This has been an extraordinary dark and dreary winter in Seattle. A dotplot of the cumulative daily Sky Cover scores for SeaTac International Airport shows the period between December 1, 2019 though January 29, 2020 far exceeds the scores for similar periods back to 2006-07.
Another way to view the darkness this winter is to look at the average daily solar radiation, measured in MJ/m^2. This data is available from Washington State University.
Extracting data from their ‘Seattle’ location – which I beleive is on the campus of the University of Washington, but subject to correction – you can see that the average daily solar radiation for this winter (December 2019-January 2020) is the lowest value for the past nine years, years in which data is available for this location.
Click to enlarge…
Finally, look at the rain pattern over this same December-January period this water year (a water year calendar runs from Oct 1 thru Sep 30 due to rain patterns in the Pacific Northwest). The line chart below shows that as of January 29, SeaTac International Airport has accumulated almost the exact normal amount of precipitation to be expected based on the 30-year average for this location (established from records for the 1981-2010 period).
But this year, the first 2-½ months of the water year were relatively dry. It wasn’t until about mid-December when the rains began catching up to the 30-year average. It’s taken a lot of steady dark, water-heavy clouds to drop this amount of rain in a relatively short period. This information supports the high level of daily cloudiness and low level of solar radiation shown in the first two charts.
Click to enlarge.