A good option is to calculate daily clear sky radiation and compare to your values. They should be close in days with no clouds.

 Clyde

Sent from my iPhone

On Feb 20, 2012, at 4:57 PM, "John S. Felkner" <[log in to unmask]> wrote:

> Dear Dssat Community:
>
> Using data on sunlight duration from a weather station in Thailand, we have
> converted sunlight duration hours to solar radiation in MJ/m2/day using the
> Weatherman sunlight duration/solar radiation conversion routine (when
> importing from the raw data import grid).  However, the resulting estimated
> solar radiation values in the Weatherman weather database that is created
> appear to be somewhat low:  none of the estimated solar radiation daily
> values exceed 20 MJ/m2/day, even during the summer months.
>
> We are wondering if anyone might have an idea why the estimated values seem
> somewhat low?  The input sunlight duration hours for a weather station in
> Surin Province, Thailand were obtained directly from the Thai
> Meteorological Department, and for 1998 range from 0 to 11.4, with a mean
> of 6.89 and a stdev of 2.72.  The estimated solar radiation values for the
> same days from Weatherman range from 4.6 to 18, with a mean of 12.5 and
> stdev of 3.02.  We specified "hours of bright sunshine" as the units in the
> Weatherman raw data import grid, and were careful to specify the correct
> latitude/longitude of the station as well as its elevation above sea level
> in meters.
>
> Thanks for any help or advice.  Is the Weatherman model for converting
> sunlight duration to solar radiation reliable?  Are there alternative
> methods for converting that anyone may be aware of?
>
> John Felkner
> University of Chicago