An algorithm is presented here for determining cloud optical depth, τ, using data from shortwave broadband irradiances,
focusing on the case of optically thin clouds. This method is empirical and consists of a one-line equation. This
method is applied to cirrus clouds observed at the Atmospheric Radiation Measurement Program Climate Research Facility
(ACRF) at Darwin, Australia, during the Tropical Warm Pool International Cloud Experiment (TWP-ICE) campaign
and cirrus clouds observed at the ACRF Southern Great Plains (SGP) site. These cases were chosen because independent
verification of cloud optical depth retrievals was possible. For the TWP-ICE case, the calculated optical depths agree to
within 1 unit with τ calculated from a vertical profile of ice particle size distributions obtained from an aircraft sounding.
For the SGP case, the results from the algorithm correspond reasonably well with τ values obtained from an average over
other retrieval methods, some of which have been subject to independent verification. The medians of the two time series
are 0.79 and 0.81, for the empirical and averaged values, respectively. Because such close agreement is likely to be fortuitous
and therefore not truly represent the performance of our method, τ values derived from our method were compared to
values obtained from lidar data. Over a three year period, the difference in median values between the two methods is
about 0.6, with the lidar optical depths being larger. This tool may be applied wherever measurements of the direct, diffuse,
and total components of the shortwave broadband flux are available at 1- to 5-minute resolution. Because these
measurements are made across the world, it then becomes possible to estimate optical depth for both liquid water and ice
clouds at many locations.