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

Our first study examines the month of October, 2010 and sunshine data from three different weather stations, one located in Seattle, one in Port Angeles, and one in Sequim. The month of 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  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 cloudy days, average hours of sun per day, and total solar radiation.

Study Highlights:

  • Sequim averaged 1.76 hours of bright sunshine per day, and had no “dreary” days

  • Sequim recorded 26 days either partly or mostly sunny days, compared to Seattle’s 23.

  • During a five day stormy period, Sequim recorded over 1.3 times the solar radiation of Seattle, with only one cloudy day compared to three.

Detailed Study Findings

The Sequim site recorded 11 mostly sunny days, to Seattle's 8 mostly sunny days. The Seattle site had more cloudy days, recording eight overcast days to Sequim's five. 

  Dreary Days Cloudy Days Partly Sunny Days Mostly Sunny Days
Sequim 0 5 15 11
Port Angeles 0 11 8 12
Seattle 0 8 15 8


On average for the month, the Sequim site had 1.76 hours of clear sunny skies per day, where as Seattle had 1.47 hours. In terms of pure solar radiation, the Sequim site for the month recorded 1.04 times as much overall solar radiation as Seattle, while Port Angeles recorded 1.03 times Seattle.

  Hours per Day Clear Sunny Total Solar Radiation (Relative Multiplier)
Sequim 1.74 1.04
Port Angeles 1.68 1.03
Seattle 1.39 1.0


Finally, let’s look at the stormy period of the 25th through the 30th. During this period the Sequim site recorded 1.31 times the solar radiation of Seattle.  Below is a graph which shows the recorded solar radiation in the three locations during this period.

Study Conclusion

  • In a relatively sunny fall month, Sequim was slightly brighter and sunnier than Seattle. 

  • Autumn stormy periods with fast moving storms may very well be dry and with quite a bit of sun in the Olympic Rain Shadow towns.

  • If you are feeling you are living in a dreary neighborhood in the eastern Puget Sound or Cascade foothills, take a road trip to the Olympic Rain Shadow, and you will likely see *a lot* more light during the darker months and stormy periods.


Full month radiation chart

Study Methodology

This study was 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, the 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. The Sequim location uses a Davis Instruments Vantage Pro 2 with optional solar sensor. This station measures solar radiation every 2 minutes and records the average over 10 minutes.

The second 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 of 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..

The third 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. Data was retrieved by downloading public information from the University of Washington’s Department of Atmospheric Sciences website.

For purposes of this study, skies were defined as follows:

“Clear Sunny” sensors record at least 60% of the maximum radiation possible for that day of that year
“Bright” sensors record between 20% and 60% of the maximum radiation for that day of that year
“Gray” sensors record between 50 w/m^2 and 20% of the maximum radiation for that day of that year
“Dark Gray” sensors record between 1 and 49 w/m^2

Days were categorized as follows:

"Mostly Sunny Day" over ½ the day had at least “bright” skies, with at least 22% of daylight hours “clear sunny”.
"Partly Sunny Day" over ½ the day had at least “bright” skies, but less than 22% of daylight hours “clear sunny”.
"Overcast Day" over ½ the day had gray or dark gray skies, but at least 22% of daylight hours “gray”.
"Dreary Day" daylight hours predominately dark gray, with less then 22% of daylight hours “gray”.

Data was aggregated, analyzed, and graphed using Microsoft Excel.


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