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

Our ninth monthly study examines the month of June, 2011 and sunshine data from three different weather stations, one located in Sequim, one in Port Angeles, and one in 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!

Study Highlights:

  • Port Angeles was 1.23 times as bright as Seattle; Sequim was 1.16 times as bright as Seattle.

  • Port Angeles saw an average of 2.57 hours of bright sun per day, compared to 2.37 hours/day in Sequim, and 1.84 hours/day in Seattle.

  • Port Angeles recorded its 2nd month in a row with slightly more overall solar radiation than the Sequim location.

Detailed Study Findings

The Sequim site recorded 10 mostly sunny days, to Seattle's 7 mostly sunny days.

  Dreary Days Cloudy Days Partly Sunny Days Mostly Sunny Days
Sequim 0 12 8 10
Port Angeles 0 9 9 12
Seattle 0 14 9 7


On average for the month, the Sequim site had 2.37 hours of clear sunny skies per day, where as Port Angeles had 2.57 hours, and Seattle had 1.81 hours. In terms of pure solar radiation, the Sequim site for the month recorded 1.16 times as much overall solar radiation as Seattle, but only 94% of Port Angeles.

  Hours per Day Clear Sunny Total Solar Radiation (Relative Multiplier)
Sequim 2.37 1.16
Port Angeles 2.57 1.23
Seattle 1.81 1.00



Study Conclusion

  • During a spring-like, early summer month, rain shadow areas recorded significantly more light than the urban Seattle area.

  • Port Angeles exceeded Sequim in sunny days and total sunshine.

  • 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 solar 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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