For the past few years a large number of autonomous continuously operating single-wavelength backscatter lidars, so called ceilometers, have been installed. Currently the assessment of their potential for aerosol remote sensing is a major research topic. This paper focusses on the need to consider water vapor absorption if ceilometers emitting at wavelengths in the 905 to 910 nm range are used and proposes a correction scheme to improve the retrieval of the aerosol backscatter coefficient.