libRadtran's input for cloud layer properties (via wc_file or ic_file) requires layer input specified as cloud water/ice path (CWP) and effective radius (r_eff). However, some applications prefer to work in terms of cloud optical depth (τ) and effective radius. In the geometric optics limit (valid in the solar spectral region), conversion between cloud water path and cloud optical depth is given by: τ=(3/2)*(CWP/r_eff)