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Anacortes in the Olympic Rain Shadow?
That is an interesting question, and one whose answer certainly has a major
impact on the weather in Anacortes. To answer this we need to look at
how much rainfall and sunshine Anacortes receives and compare that to locations
we know are in the rain shadow and locations we know are not. It is also
important to look at seasonality, as the rain shadow typically forms during the
winter and spring months.
In terms of precipitation1, averaged over a 50 year period, Anacortes
receives 26.28” of precipitation, compared to 35.86” in Seattle, and 25.57” in
Port Angeles1. From this data alone, one might conclude that
Anacortes likely has a climate similar to Port Angeles, which we have seen is
definitely in the rain shadow from a sunny day perspective. But rainfall really
doesn’t tell the whole story. For example, there is a possibility that a place
with more rainfall may also have more sun, if the storms are more intense and
short lived, etc. In looking at precipitation by month, it is interesting that
Port Angeles receives significantly more precipitation than Anacortes in the
darker stormy months of November – February.
So what about sunshine? Luckily Vince Streano, of
anacortesweather.com has been
recording the daily sunshine in the Anacortes area since 1993. At first he did
his observations entirely manually, but beginning in 2005 he starting recording
daily solar radiation data. Vince graciously provided me with his radiation data
for Anacortes for the same 12 month period (October 2010-September 2011) we’ve
studied Seattle, Sequim, and Port Angeles; so we were able to run an
apples-to-apples sunshine comparison over a 12 month period2. Note,
to estimate whether a day was Sunny, Cloudy, etc we used a complex model that
examines solar radiation recorded and averaged every 30 minutes, categorizing
those samples as bright sunshine, partial sunshine, etc. The model then looks at
the entire day, and categorizes it mostly sunny, partly sunny, etc based on the
hours-per-day of the different samples.
Our results showed that Anacortes is significantly sunnier than
Seattle but not as bright as either Sequim, or surprisingly Port Angeles; this
confirmed our suspicion that Anacortes is located on the north eastern fringe of the
Olympic Rain Shadow. Here are the data and analysis that led to our conclusion.
Annual Totals: October 2010-September 2011
The table above shows that
during the study period, Anacortes exceeded Seattle in number of bright sunny
days by 26 % (111 vs. 88), and had 16 % fewer cloudy or dreary days (138 vs.
160). In terms of total solar insolation, Anacortes exceeded Seattle by 4%.
While Anacortes was quite a bit brighter than Seattle it clearly was not as
bright as the Sequim or Port Angeles location. For a better understanding of the
situation, let’s look at some seasonal totals.
During the spring months, the rain shadow effect
was quite dramatic in Anacortes. In terms of the overall sunny day profile,
Anacortes was quite close to Port Angeles, not that far off Sequim, and
outdistanced Seattle with 38% more bright sunny days and 22% fewer cloudy or
During the winter months, the rain shadow effect in was not as dramatic, but
still Anacortes recorded more than 2X the bright sunny days of Seattle (10 vs
4), and fewer cloudy days (31 vs. 34). However Sequim had 2X the bright sunny
days of Anacortes, and dramatically fewer really dark and dreary days. With
respect to Port Angeles, it is interesting that although that city typically
gets quite a bit more precipitation than Anacortes in the winter months, and did
get more precipitation during the “study” period (16.4” vs. 14.53”), it also
recorded more sunshine, showing rainfall does not always predict sunshine.
What is most striking about our results is their month-to-month consistency. In
eight stormy months (Oct-May), in every month, the ranking in terms of total
solar insolation is 1st Sequim, 2nd Port Angeles, 3rd
Anacortes, and last Seattle. During this same 8 month period, Sequim ranks
1st for number of mostly sunny days in 7 of the 8 months, while
Anacortes ranks 3rd in 6 of the 8 months, and Seattle is last for 7
of the 8 months.
The consistency of the results from month-to-month, along with the evidence that
during stormy winter and spring months, Anacortes consistently had more sunshine
than Seattle, but less than Sequim or Port Angeles, confirms our suspicion that
Anacortes lies on the north eastern fringe of the Olympic Rain Shadow.
Rain Shadow Annual Days of Sunshine Report to view
a summary of Sequim vs. Seattle sunshine, with more detailed analysis, links to
monthly reports, and study methodologies.
1 Precipitation data from the Western
Regional Climate Center,
2 This study is only over the one year
period where radiation data is available for the sites looked at. In November 2010, some Anacortes measurements were missing from the log, and we
relied on manual measurements and some estimating for those days. We are not
overly concerned whether 2011 was an especially wet year, since we are most
interested in the relative sunshine, not the absolute sunshine, In fact in some
regards, the wetter the year, the better the year for studying the rain shadow,
since it only forms during stormy periods. Still, though as more data becomes
available, we will continue to track things.