Fig 1.
The sky-scanner system that conducted the all-sky radiance measurements was programmed to sample the radiance at intervals of 15 degrees along both spherical coordinates (θ and φ) such that a total of 150 measurements resulted from each scan. A single-wavelength scan took roughly 8 minutes. In the case of downtown Santiago de Chile (33.4467°S, 70.6827°W), the gray sky in the picture is largely attributable to urban pollution. Thick clouds are frequent on King George Island (62.2013°S, 58.9658°W). The snow on Union Glacier (79.7669°S, 82.9144°W) is one of the cleanest on Earth, which makes the local albedo extremely high. Photographs taken by the authors. Plots and maps were generated by using Python’s Matplotlib Library [54].
Fig 2.
Radiance distribution on King George Island (early summer).
Frequent clouds make the zenith radiance larger (relative to cloudless conditions). a) Radiance distribution (450 nm) measured under broken cloud conditions on 9 Dec 2016. Circles indicate the measuring points except for the black filled-in circle that stands for the sun position. The solar zenith angle (SZA) and solar azimuth angle (SAA) at the moment of the measurements were 40° and 61°, respectively; b) Radiance distribution (450 nm; SZA = 39°; SAA = 97°; 9 Dec 2016) measured under overcast conditions; c) Angular distribution of the radiance in the plane “West-Zenith-East”. This plot shows the radiance measured changing the zenith angle (θ) while keeping the azimuth angle (ϕ) constant at 0°. Radiance data were obtained from plots a) and b). If the radiance were perfectly isotropic, this plot would show semicircles; d) Radiance distribution (550 nm; SZA = 39°; SAA = 91°) computed by using the UVSPEC model assuming cloudless conditions; e) Radiance distribution (550 nm; SZA = 39°; SAA = 91°; 10 Dec 2016) measured under overcast conditions; f) Angular distribution of the radiance in the plane “West-Zenith-East”. Radiance data were obtained from plots d) and e). Coordinates x and y in plots c) and f) were computed considering that the radiance (r) and the zenith angle (θ) are polar coordinates: x = r.cosθ y = r.sinθ. Plots were generated by using Python’s Matplotlib Library [54].
Fig 3.
Radiance distribution in Santiago (summer).
Urban pollution makes zenith radiance larger (relative to unpolluted conditions). a) Radiance distribution (450 nm; SZA = 15°; SAA = 105°; 20 Jan 2015) measured under cloudless conditions when the AOD was 0.34. Circles indicate the measuring points except for the black filled-in circle that stands for the sun position; b) Radiance distribution (450 nm; SZA = 21°; SAA = 135°; 22 Jan 2015) measured under cloudless conditions when the AOD was 0.18; c) Angular distribution of the radiance in the plane “West-Zenith-East”. This plot shows the radiance measured changing the zenith angle (θ) while keeping the azimuth angle (ϕ) constant at 0°. Radiance data were obtained from plots a) and b). If the radiance were perfectly isotropic, this plot would show semicircles; d) Radiance distribution (550 nm; SZA = 18°; SAA = 91°; 22 Jan 2015) measured under cloudless conditions when the AOD was 0.14; e) Radiance distribution (550 nm; SZA = 18°; SAA = 90°) computed using the UVSPEC model assuming unpolluted conditions (AOD = 0.03); f) Angular distribution of the radiance in the plane “West-Zenith-East”. Radiance data were obtained from plots d) and e). Coordinates x and y in plots c) and f) were computed considering that the radiance (r) and the zenith angle (θ) are polar coordinates: x = r.cosθ y = r.sinθ. Plots were generated by using Python’s Matplotlib Library [54].
Fig 4.
Radiance distribution on Union Glacier (summer).
a) Radiance distribution (350 nm; SZA = 75°; SAA = 330°; 2 Dec 2014) measured under cloudless conditions. Circles indicate the measuring points except for the black filled-in circle that stands for the sun position; b) Radiance distribution (450 nm; SZA = 75°; SAA = 330°; 2 Dec 2014) measured under cloudless conditions; c) Angular distribution of the radiance in the plane “West-Zenith-East-Nadir” conducted when the SZA was 62° (350 nm, see blue line) and 67° (450 nm, see red line). This plot shows the radiance measured changing the zenith angle (θ) while keeping the azimuth angle (ϕ) constant at 0°. If the radiances were perfectly isotropic, this plot would show circles. d) Ratio between the horizon radiance (θ = 90°; φ = 180°) and the zenith radiance (θ = 0°) measured on Union Glacier; e) Ratio between the nadir radiance (θ = 180°) and the zenith radiance (θ = 0°) measured on Union Glacier; f) Angular distribution of the radiance in the plane “West-Zenith-East-Nadir” conducted on Union Glacier (early summer) when the SZA was 66°. Plots d) and e) are based on spectral measurements shown in the supporting information (S10 Fig in S1 File). The solar zenith angle (SZA) and the solar azimuth angle (SAA) at the moment of the measurement are indicated in the plots. Coordinates x and y in plots c) and f) were computed considering that the radiance (r) and the zenith angle (θ) are polar coordinates: x = r.cosθ y = r.sinθ. Plots were generated by using Python’s Matplotlib Library [54].