Updated: December 29, 2021, 6:30 AM PDT
Figure 1 updated with most recent data.
Figure 2 has been updated. Note that the minimal value on the x-axis begins at 320 MJ/sq.meter.
Figure 3 has been updated. Note that the minimal value on the y-axis begins at 300 MJ/sq.meter.
Figure 4 has been added to show the daily temperature departure for 2021 to date.
Figure 5 has been added to show the daily temperature departures since 2000 at SeaTac airport.
If it seems like it has been pretty dark and rainy this autumn and early winter in Seattle you would be right. I’ve already posted about the high level of precipitation we’ve seen in Seattle and the extended Western Washington area since the start of the 2021-22 water year on October 1. The areas surrounding Bellingham and Vancouver B.C. have been especially hit hard with rain.
But it has seemed very dark in recent months as well, and by dark, I mean low levels of light. Of course all this rain comes in leaden skies. And looking at solar radiation data collected near Husky Stadium and Union Bay (47.66, 122.29) in Seattle confirms this has been the darkest start to winter in the past ten. Washington State University operates a solar collection station in this area and exposes the processed data to the public on their web site. A link to this data is listed at the bottom of this page.
Figure 1 shows the cumulative daily totals of solar radiation (MJ/sq.meter) from October 1 through December 31 for 2013 through the present year. At this writing we are only partly through December this year. But it is clear that this year has been quite dark compared to the other years. Lower traces indicate lower levels of solar energy measured. This year’s trend line is the heavy line. The Seattle area began with a normal October, but around mid-month the rate of cumulative solar energy slowed considerably.
I choose a start date of October 1 since this is the traditional start date of the Northwest water year, the time of year when storms begin to blow off the Pacific with increasing frequency. This date provides a good start or “zero point” for the start of the wet season.
Figure 1. Solar radiation measured at Seattle’s Union Bay data collection site.
Figure 2 was added on December 15. This figure is a dot plot showing the total cumulative solar radiation received at WSU’s Solar Station in Seattle. The dot plot is sorted by total solar daily accumulation for the stated period.
This year’s accumulation of sunlight since the start of October is marked by the black dot in the lower left corner. It is substantially lower than the sunnier years at winter’s start. It is substantially lower than the median value for all years shown. The median value is shown by the dash vertical line.
Looking at the years’ positions on the y-axis, there does not appear to be any pattern to the order for the very limited number of years for which data exists.
Figure 2. Dot plot showing cumulative solar radiation sorted by year.
Figure 3 essentially shows the same data as Figure 2. In this bar chart, the years are in order along the X-axis (horizontal). It’s a little easier for the mind to see the lack of any pattern by time in this small sample size – we are more often used to seeing date-format data along the horizontal axis, increasing in time from left-to-right. It’s also clear to see how low sun levels have been this Oct-Dec period compared to recent years. All years show data through the date in the title.
Figure 3. Cumulative Solar Radiation, Oct 1 through December, sorted by years, 2012-21.
In addition to lower solar radiation levels in Seattle since roughly mid-October, it has been a relatively cool autumn and start to winter. September had mare days that were cooler than normal than days warmer than normal. Same for October. November was pretty mild with a series of atmospheric rivers coming in from the tropics. Those brought warmish, moist air with them. And plenty of flooding in parts of the Pacific Northwest and southern British Columbia. December has been relatively colder than most recent Decembers. This can be seen in the large number of days where average daily temperatures have been below 30Y climatic normals.
Figure 4 shows the daily temperature departures for 2021 up through the most recent date.
Figure 4. Daily Average Temperature: Departure from 30Y normals. Click to enlarge.
Figure 5 shows similar data as Figure 4, daily average temperature departures from 30-year normals going back to January 1, 2000. It appears in recent years, the trend is back towards the climatic normals of the past 30 years after a warming period for several years in the mid-2020s.
The 30Y normal daily average temperature reference values for all years except 2021 are based on the years 1981-2010. In 2021, the comparison is against the normal daily temperature range from 1991-2020. It’s possible that the 1991-2020 30Y normals cycles have increased from the previous 1981-2010 30Y cycle and this year’s lower trend is simply reflecting that change - departures may be comparing against a high reference line..
This data is for the National Weather Services’s Seattle-Tacoma International Airport site.
Figure 5. Daily normal temperature departures for SeaTac airport from 2000 to present. Click to enlarge.