from Summit Increasing Over Time
By Brian Fitzgerald, Director of Science & Education | June 15,
Observer Sam Robinson stands above the clouds in December 2020. Observers
record prevailing visibility at each of their 24 hourly observations, every day
of the year.
Visibility appears to be generally increasing over time on the summit
of Mount Washington since our continuous record of visibility began in 1943.
This noteworthy finding comes in the wake of an initial data
exploration and analysis of Mount Washington Observatory’s (MWO) long-term
visibility records, completed by Weather Observers Jay Broccolo and Sam
Robinson in Spring 2021.
The project report, entitled “A Data Exploration of Visibility at Mount
Washington Observatory (1943-2020), KWMN: Key Findings” was recently published to help the public understand the background,
motivation, relevance, findings and suggestions for further study.
The exploration grew out of public and staff interest initially related
to the COVID-19 pandemic and its effect on the environment. Back in spring
2020, our staff fielded a number of inquiries asking if the weather observers
were able to see farther than usual due to the global reductions in industry,
transportation and overall aerosol emissions. It was a tricky question to
answer, because in order to respond with any confidence, one would have to know
what “usual” visibility is, and would certainly require a lot of visibility
data over time to compare against.
For those wondering how visibility is measured in the first place,
understand that MWO’s weather observers record something known as “prevailing
visibility” at every single hourly observation, both day and night, by
determining which known landmarks and distances that they can see on the horizon.
As the highest point in the Northeast with a treeless summit, Mount
Washington’s position allows for an unobstructed view in all directions
(provided you’re not in a cloud). The naked-eye view from the summit can reach
up to 130 miles to Mt. Marcy in NY, for example, 79 miles to Camel’s Hump in
VT, or 67 miles to the Atlantic Ocean off the Maine coast.
At night, observers use many of the same natural features plus the
lights of known locations to assess the visibility. For context, most weather
stations that report visibility are at airports with automated instrumentation
to help pilots and traffic controllers assess take-off and landing conditions.
Visibility at these stations will only report up to 10 miles as the highest
value of interest.
map of concentric circles around the summit with visibility markers indicated.
The inner-most circle is 20 miles, while the outer-most circle is 130 miles
For nearly a year, Broccolo and Robinson dug into paper and digital
visibility data, gathering any related information including measurement
standards and observer training materials.
Two pieces of critical information were discovered early on: 1)
24-hourly visibility observations began continuously in 1943, with reporting
being more intermittent previously, and 2) prevailing visibility records before
2008 exist only on paper, and digitally from 2008 to present.
Without taking on the hugely exhaustive process of manually entering
data from paper forms into a digital database, Broccolo and Robinson were able
to use a nearly identical visibility variable: lowest prevailing visibility.
This value had been digitized along with all values on the daily B-16 Form
during a recent project by observers and interns.
Lowest prevailing visibility is what it sounds like. It essentially
means the lowest prevailing visibility reported in the last two observations.
So if two hours ago the visibility was reported at 1/16 of a mile, and this current hour the visibility improved to 50
miles, the lowest prevailing visibility for this hour would be 1/16 of a mile.
Through a direct comparison of lowest visibility versus prevailing visibility
from 2008 to present, it was found that both of these variables tracked very
closely, making lowest visibility an acceptable proxy.
With data in hand, Broccolo and Robinson examined seasonal and annual
average lowest prevailing visibility to explore what long-term trends or
anomalies may have been present. The initial analysis appears to show a steady
increase in visibility first appearing in the 1950s and then increasing at a
higher rate starting in the early 2000s. Public interest in whether average
visibility was found to be increasing due to the COVID-19 pandemic in 2020 was
not easily discernible.
averages of lowest visibility (miles) at KMWN (December, 1942 through November,
On average, meteorological summer (June, July, August, or “JJA” as
noted on the figure 2 above) showed the lowest visibility of all of the
seasons. Higher humidity levels and a more active southerly flow of air from
major areas of industry may be reasons for this difference, though more
analysis is needed to assess any correlation.
In future investigations, a number of different questions may be
explored, including what if any difference exists between day and nighttime
visibility observations, or how these visibility data compare against other
atmospheric measurements such as particular matter concentrations or wind speed
“The next steps would be to analyze the data deeper and corroborate the
findings with other measurements,” Broccolo said. “Further statistical analysis
is required to show the quality of data. Measurements of air quality affecting
particulates at a similar geographic location and elevations or the development
of a measuring system at the summit of Mt. Washington would also be of interest
in order to compare visibility and air quality.”
Regardless of which investigations come next, the value of MWO’s
visibility dataset is immense. No other record of similar location, length,
resolution or quality truly exists, which makes it a tremendous asset for
studying the relationships between visibility and air quality over time in
Northern New England.
Whether examined internally or through partnership, the possibilities
for future work are clear to see, so to speak. Read
the full project summary report.