16 Nebraskaland • October 2019
As author Olivia Dresher wrote, "Fog is rain that whispers."
Fog is our opportunity to experience the mystical
environment of a cloud without ever leaving the ground. It
provides a picturesque backdrop for those wishing to capture
a moment in time. It is generally short-lived, which can make
capturing these moments somewhat tricky.
Fog can also, unfortunately, lead to automobile accidents
when visibility is significantly reduced. By definition, fog
causes a reduction in horizontal visibility to a distance of
about half a mile or less. One way we characterize clouds is
by their distance from the surface, therefore what makes fog
different than a cloud is simply altitude.
The normal recipe for a cloud includes three ingredients:
water vapor, a particle such as dust for the vapor to condense
onto, and an upward motion of air. In the case of fog, the
upward motion that would lead to cooling and condensation
of those water droplets is not actually necessary. There are
quite a few types of fog; the only thing that distinguishes
one type from another is how those droplets formed.
When winds are light, the skies above are clear and there
is a shallow layer of moist air at the surface, fog can occur
in the late evening through early morning. This is the time
of day when the air temperature dips close to the dewpoint
temperature, or the temperature at which dew begins to
form. As the warmth of the day takes hold, these droplets,
termed radiation fog, normally make an exit and dissipate.
This type is normally seen in fall and winter.
Another common type is advection fog. Often seen over
cool waters, this fog occurs as moist air advects (a fancy
word for moves) over a cold surface. The vapor-rich air cools
and condensation occurs so that a cloud is formed.
On really cold winter days, fog also can form when water
vapor is added by either a natural or artificial source. This
could be a lake that has held onto some warmth or steam
from a power plant. And in the case when temperatures
are well below freezing, those supercooled water droplets –
water that exists in liquid form even when the temperature
is less than 32 F – can deposit on whatever surface they
contact.
The next time you're emerging from the warmth of a cabin
into the crisp fall air, and you are met by stillness and the
translucent white blankets filling the depressions in the
landscape, soak in the fleeting whispering rain.
Martha D. Shulski, at the University of Nebraska-Lincoln, is
the director of the Nebraska State Climate office.
IN THE FIELD
CEDAR
CANYON
NORTHWEST
OF
CRAWFORD,
PHOTO
BY
BOB
GRIER
FOG
By Martha D. Shulski
