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Sunshine Analysis – Olympic Rain Shadow vs. Downtown Seattle
Our 12th monthly study examines the month of
September, 2011 and sunshine data from three different weather stations, one located
in Sequim, one in Port Angeles, and one in 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
On balance though, September was a reasonably
sunny month in all locations, with substantially more sunny days that either May
Sequim was 1.14
times as bright as Seattle; Port Angeles was 1.13 times as bright as Seattle.
Sequim saw an average of
2.94 hours of bright sun per day, compared to 2.90 hours/day in
Port Angeles, and 2.54 hours/day in Seattle.
Sequim exceeded Port
Angeles for the first month since April with slightly more overall solar radiation than the
Port Angeles location.
The Port Angeles site recorded
17 mostly sunny days, to Seattle's 14 mostly sunny days.
Partly Sunny Days
Mostly Sunny Days
On average for the month, the
Sequim site had 2.94 hours of clear sunny skies per day, where as Port Angeles
had 2.90 hours, and Seattle had 2.54 hours. In terms of pure solar radiation,
the Sequim site for the month recorded 1.14 times as much overall solar radiation as Seattle
and 1.01 times Port Angeles
Hours per Day Clear Sunny
Total Solar Radiation (Relative Multiplier)
During a fall transition month, weather
overall was good, but rain
shadow areas started to see more light than the urban Seattle area.
The second half of the month was much cloudier
than the first half, and the rainshadow made its presence felt.
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
Full month solar radiation chart
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
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”.
aggregated, analyzed, and graphed using Microsoft Excel.