To better understand the characteristicresource, a preliminary study was undertaken of trends in
direct normal irradiance from three sites around Oregon
over a period of 25 years. An overall increase of about 13-
16% over the 25 years was found. Seasonal as well as yearly
trends were studied. During the summer months an increase
was observed while during the winter months a decrease
was observed. Changes in clear noon values are also
examined. This article analyzes direct normal data unlike
other recent studies which find decreasing trends in global
radiation data worldwide. The data analysis in this study is
the first step in testing and refining regional climate models
in the Pacific Northwest. A better understanding of the
regional climate models can be achieved by utilizing longterm
solar radiation data trends with changes in cloud cover,
aerosol optical depth, and other meteorological changes.s of the region