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Is 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 dreary days.

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.

See our 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, http://www.wrcc.dri.edu/

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.

 



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