- Summertime low clouds are common in the Pacific Northwest (PNW), but spatiotemporal patterns have not been characterized. We show the first maps of low cloudiness for the western PNW and North Pacific Ocean using a 22‐year satellite‐derived record of monthly mean low cloudiness frequency for May through September and supplemented by airport cloud base height observations. Domain‐wide cloudiness peaks in midsummer and is strongest over the Pacific. Empirical orthogonal function (EOF) analysis identified four distinct PNW spatiotemporal modes: oceanic, terrestrial highlands, coastal, and northern coastal. There is a statistically significant trend over the 22‐year record toward reduced low cloudiness in the terrestrial highlands mode, with strongest declines in May and June; however, this decline is not matched in the corresponding airport records. The coastal mode is partly constrained from moving inland by topographic relief and migrates southward in late summer, retaining higher late‐season low cloud frequency than the other areas.
- In the Pacific Northwest (PNW), low clouds, including fog, are a visible feature of the region's climate. PNW summers are dry, and ecosystems, cities, and agriculture rely on cooling effects and additional moisture provided by low clouds. We seek to understand when and where summertime low cloud frequency varies across the PNW by analyzing 22 years of satellite imagery. Using a statistical technique to isolate unique spatial and temporal groups, we identified four areas exhibiting unique patterns, which we call oceanic, terrestrial highlands, coastal, and northern coastal. Each pattern exhibits unique characteristics. Although low clouds over land are the least frequent, they steadily declined in the 1996–2017 satellite record, particularly in May and June, a trend not apparent in the other three regions or in the airport records. Oceanic low clouds occur the most frequently. Coastal low clouds migrate southward throughout summer and have higher frequencies in August and September relative to the rest of the PNW.