The aerosol extinction is a source of uncertainty in predictions of the atmospheric attenuation of infrared radiation. We model the dependence of aerosol extinction in the atmospheric boundary layer on meteorological variables such as visibility, humidity, and air mass type by accounting for their influence on a bimodal particle size distribution consisting of two log-normal distributions, and calculating the extinction from Mie theory. The model is fitted to results from simultaneous measurements near the ground with a multiwavelength transmissometer and meteorological sensors, including a nephelometer and a rain gauge. The transmissometer measures the atmospheric extinction along a 500-m horizontal path in 15 bands in the 0.5–14 μm wavelength interval. A complete sequence of transmissometer data and meteorological values is collected automatically every 10-min period. Preliminary results show the anticipated increase of the aerosol extinction with increasing relative humidity and with decreasing wavelength to be in general consistency with the model. Whether the observed variations in wavelength dependence of the aerosol extinction can be interpreted in terms of weather influence in accordance with the model cannot be established until a larger data base has been obtained.