We analyzed how the asymmetry factor of the atmospheric aerosol phase function can be estimated from spectral measurements of light attenuation and scattering. An empirical formula is suggested to parameterize the asymmetry factor as a function of the Ångström exponent and complex refractive index. Several approaches to determining the asymmetry factor from data of integrating nephelometers are considered. An analytical formula, relating the asymmetry factor to the nephelometer-measured ratio of the radiative fluxes scattered into the forward and backward hemispheres, is derived. The asymmetry factor at three wavelengths is calculated according to data of field measurements in St. Petersburg suburbs in 2015.