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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.
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.
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.
Partly Sunny Days
Mostly Sunny Days
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)
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.
a relatively sunny fall month, Sequim was slightly brighter and sunnier
stormy periods with fast moving storms may very well be dry and with quite a
bit of sun in the Olympic Rain Shadow towns.
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 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.