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Wednesday, December 20, 2017 The Nugget Newspaper, Sisters, Oregon
Icy fog is an occasional visitor
By Ron Thorkildson
Correspondent
It’s a fact of nature that
the ability of air to contain,
or “hold,” water in the gas-
eous state is dependent upon
temperature. When the atmo-
sphere is cooled, that capacity
is reduced. Air is said to be
saturated when it can’t “carry”
any more water vapor. This is
the process by which clouds
are born.
Normally, clouds form at
some height above the ground
— but not always. Clouds that
like to hang out on the ground
we call fog. And this form of
condensation imposes a whole
set of challenges to safe trans-
portation because of reduced
visibility.
Fog is a frequent visitor
to the coasts of Oregon and
Washington, not only dur-
ing the winter but sometimes
the summer months, too.
Persistent northerly winds
during the summer tend to
promote strong upwelling just
off the coast. When warm,
moist air moves over this
colder water, surface fog can
often result. Then, if interior
regions of the two states heats
up, the fog and low clouds are
sucked onto the coastal areas
that sometimes can penetrate
inland for several miles.
Mid-to-late autumn is the
ideal season for the forma-
tion of fog in the western val-
leys. Before the progression
of winter storms normally
kicks in, high pressure tends
to settle in over the Pacific
Northwest, producing sunny
days but cool, clear and longer
nights. As the ground radiates
heat into space, it chills the air
above it. If the cooling is suf-
ficient, fog will form before
sunrise the next morning.
Potentially the most
impactful fog is the subfreez-
ing variety that sometimes fills
the Columbia Basin of eastern
Washington and north-central
Oregon during late fall/early
winter. It takes a special set
of atmospheric conditions to
produce it, and it can be hard
to get rid of once it forms;
hanging around for up to two
weeks in some cases.
The primary ingredient in
concocting this foggy brew is
the development of a strong
ridge aloft along the western
U.S. that serves two purposes.
First, it causes the jet stream
that carries storms across the
Pacific Ocean to be diverted
far to the north and east, thus
bypassing the Northwest. This
results in mostly clear skies, at
least initially, allowing ground
heat to be radiated into space
during the long nights.
The second consequence of
the ridge is to produce a strong
inversion that suppresses con-
vection, the vertical motion of
air. So the cold air that’s being
chilled by the ground stays
put, capped by the warmer air
aloft.
Eventually, if the air isn’t
too dry, it will cool sufficiently
for condensation to occur.
As the fog overspreads the
basin and thickens, the weak
daytime sun is insufficient to
burn it off. If the temperature
is at freezing or below, rime
ice will begin to collect on
exposed surfaces.
Several high-voltage elec-
trical transmission lines and
even more lower-voltage
utility lines in the Columbia
River Basin are susceptible
to this type of ice damage.
When ice accretion becomes
great enough, towers buckle
or collapse and current-carry-
ing wires break and fall to the
ground, interrupting electrical
service.
A related, but less
PHOTO BY JIM CORNELIUS
Red-belting is a phenomenon associated with persistent winter fog.
catastrophic, phenomenon
called “red-belt” sometimes
occurs when the freezing fog
moves into Central Oregon.
Evergreen trees that are on a
hill that pokes above the fog
layer are vulnerable. The top
of the fog deck acts like a mir-
ror in reflecting the low angle
sunlight onto the needles of
south-facing trees just above
the fog. As this weather condi-
tion persists, the concentrated
energy from the sun dries
out the needles, destroying
their cell structure. The dam-
age isn’t evident until several
months later when tempera-
tures warm and the sap starts
flowing again. A narrow red-
dish band of dead needles
is left behind on the trees
or
A lab
slightly above where the top
of the fog had been. The trees
are not permanently affected
and recover in a year or two.
Local naturalist Jim
Anderson has studied occur-
rences of red-belting on Black
Butte, and elsewhere. His
article on the subject is found
in The Nugget Newspaper,
March 11, 2009.
The last time power lines
were damaged by rime ice in
the Columbia Basin of central
Washington was in January of
2009 when a very strong bub-
ble of warm air settled over
the region, trapping freezing
fog there for a period of about
10 days. That was also the last
time red-belt has been seen on
Black Butte.
e
of lov
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