**Converting Solar
Radiation to Hours of Bright Sunshine **

By R.L. Snyder

Copyright - Regents of the University of California

Created – November 1, 2001

Last Revision –

Hours of bright sunshine (*n*) are often used
in biological models. In the past, a
measurement of the bright sunshine hours was accomplished using a sunshine
recorder. Then the ratio (*n*/*N*) of actual bright sunshine to
potential bright sunshine (*N*) was used to estimate the ratio of surface
to extraterrestrial solar radiation (*R _{s}*/

In some cases, it is desirable to convert back from
irradiance to sunshine hours. This
involves first estimating extraterrestrial radiation and the potential
(maximum) sunshine hours, which both depend on the astronomical
calculations. The ratio *n*/*N*
is calculated as a function of *R _{s}*/

Then the actual bright sunshine hours is determined
by multiplying the n/N by the N estimated from astronomical calculations. Click on Wton to upload
the Excel program Wton.XLS that makes these calculations. To use the program, input the site
information in the open boxes near the top and then enter the solar radiation
data (*R _{s}*) in the units MJ m

To convert from the mean power over the 24-hour day
in W m^{-2}d^{-1} to MJ m^{-2}d^{-1}, multiply
by 0.0864.