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April 2011 Sunshine Analysis – Olympic Rain
Shadow vs. Downtown Seattle
Our seventh monthly study examines the month of
April, 2011 and sunshine data from three different weather stations, one located
in Sequim, one in Port Angeles, and one in Seattle. April in the pacific
northwest is typically a solid spring month with increasing numbers of sunny
warm days, interspersed with occasional cool stormy periods. This April was
exceptionally cool and stormy, in fact Seattle recorded the coldest average high
temperature every recorded for a month of April. This month Port Angeles lead
the way in sunny days, with 13 mostly sunny days which compared very favorably
to Seattle's 4 sunny days. Sequim was very close with 12 sunny days.
Sequim was 1.20 times as
bright as Seattle; Port Angeles was 1.19 times as bright as Seattle.
Sequim saw an average of
2.34 hours of bright sun per day, compared to 2.37 hours/day in Port
Angeles, and 1.56 hours/day in Seattle.
While this was a cold and
stormy spring month, Seattle's weather improved vis-a-vis March, and the
differences in solar radiation started to narrow.
While this is not a rigorous
scientific study, and the sites for the weather stations are not identical in
terms of exposure, we feel the results of this study are still valid and very
interesting. For more information about the weather stations and the methodology
of the study, see the appendix located at the end of this report.
Detailed Study Findings
The Sequim site recorded 9
mostly sunny days, to Seattle's 3 mostly sunny days. Seattle had 5 dreary days,
compared to 4 in Sequim, and only 1 in Port Angeles.
Partly Sunny Days
Mostly Sunny Days
On average for the month, the
Sequim site had 2.34 hours of clear sunny skies per day, where as Port Angeles
had 2.37 hours, and Seattle had 1.56 hours. In terms of pure solar radiation,
the Sequim site for the month recorded 1.20 times as much overall solar
radiation as Seattle, but virtually the same as Port Angeles.
Hours per Day Clear Sunny
Total Solar Radiation (Relative Multiplier)
During and exceptionally cold and stormy
spring month, rain shadow areas recorded significantly more light than
the urban Seattle area.
Port Angeles for the first time was very close
to Sequim in terms of total sunlight, and tied Sequim in terms of sunny
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
during the darker months and stormy periods.
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.