National Oceanic Atmospheric Administration (NOAA)
"Aerosol Optical Thickness" is the degree to which aerosols prevent the transmission of light in the atmoshpere. An aerosol optical thickness of less than 0.1 indicates a crystal clear sky with maximum visibility, whereas a value of 4 indicates the presence of aerosols so dense that people would have difficulty seeing the Sun, even at mid-day!
source: http://serc.carleton.edu/usingdata/datasheets/aerosolopticalthick.html
source: http://serc.carleton.edu/usingdata/datasheets/aerosolopticalthick.html
