Sketch of the averaging procedure of the aerosol optical depth, T_aer, over an assumed air-shower light emission profile. In this case, some aerosol extinction due to the ground layer is shown, and a cloud is located at a typical height of 9 km above ground. The left vertical axis shows T_aer, and the right axis denotes the normalized distribution, ϵ(h), of the number of Cherenkov photons that can reach the IACT telescope and camera. In red, ϵ(h) is shown and in blue, the product of ϵ(h) · T_aer(h).