|
|
|
|
Ice Fog
Wed Mar 10, 2004
Listen in RealAudio 
Ice fog presents a major visibility problem, and potential health hazard, to residents of
northern communities such as Fairbanks, Alaska and International Falls, Minnesota, and humans
play a big role in its formation.
Hi, I'm Bryan Yeaton for The Weather Notebook.
Ice fogs form when excessive water vapor enters bitterly cold air, and, rather than condense
into water droplets as in regular fog, forms ice crystals directly, a process called
"deposition."
In very clean air conditions, water can remain as a vapor to temperatures as low as minus 40,
but if the air is dirty -- containing, say, dust or other particles from industry or
transportation -- ice crystals may form spontaneously at temperatures as warm as minus 20
Fahrenheit. The crystals produced are so small that ten can fit side by side on the edge of a
piece of paper.
Tailpipe and smokestack emissions produce not only the particles needed to seed ice fogs, but
they also are a major source of the water vapor. Crowds of people and animal herds may also
add vapor from their breath. Recall seeing your breath on a very cold winter day then multiply
it by a herd, and you'll get the picture. Native hunters in arctic regions often used these
clouds of ice fog hanging over a caribou herd to find supper.
Valleys and other low terrain are conducive to ice fog formation because they can trap the
cold air that pools to the bottoms. This temperature inversion, warm air lying over cold,
keeps the air stagnant, allowing pollutants and water vapor to build up. Denver is well known
for its inversions, but this January, Salt Lake City suffered through most of the month in the
nasty air.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook is
generously supported by the National Science Foundation and Subaru of America.
Today's Links
Alaska Science Forum:
http://www.gi.alaska.edu/ScienceForum/ASF13/1319.html
Some Photos:
http://www.alaskaphotography.com/photos/landscapes/icefog_lupton.html
|
|
