24][25][26], and a variety of data-processing techniques that include radiometric images of daylight and starlight [24], visible-wavelength ratios [20,25], polarimetry [27][28][29][30], and neural networks [31]. In addition to the most common visible and near-infrared (VNIR) cloud imagers, longwave infrared (LWIR) systems are being developed with the advantage of providing equal day and night sensitivity with the same cloud retrieval algorithm, as long as clear-air emission is properly estimated and removed (this is done primarily with an estimate of atmospheric water vapor, but does not vary with aerosols [37]).…”