The exact formulas are presented that allow for the effect of Doppler-broadened Rayleigh backscattering (DBRS) in DIAL humidity sensing. Numerical experiments have shown that the DBRS effect imposes limitations on the applicability of DIAL technique to sensing the humidity in the stratosphere with a ground–based lidar. As to a spaceborne lidar, the DBRS effect results in the error in H2O measurements no more than 16% within the altitude range 0–20 km with no aerosol layers.