Relating Solar Irradiance Variations and Weather across the Northwestern United States

The northwestern United States is not typically thought of as a promising location for solar energy production because of its high latitude and frequent cloud cover. However, conditions vary widely across this region, and even those areas prone to extended cloudiness have high solar potential during the summer, since the days are long (up to 16 hours) and the weather is generally clear. Therefore, an assessment of the character of the area's solar resource is warranted. Here we present results of a study of irradiance data from five sites spread from western Oregon to southwestern Montana that range in climate from wet lowland coastal to semiarid high plains to intermountain valley, and elevations from 150 to 1500 m. Long term local solar resource availability will first be discussed, followed by a description of the variability of this resource at temporal scales ranging from minutes to days that was determined using several analytical techniques. The observed differences in overall resource and temporal variability among the sites are then related to weather regimes, which vary geographically as a function of climate. The irradiance variability patterns are also compared to those in other areas of the country, including the central plains, Colorado, and the Eastern Seaboard, where other weather regimes prevail.