We have analysed the ability of the Angstrom power law to model the spectral aerosol optical depth, tau(a lambda), for the 400-670 nm band, obtained from spectral direct irradiance measurements at normal incidence. The spectra were registered at ground level in Valencia, Spain, using a Li-cor 1800 spectroradiometer. The results obtained showed that the fitting method that introduces lower errors in the determination of the Angstrom power law coefficients is to adjust directly the spectral experimental data In this way the errors obtained for the turbidity coefficient, beta, were about 0.004 and for the wavelength exponent, alpha, 0.07. The correlation coefficient was always greater than 0.95. These values of the correlation coefficient could be improved by parameterisation of tau(a lambda) using an alternative function of the wavelength to the Angstrom power law. But this may not be justified for the turbidity Values attained in Valencia.