[MLO Apparent Transmission Graph]

Shown above is the most current version of the Mauna Loa Apparent Transmission record. Monthly values are plotted in time series along with a 6-month smoothed LOWESS fit in red. The downloadable data file (available here) includes monthly mean values of the apparent transmission. In 2019 new processing methods were developed for computing apparent transmission from the raw radiation data from Mauna Loa. Efforts were made to ensure that the new transmission processing emulated the previous methods. In updating the time series, the historic apparent transmission data was retained through August 2017 and the new processing was applied thereafter.

The legacy time series from 1958 through Aug. 2017 maintains past improvements:

  1. a refinement to the time-to-integer-airmass interpolation scheme between 1993 and 2012,
  2. a minor revision of the clear-sky selection criteria used between 1993 and 2012, and
  3. the removal of spurious contributions from a backup instrument between 2000 and 2002.
These changes did not significantly alter previously published conclusions concerning this dataset by NOAA/ESRL authors, and strengthen the case for a general decline in transmission (or transmittance) between 2000 and 2009.

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