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September 2011 Sunshine Analysis – Olympic Rain Shadow vs. Downtown Seattle

September in the Pacific Northwest is typically a transition month from summer to fall weather. This September was a dramatic example of how rapidly that transition can take place. The first eleven days of the month were hot and sunny, with all stations recording more than 10 sunny days, and near equal overall solar radiation.

Then, on the 12th, fall arrived, as if overnight, with the onset of cooler temperatures, some rain, wind, and fewer sunny days. This allowed the rain shadow to start to kick into gear. For example during the stormy period from the 12th to the 17th, Sequim recorded 5 at least partly sunny days, while Seattle only recorded 2. During the last 19 days of the month, Sequim recorded 17% more sunlight than Seattle, while it was near equal for for the first 11 days. For the first time since April, Sequim's overall solar radiation exceeded Port Angeles, marking a reversal in a summer trend we had observed.

On balance though, September was a reasonably sunny month in all locations, with substantially more sunny days that either May or June.

We've now completed a year with this study. We have taken this opportunity to use a much more accurate algorithm to calculate sunny days which a) normalizes radiation between different sites with different equipment, and b) looks at radiation thresholds on daily basis rather than a monthly basis.  We've scrubbed the monthly data, and re-posted all monthly reports. Stay tuned for a exciting annual summary.

August 2011 Sunshine Analysis – Olympic Rain Shadow vs. Downtown Seattle

August in the pacific northwest is typically high summer with plentiful sunshine, little precipitation, warm temperatures, and frequent marine layer influence. Sometimes in late August we may see an early fall storm pass through.

This August was definitely high summer with only two hundredth of an inch of rain falling on the beach is Sequim. This was a *very* welcome change from the abnormally stormy and cool months that characterized 2011. It also served as a great month to see if the North Olympic topography has any benefit in terms of sunshine vis-a-vis Seattle in high summer months. 

Data showed that all locations recorded dramatically more bright sunny days than any month this year, with Port Angeles leading the way at 21. Seattle had 17 and Sequim had 16 bright sunny days; however, Seattle had 3 cloudy days, while Sequim and Port Angeles had only 1.

The data confirmed our suspicion that in a sunny summer month, the rainshadow effect would be at its seasonal minimum.

Next month will mark a complete year for this study. We will be taking that opportunity to summarize the year's findings. During the year we have been continually improving our algorithm which estimates sunshine from solar radiation. In particular we have been looking at seasonality, equipment uncertainty, and match to empirical observations. Next month we will incorporate improvements to the algorithm for the whole years data set.

We are also approaching 100 fans on facebook, which is a big milestone for a small website, so if you appreciate this site, help us hit that milestone by "liking us" and/or sharing our findings with your sun-loving friends, rain shadow residents or admirers.



July 2011 Sunshine Analysis – Olympic Rain Shadow vs. Downtown Seattle

July in the pacific northwest is typically high summer with plentiful sunshine, little precipitation, warm temperatures, and frequent marine layer influence. Sometimes early July may see a weak weather system, with clouds, and cool temperatures, but it is very unusual to see significant weather systems during the 2nd half. During July, one would expect the impact of the rain shadow to be at its minimum.

This July started out as high summer, with a string of sunny days at all locations, but then became atypically wet and cloudy during the middle of the month, resulting in a quite a cloudy and cool July overall. In this regard, the rain shadow did have some impact, as Sequim saw only 6 cloudy days, vs. 11 in Seattle.

During the middle of the month, the weather reverted to abnormal spring-like weather, with weak low pressure systems, some rain, coupled with some marine fog. During the 13 day period from the 7th to the 19th, Seattle saw seven cloudy days, and only one sunny day. The rain shadow provided some relief during this period, with Port Angeles recording 1.25x the sunshine of Seattle, and four sunny days.

June 2011 Sunshine Analysis – Olympic Rain Shadow vs. Downtown Seattle

June in the pacific northwest is typically an early summer month with significant sunshine, warm temperatures, building marine layer influence, and several days of rain.  During this June,  which was exceptionally cold and stormy, Port Angeles recorded 12 mostly sunny days, while Sequim recorded 10, and Seattle only 7. We are starting to see the equalizing effect of summer, with the difference in sunny days between rain shadow areas and urban Seattle lower than in more active winter and spring months. For example, in December, Sequim recorded 9x the sunny days of Seattle, where as in June, that ratio was only 1.4. Also, this June, Port Angeles continued to lead Sequim slightly in terms of overall sunshine, likely due to marine layer effects.

After nine months of measurements, using our methodology, Sequim has now recorded nearly double the bright sunny days of Seattle, 84 vs. 44, and has recorded 1.48 times the average hours per day of sunshine!

 May 2011 Sunshine Analysis – Olympic Rain Shadow vs. Downtown Seattle

May in the pacific northwest is typically transition month between cooler spring months with active storms coming through and a calmer weather pattern with more marine layer influence.  During this May,  which was exceptionally cold and stormy, both Sequim and Port Angeles recorded 14 mostly sunny days, the highest number of sunny days since October; Seattle only recorded 9 sunny days. We also started to see the influence of maritime fog. There were quite a few cloudy days in Sequim which may have been the result of of fog. In fact in May, Sequim recorded less overall solar radiation than Port Angeles, likely due to the sensor's location on the shoreline. Close examination of daily radiation shows several days where Port Angeles was sunny in the AM and Sequim was not.   

April 2011 Sunshine Analysis – Olympic Rain Shadow vs. Downtown Seattle

April in the pacific northwest is typically a solid spring month with increasing numbers of sunny warm days, interspersed with occasional cool stormy periods. This April was exceptionally cool and stormy, in fact Seattle recorded the coldest average high temperature every recorded for a month of April. This month Port Angeles lead the way in sunny days, with 13 mostly sunny days which compared very favorably to Seattle's 4 sunny days. Sequim was very close with 12 sunny days.  


March 2011 Sunshine Analysis – Olympic Rain Shadow vs. Downtown Seattle

March in the pacific northwest is typically a spring month with rapidly increasing solar radiation, but frequent stormy periods. 

This March was exceptionally cool and stormy. Following a very cold end of February, the weather moderated, and starting on Monday March 7th, we saw a parade of very fast moving cool storms march across the Pacific toward the Northwest, producing abundant snow, rain, and a pronounced rain shadow. This pattern lasted until the 15th, after which we experienced a period of dryer weather with weaker more seasonal storms. The last three days of the month included a rare spring pineapple express, which obliterated the rain shadow. 

Despite the very stormy weather pattern, rain shadow locations saw up to 23 partly or mostly sunny days where as Seattle saw only 13 such days. In fact, Seattle had a quite dark and wet month, with a decrease in sunny days vis--vis February, despite the longer month, and generally improving weather.



February 2011 Sunshine Analysis – Olympic Rain Shadow vs. Downtown Seattle

February is a month of rapidly increasing solar radiation; it is also typically a transition month from more winter-like weather to more spring like weather. This February started out in the same pattern as January 2011, which was characterized by weak, warm weather systems, and generally cloudy/dreary weather. Typical to Februaries, the sun broke through on the 8th, and next 14 days included 7 bright sunny days and 5 partly sunny days at the Sequim location. The high pressure opened the gates to an arctic air mass during the end of the month, with record cold temperatures and accumulating snow in Sequim and Port Angeles.

The unusual weather pattern still produced a much brighter month than January, with 20 mostly or partly sunny days in Sequim, vs. only 11 in January. Port Angeles saw 16 days of at least partial sunshine, while Seattle only experienced 13. Sequim outpaced both Seattle and Port Angeles dramatically this month with 10 clear days, vs. only 4 in Port Angeles and 7 in Seattle.


January 2011 Sunshine Analysis – Olympic Rain Shadow vs. Downtown Seattle

January saw a uniquely dreary combination of cloudy dry days and warm, wet storms.  Most precipitation fell as rain even at high elevation, with a dramatic lack of new snow accumulation in the mountains. With a persistent ridge of high pressure diverting the passage of strong storms to the north and south of our area, the rain shadow engine was a bit short of fuel. Still rain shadow towns were brighter than urban Seattle, which was very dark, with 7 dreary days and 16  cloudy days.

Port Angeles and Sequim were quite similar in terms of overall sunshine and sunny days this month, with Sequim recording 24% more and Port Angeles 16% more sunshine than downtown Seattle.

What was especially interesting about this month, was the impact the weak weather pattern had on Sequim's overall sunlight. Sequim's overall sunlight was lower than that recorded in either November or December, and was only 89% of the month of December, during a period of increasing incident solar radiation.

December 2010 Sunshine Analysis – Olympic Rain Shadow vs. Downtown Seattle


December is a typically cold, wet, and stormy month. This December saw a very active, stormy weather pattern, with a major Pineapple Express event on the 11th and 12th, with additional stormy periods almost every week during the month. Measured weather was wetter and warmer than normal in greater Seattle, yet the Olympics created quite a shadow! Overall sunny days in Sequim were actually up from 7  in November to 9 in December, while Seattle only recorded 1 sunny day. Overall, Seattle was remarkably dark in December, and Sequim saw 1.36 times the recorded sunlight as Seattle.

Once again, Port Angeles also recorded brighter overall weather than Seattle, but was outpaced dramatically by Sequim.

November 2010 Sunshine Analysis – Olympic Rain Shadow vs. Downtown Seattle

As is common in the Pacific Northwest, November was wet, cold, and stormy. Overall sunny days in Sequim were down from 11 in October to 7 in November, and total recorded sunlight in Sequim was only 50% of the month of October. Seattle fared even worse, recording only 41% of the sunlight recorded in October.

October 2010 Sunshine Analysis – Olympic Rain Shadow vs. Greater Seattle


October 2010 in northwest Washington State was not atypical. It had stretches of clear days during the early and middle of the month, with two pronounced stormy periods, one from the 7th to the 10th, and another from roughly the 22nd onward. Overall this was a good month to observe the rain shadow due to the diversity of weather conditions. During the month, and especially during the stormy periods, the rain shadow effect did have a clear impact on the overall sunshine in the rain shadow area, measured by number of sunny days, lack of dreary days, average hours of sun per day, and total solar radiation.

Study Methodologies

These studies are conducted by examining incident solar radiation. Solar radiation is measured in watts per square meter. This measurement is directly related to illuminance, a measure of how much light falls on a given area.

We used data from three different weather stations.

The first station, our Sequim station is located on the roof of a single story home on Jamestown Beach Rd, in Sequim, directly on the shores of the Strait of Juan de Fuca. This location may be very close to the epicenter of the Olympic Rain Shadow. As with many locations on the Dungeness plain in Sequim, this one is not shaded by tall trees, nor does it have any hills affecting its exposure. When the sun rises in the morning, it clears the horizon almost immediately as the areas to the south and east are open water. When the sun sets in the evening, it sets to the west, over the Dungeness plain, so stays above the horizon for quite some time. This station measures solar radiation every 2 minutes and records the average over hour.

The second station is atop the Atmospheric Sciences Department building of the University of Washington, in the University District of Seattle. This seven story building, is not in the classical Olympic Rain Shadow area, but is still slightly shadowed by the Olympics. Given that the sensor is located atop a tall building, this location has ideal exposure and receives maximum solar radiation. This station records solar radiation every minute.

The third station is on a 10 foot pole on a piece of property about 3/4 of a mile West of Lake Sammamish in Redmond, close to the border of Bellevue. The site is typical of many home sites in the area, with hilly terrain to the south and west, and with some trees on the property; this sensor does though have clear exposure to the south. The Redmond weather station records radiation every 10 minutes.

The forth station is in downtown Port Angeles, at Lincoln High School. The school's Davis Instruments Vantage Pro2 Plus sensors are mounted on the rooftop of the high school,  about a mile southwest of ferry dock to Victoria B.C. at an elevation about 200 feet. There is excellent exposure clockwise from northeast to southwest and good exposure for the other directions. The sensors record solar radiation every five minutes. Special thanks to Peter Alexander, his math classes, and Lincoln High School for the data and support.

Data was collected in Sequim using a Davis Instruments Vantage Pro 2 with optional solar sensor, in Redmond using a Better Generation PowerPredictor, and from Seattle via downloading public information from the University of Washington’s Department of Atmospheric Sciences website.

Additional detail on the definition and methodologies we use for calculating sunny days, bright sunshine, etc. are located in the individual monthly reports. While this is not a rigorous scientific study, and the sites for the weather stations are not identical in terms of exposure, we feel the results of this study are still valid and very interesting.


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