The spectral attenuation of sunlight passing through the atmosphere was determined with the Langley method for 110 clear days and at 11 wavelengths to an accuracy of δτ=±0.002 (τ is the optical thickness) at the Mauna Loa Observatory, Hawaii. Suspended aerosols above the observatory attenuated light by an average of 1.9% (in the vertical direction) at a wavelength of 5000 Å, and the average attenuation varied with wavelength as λ−1.6. Air masses from northerly directions were most turbid, τ¯=0.021±0.015, white those from southwesterly direction were least turbid, τ¯=0.017±0.005. The lowest values of optical extinction varied as λ−1.6 while those from directions of nearest continents varied as λ−2.5; the larger values of wavelength exponent for the continental aerosol is what would be expected for an aerosol cloud that had been carried by the winds from distant continents. It is deduced that aerosol from North America and Asia occasionally reaches the Hawaiian Islands. The explosive eruptions of A... Abstract The spectral attenuation of sunlight passing through the atmosphere was determined with the Langley method for 110 clear days and at 11 wavelengths to an accuracy of δτ=±0.002 (τ is the optical thickness) at the Mauna Loa Observatory, Hawaii. Suspended aerosols above the observatory attenuated light by an average of 1.9% (in the vertical direction) at a wavelength of 5000 Å, and the average attenuation varied with wavelength as λ−1.6. Air masses from northerly directions were most turbid, τ¯=0.021±0.015, white those from southwesterly direction were least turbid, τ¯=0.017±0.005. The lowest values of optical extinction varied as λ−1.6 while those from directions of nearest continents varied as λ−2.5; the larger values of wavelength exponent for the continental aerosol is what would be expected for an aerosol cloud that had been carried by the winds from distant continents. It is deduced that aerosol from North America and Asia occasionally reaches the Hawaiian Islands. The explosive eruptions of A...