SURFRAD includes ancillary data (e.g., cloud cover, moisture) that affect the transfer of solar and thermal infrared radiation to and from the surface. An aerosol optical depth product has been recently added.
Aerosol optical depth is a measure of the extinction of the solar beam by dust and haze. In other words, particles in the atmosphere (dust, smoke, pollution) can block sunlight by absorbing or by scattering light. AOD tells us how much direct sunlight is prevented from reaching the ground by these aerosol particles. It is a dimensionless number that is related to the amount of aerosol in the vertical column of atmosphere over the observation location.
A value of 0.01 corresponds to an extremely clean atmosphere, and a value of 0.4 would correspond to a very hazy condition. An average aerosol optical depth for the U.S. is 0.1 to 0.15.
Global solar SURFRAD measurements marked with * in the daily time series above were identified as times of cloud-free skies. The afternoon clear-sky times are cross referenced with coincident MFRSR data to produce the calibration Langley plot on the right.
Blue points are MFRSR 500 nm measured MFRSR voltages that correspond to the afternoon cloud-free times on the plot to the left.
These automatically generated calibration Langley plots are constructed over one, or two-month periods and composited to identify a representative Vo for that period.