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

Our tenth monthly study examines the month of July, 2011 and sunshine data from three different weather stations, one located in Sequim, one in Port Angeles, and one in 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.

Study Highlights:

  • Port Angeles was 1.13 times as bright as Seattle; Sequim was 1.11 times as bright as Seattle.

  • Port Angeles saw an average of 3.04 hours of bright sun per day, compared to 2.65 hours/day in Sequim, and 2.68 hours/day in Seattle.

  • Port Angeles recorded its 3rd month in a row of with slightly more overall solar radiation than the Sequim location, likely due to its higher elevation and lower incidence of maritime fog.

Detailed Study Findings

The Port Angeles site recorded 17 mostly sunny days, compared to Seattle's 13 mostly sunny days.

  Dreary Days Cloudy Days Partly Sunny Days Mostly Sunny Days
Sequim 0 6 13 12
Port Angeles 0 8 6 17
Seattle 0 11 7 13


On average for the month, the Sequim site had 2.65 hours of clear sunny skies per day, where as Port Angeles had 3.04 hours, and Seattle had 2.68 hours. In terms of pure solar radiation, the Sequim site for the month recorded 1.11 times as much overall solar radiation as Seattle, but only 98% of Port Angeles

  Hours per Day Clear Sunny Total Solar Radiation (Relative Multiplier)
Sequim 2.65 1.11
Port Angeles 3.04 1.13
Seattle 2.68 1.00



Study Conclusion

  • During an atypical, cloudy, summer month, rain shadow areas recorded slightly more light than the urban Seattle area.

  • Port Angeles exceeded Sequim in sunny days and in 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, especially 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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