The atmospheric aerosol optical depth (AOD) weighted over the solar spectrum is equal to the monochromatic AOD at a certain wavelength. This key wavelength is ~0.7 μm, which is only slightly influenced by air mass and aerosol content. On the basis of this result, simple relations are proposed to predict monochromatic AOD from pyrheliometric data and vice versa. The accuracy achieved is close to ∓0.01 units of AOD at ~0.7 μm, estimated from simultaneous sunphotometer data. The precision required for the estimation of the precipitable water-vapor content is approximately ∓0.5 cm.