The variation of the optical depth of atmospheric aerosols has been analysed based on measurement data under different atmospheric conditions. The accuracy of Angstrom formula is estimated. The agreement of Angstrom formula with measurement data and the valid range of the Angstrom formula are discussed. The result shows that: 1) in the visible and near-infrared spectral regions only 20% of the measurement data are in close agreement with Angstrom formula; 2) when aerosol optical depth is smaller than 0.1, t...