February 12, 2022 - Valley Fog in Oregon

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Dense fog blanketed Oregon’s Willamette Valley on February 10, 2022,
stretching from Canyonville to Portland and snaking into adjacent
valleys. On February 9, the National Weather Service (NWS) issued a
warning for the Portland area, advising that areas of dense fog and icy
conditions could create hazardous traveling conditions through the
morning of February 10. According to the NWS, Cold and damp air
remained trapped under a very strong inversion, resulting in widespread
low clouds and fog. Areas of dense freezing fog were expected to form
in areas between 500- and 1,500-feet elevation, with the potential of
development fog below in a few places below 500 feet elevation.

The false-color image acquired by the Moderate Resolution Imaging
Spectroradiometer (MODIS) on the morning of February 10 proved the
predictions right on-target. This type of image uses a combination of
infrared and visible light which helps separate fog (low cloud) from
snow and ice. Snow appears electric blue while cloud/fog looks white.
When fog or cloud is very cold and contains ice crystals, it may appear
very pale electric blue. Open land looks brown; vegetation is bright
green and deep water looks black. With this combination, it’s easy to
see that cold cloud sits over the Pacific Ocean, low cloud drapes over
the inland valley, and snow sits atop the high elevations in the
Cascades. Brick-red areas in the greenery of the Cascades are burn
scars, left behind by recent fires.

Fog is a low-lying type of cloud composed of tiny water droplets
suspended in the air. The main difference between a cloud and fog is
that the base of fog reaches the ground, while the base of a cloud is
well above the surface. The difference between fog and mist relates to
how much the suspended water droplets reduce visibility. Fog reduces
visibility to less than 1 kilometer (0.6 miles). Mist also reduces
visibility but never below 1 kilometer.

Radiation fog usually forms in clear, calm conditions at night. The
cooling of the land surface chills the air close to the surface,
reducing the air’s ability to hold moisture and trigging condensation
and fog formation. In low-elevation areas, radiation fog usually fades
as the Sun warms the land surface, but in higher-elevation areas valley
fog can linger throughout the day.

Image Facts
Satellite: Terra
Date Acquired: 2/10/2022
Resolutions: 1km (342.1 KB), 500m (935.8 KB), 250m (599 KB)
Bands Used: 7,2,1
Image Credit: MODIS Land Rapid Response Team, NASA GSFC



https://modis.gsfc.nasa.gov/gallery/individual.php?db_date=2022-02-12

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