Cloud Cover Obtainment

Is there an equation or method to obtain cloud cover from short-wave solar radiation data? I have hourly short-wave solar radiation data which I will put into the met file however cloud cover is still required in the met file for calculating long-wave solar radiation as mentioned in a past Forum topic (http://w2forum.cee.pdx.edu/?q=node/345). There is also a mention of using Brunts equation to calculate long-wave solar rad in the manual ("The long wave atmospheric radiation is computed from air temperature and cloud cover or air vapor pressure using Brunts formula."). However I am confident that this would not replace the need for a cloud cover input in the met file since there is no place to input long-wave solar radiation as a variable in the met file. Please let me know of a way to obtain cloud cover. Thank you for your time, Steven