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Sunshine Analysis – Olympic Rain Shadow vs. Downtown Seattle
Our sixth monthly study examines the month of
March, 2011 and sunshine data from three different weather stations, one
located in Sequim, one in Port Angeles, and one in Seattle. March in the pacific
northwest is typically a spring month
with rapidly increasing solar radiation, but frequent stormy periods.
This March was exceptionally cool and stormy.
Following a very cold end of February, the weather moderated, and starting on
Monday March 7th, we saw a parade of very fast moving cool storms march across
the Pacific toward the Northwest, producing abundant snow, rain, and a
pronounced rain shadow. This pattern lasted until the 15th, after which we
experienced a period of dryer weather with weaker more seasonal storms. The last
three days of the month included a rare spring pineapple express, which
obliterated the rain shadow.
Despite the very stormy
weather pattern, rain shadow locations saw up to 23 partly or mostly sunny days
where as Seattle saw only 13 such days. In fact, Seattle had a quite dark and
wet month, with a decrease in sunny days vis-à-vis February, despite the longer
month, and generally improving weather.
Sequim was 1.26 times as
bright as Seattle; Port Angeles was 1.06 times as bright as Seattle.
Sequim saw an average of
1.79 hours of bright sun per day, compared to .92 hours/day in Port Angeles,
and .82 hours/day in Seattle.
During the nine day
period from 7th to the 15th of March, Seattle had seven cloudy, rainy days,
and 3.42 " of rain.
During this same
period, Sequim recorded 8 of 9 days at least partly sunny, with only 1/8th
the rainfall of Seattle, and a total of 1.5 times the sunlight! Details
radiation and rainfall during stormy period from March 7-15th
The Sequim site recorded 7
mostly sunny days, to Seattle's 5 mostly sunny days. Seattle had 17 cloudy days,
compared to 8 in Sequim, and 12 in Port Angeles.
Partly Sunny Days
Mostly Sunny Days
On average for the month, the
Sequim site had 1.70 hours of clear sunny skies per day, whereas Port Angeles
had .87 hours, and Seattle had .99 hours. In terms of pure solar radiation, the Sequim site for the
month recorded 1.26 times as much overall solar radiation as Seattle, and 1.19 times as much as the Port Angeles site.
Hours per Day Clear Sunny
Total Solar Radiation (Relative Multiplier)
In a very active spring month, rain
shadow areas recorded significantly more light than the urban Seattle area.
During the most active stormy period, rain
shadow areas clearly differentiated themselves from the Seattle area
regarding sunlight and rainfall.
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.