The scattering ratio is calculated from the elastic-backscatter lidar signal at 532 nm due to molecules and particles and the Nitrogen Raman scattering with a shifted wavelength (at 607 nm) proportional to air density.35 The scattering ratio is calibrated in the altitude range from 20 to 25 km where we assume aerosol-free conditions. During the preceding years, no major volcanic eruption occurred and consequently, the background stratospheric aerosol concentration is very low. The optical depth is calculated for the altitude range from 7 to 14 km where most of the cirrus clouds were observed.26 The LR can be directly derived from the lidar extinction profile and backscatter coefficient, however, large uncertainties and biases concerning the retrieved values are likely obtained35 in case of subvisible cirrus and thick clouds (when no Rayleigh scattering above the cloud is detected). The LR depends on the optical characteristics of the cirrus particles including size, shape, and particle composition and thus can vary vertically within a cloud. Usually, the value of the LR is assumed constant within the cloud while we know that bigger particles are found at the bottom of the cloud and smaller particles at the top. The attempts to systematically retrieve LR directly using the lidar signal itself have been obtained with large uncertainties leading to a wide spread of values.36 The laser light is attenuated during its passage through the cloud. This is detectable on the lidar signals, however, for subvisible clouds, the attenuation is low and in that case, the optical depth is retrieved with large uncertainties. This is especially true for short integration times such as those we used in this study with initial averaging of 160 s. For this reason, in the previous studies, the LR was set to a constant value of 18.2 sr.37 Recent studies report mean values of 30 sr.38,39 Most of these past investigations have been performed for optically thick cirrus and it was not demonstrated at that time that the cirrus clouds of different types have different LRs. Chen et al.40 show that the LR varies with the cirrus optical thickness. Subvisible clouds have LR around 20 sr close to what have been used in previous studies26,39 and show values larger than 30 sr for thicker clouds. To derive optical depth distributions, the latter two values were used according to the cirrus classes. This is why, in this study, optical depths for both classes corresponding to thin clouds (C1 and C3) were retrieved using the Platt’s value, and for thicker clouds (class C2) have used a larger value of 30 sr. The air density number concentration is calculated from the Mass Spectrometer Incoherent Scatter-Extented atmospheric model of 1990 (MSISE-90).