Sky brightness in the solar almucantar in the presence of thin cirrus clouds

Measurements of the sky brightness at wavelengths of 0.63 and 1.01 μm in the solar almucantar in the presence of optically thin cirrus clouds were taken at the Zvenigorod Scientific Station of the Institute of Atmospheric Physics, Russian Academy of Sciences, with the aid of a scanning photometer. The method of measurements allowed continuous monitoring of the optical state of the atmosphere and, in particular, monitoring of its optical thickness τ. Experimental values of τ are used to compare theoretical and experimental data on the sky brightness in the solar almucantar. It is shown that information about possible sizes and possible orientations of ice crystals in space can be gained only if the content of ice crystals with definite shapes and sizes in clouds is relatively high.