A method for the determination of the spectral dependent fraction of the atmospheric extinction for an arbitrarily observed star in the case of broad-band photometry is suggested. This method involves measurements in an auxiliary photometric passband, defined by an additional filter with the spectral transmittance curve similar to that of the atmosphere. In this case, the atmospheric extinction is a linear function of the color index (we name it as D--index) obtained from the flux measurement in these two passbands. This method is estimated to provide an accuracy which is better than 0.002 mag for the ultraviolet W passband. An example for the practical realization of such ``atmospheric'' filters is constructed from colored glasses. The D--index may be also used for photometric classification of stars and for transformation of photometric systems.