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Annual Days of Sunshine Report: Downtown Seattle vs. Olympic Rain Shadow, Oct
report is a comparison and analysis of the sunshine in
downtown Seattle, Washington vs. areas, including Sequim, located in the Olympic rain
shadow. The number of sunny days per year in Seattle is reasonably well
understood and documented, but the corresponding data for Sequim, or for that
matter, any location in the rain shadow, had not been measured to our knowledge
prior to this study. We used recorded solar radiation to estimate sky conditions
using a sophisticated algorithm we put together and refined during the yearly
- Winter (Nov-Jan) saw 5X as
many mostly sunny days in the shadow vs. Seattle.
- Winter saw only 1/4 as
many dreary days in the shadow vs. Seattle.
- Spring (Feb-May) saw the
highest number of rain shadow days per month, at nearly 8!
- Summer (Jul-Sep) saw rain
shadow areas and Seattle with nearly equal mostly sunny days.
- Port Angeles was
definitely *in* the rain shadow, with quite similar benefits to Sequim.
Our data shows that the climate in the rain shadow was clearly brighter and
sunnier than downtown Seattle, with 127 mostly sunny days vs. only 88 in Seattle, fewer cloudy days, and dramatically
fewer dreary days.
(note, a "rainshadow" day is
a cloudy or dreary day in Seattle when is it partly or mostly sunny in Sequim or
However, it was on a seasonal basis where the more dramatic variations arose.
During the darkest and stormiest winter months of November through January,
Sequim recorded 51 of 92 days at least partly sunny, with 20 mostly sunny days,
and only 4 dreary days. During this same period in downtown Seattle, only 4 of
92 days were mostly sunny, with 19 dreary days. It is important to note that
during this period, there were stretches of days where there was "not place to
run, no place to hide". For example, January in Sequim was particularly bad,
with only4 mostly sunny days, and 20 out of 31 days cloudy or dreary.
But right on the heals of January, the rain shadow really kicked into gear
during the early spring months of February through April, In Sequim, 63 of 89 days were at least partly sunny. March in particular was a very
stormy and dark month in Seattle, with 18 of 31 days cloudy, while Sequim
recorded only 8 such days.
In the typically bright “summer” months of July, August, and September, the rain
shadow effect was at its minimum. Total mostly sunny days were close to
equivalent in Sequim and Seattle, at 43 and 44 respectively. Still, even in the
dry months, there were some stormy periods, especially in July and September,
and the rain shadow produced 13 rain shadow days during this period.
While this report focuses on Sequim vs. Seattle, thanks to Lincoln High School,
we also analyzed Port Angeles weather at the same level of detail as Sequim and
Seattle. On an annual basis, Port Angeles recorded virtually the same number of
mostly sunny days as Sequim and nearly identical the overall solar radiation; so
Port Angeles is most definitely “in the shadow”. However there were
some seasonal differences between the two locations. For more detail, see our
Port Angeles Sunshine.
Olympic rain shadow has a dramatic impact in the amount of incident light and
number of sunny days recorded in locations like Sequim when compared to downtown
Seattle. The weather differences are most pronounced during
the winter and spring seasons. Port Angeles also benefits strongly from the rain
shadow, as do likely other areas such as Victoria, the southern San Juan
Islands, and western Whidbey Island. While this study focused on downtown
Seattle, it is highly likely, if not certain, that many suburban areas of
Seattle, especially those to the east, are even more dramatically less
sunny than Olympic rain shadow areas during the winter and early spring.
We also have archived
monthly rain shadow climate
summaries and individual monthly reports with daily sunshine charts
and radiation plots.
Methodologies, Weather Stations, etc.
The studies and live sunshine conditions on this
website are based on examining incident solar radiation. Solar radiation is measured in watts per square meter
Solar radiation is directly related to illuminance, a measure of how much light
falls on a given area.
Live sunshine conditions are calculated as
Sunshine” sensors record at least 60% of the maximum radiation possible
for that day of that year
“Partial Sunshine” 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 are categorized as follows:
Sunny" over ½ the day had at least “partial
sunshine”, with at
least 22% of daylight hours “bright sunshine”.
"Partly Sunny" over ½ the day had at least “partial
sunshine”, but less
than 22% of daylight hours “bright sunshine”.
"Overcast" over ½ the day had gray or dark gray skies, but at least
22% of daylight hours “gray”.
"Dreary" daylight hours predominately dark gray, with less then
22% of daylight hours “gray”.
The website uses data from four 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
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.
The fourth station is in downtown Port Townsend at
a private residence. The Davis Instruments Vantage Pro 2 Plus with option solar
sensor is located on the roof of the home and has clear exposure to all
directions. It elevation is roughly 100 feet above sea level, and it is about
1/4 mile from the shoreline.
For the year long study, we made a number of
improvements to our model which estimates sky conditions from solar radiation.
First, we and others noticed that the UW equipment in
Seattle was uniformly recording lower radiation than the two other stations,
even on completely clear, sunny days. So we normalized the radiation on the Y
axis such that that all stations were recording nearly identical radiation on
clear sunny days. This reduced, but in no way eliminated, the advantage rain
shadow locations had over Seattle.
Next, we made our seasonality model much more
sophisticated. Originally, we used radiation and "bright sunny hours" thresholds
computed on a monthly basis to estimate sunny days. The problem is those
thresholds are very different at the beginning of a month like February than at the
end. So we moved to a daily threshold for these values, which is much more
accurate. This change made the model more accurate, but did not change overall
comparative results dramatically.
Finally we compared our model's results for
Seattle to the National Weather Service evaluations of average daily sky
conditions to check for consistency. The weather service categorizes days as on
average as, "Cloudy", "Partly Cloudy", or "Clear", based on
observations. We found that our totals for cloudy, and the sum of partly and
mostly sunny days, lined up quite well with their observations; however, our category of "mostly sunny"
is a little broader than their category of "Clear". Overall though, the
consistency was encouraging. Of course the weather service only does sky
condition summaries for a small number of cities, none in locations such as
Sequim, Port Townsend, or Anacortes, so
that is why we went with the radiation approach.
Study data was aggregated, analyzed, and graphed using