Infrared optical properties of the marine boundary layer are basic in the performance of thermal imaging systems, such as forward looking infrared (FLIR) sensors, over the ocean. To aid in evaluating the performance of these sensors, spatial distributions of infrared sky radiance in the 3- 5 μm mid-wavelength infrared (MWIR) and 8 - 12 μm long wavelength infrared (LWIR) spectral bands were measured simultaneously at low elevation angles above the sea surface. Calibrated AGA, Model 780, dual scanning systems functioned as imaging infrared radiometers. Infrared sky radiance and meteorological parameters were recorded concurrently in a series of four data sets during one diurnal cycle starting 15 April 1986 at 1500 Pacific Standard Time (PST) and ending 16 April 1986 at 1730 PST. Radiosondes were released from the deck of the US S POINT LOMA, about 7.6 μm above the ocean, at a range of 5 km due west of the coastal sensor site at Naval Ocean Systems Center, San Diego, CA. Wind speed, direction, sea temperature, and cloud conditions were also recorded on board the ship. Sequential images of radiance distributions provided control data for monitoring the stability or variability of atmospheric conditions throughout the time for radiosonde ascent to about 6 km altitude. Measured IR sky radiance distributions were compared with corresponding clear-sky radiance using the LOWTRAN 6 computer code. Cloud radiance and scattered solar radiation restricted the comparison to elevations close to the optical horizon where aerosol attenuation would be greatest. Infrared aerosol transmittance was inferred from the ratio of measured radiance to calculated clear-sky radiance along the horizon line of sight (LOS). Equivalent temperatures for blackbody radiance at the horizon were either less than or equal to the ambient air temperature near the sea surface, except when the MWIR band included scattered solar radiation; consequently, only the LWIR band could be used to infer aerosol transmittance reliably. Radiance along the optical horizon originated mainly in the lowest 100 m of the atmosphere; therefore, reasonably accurate horizon radiance or transmittance predictions could be made from meteorological data within this low altitude. These results indicate that a LWIR aerosol transmissometer could be developed by computing the ratio of measured horizon sky radiance to calculated clear-sky radiance using local ambient meteorological data.