Crestone Eagle, January 2004:
Ice fog and Fata Morgana
here on the edge of both the Sangre De Cristo Mountains and
the San Luis Valley, we sometimes get to see different weather
phenomena that most areas of the world rarely witness. I've
been asked about two phenomena that take place here in the
winter months: “ice fog” and the “Fata Morgana”.
Let’s start off with ice fog, since this is a weather
condition that the Valley experiences on many winter mornings.
Just for general interest, ice fog has several different names,
for example, the Indians called it "pogonip". There
are actually a few different explanations on what ice fog
is and how it happens. First, don't get ice fog confused with
freezing fog—they’re not the same thing. Basically,
freezing fog is just that, fog that forms when the temperature
is below 32°F, causing water particles to freeze on the
Ice fog forms in a different way. On very cold, clear nights,
the ground radiates infrared energy out to the atmosphere
and space. As the ground gets colder, the air in contact with
the ground also cools. On a still night, when there's enough
moisture in the air, the air temperature may go down to the
“dew point"—the temperature at which condensation
occurs. But first, the temperature needs to be cold enough
for the moisture in the air to form tiny ice crystals. You
also need just enough air motion to stir the air gently, so
that more air can be cooled. What you get has been described
by some as “tiny floating, shimmering crystals”
(not to be confused with snowflakes!). These crystals are
highly visible due to the light's reflection; they are usually
Sometimes you will also see light that has been described
as "emanating vertically upwards. . . a shaft of something
like light, but more like a shimmering glow." These are
caused by columns of light extending upward from streetlights,
etc., and called "light pillars". When sunlight
is the source of the light, they're called "sun pillars".
They happen when light reflects off the tops and bottoms of
the ice crystals, as the crystals flutter down like slowly
The ice crystals in a light pillar may have various shapes.
The shape of an ice crystal depends on the temperature of
the air where it forms and grows, the amount of water vapor
present, and the water vapor pressure. Light pillars have
been known to form with hexagonal plates, capped columns and
column crystals. Column crystals have the hexagonal shape
of a standard lead pencil and are long like the pencil.
A second, more scientific explanation as to what ice fog
is comes from the Geophysical Institute, University of Alaska
at Fairbanks: “As the sun retreats to near or below
the horizon, less heating of the ground surface and the near-surface
air occurs. If the sky is clear, the earth radiates its heat
energy to the frigid reaches of space and then cools the air
in contact with it. Cold, stagnant air near the ground results,
often inverting the normal trend for decreasing air temperature
with increasing altitude. Sometimes extreme inversions develop….”
What creates that “Fata Morgan" effect when
you look down from the Baca or while traveling down T road,
toward the Valley floor, and buildings in Moffat or Hooper
look to be 6 stories tall? Sometimes I've been able to see
the buildings as far away as Center because of this effect.
What you are seeing is called a “superior mirage”.
This mirage forms when cold air lies beneath warmer air, a
condition known to meteorologists as a temperature inversion.
In this condition, light rays refract, or bend, toward the
colder (and denser) air—that is, downward. This bending
causes the image of the object to appear to us to be above
its actual position because our brains assume the light rays
have taken a straight path from the object to our eyes.
The rate of increase of temperature with height (the temperature
gradient) affects how the light rays travel from the object
to our eyes and thus how we see the resulting image pattern.
The superior mirage may cause the image (or parts thereof)
of an object to appear visible even though the object is actually
located below the horizon. The object can also appear lifted
well above its actual position, and taller, larger or closer
than it actually is; or shorter, smaller or further away than
it actually is. When an image appears much higher in the sky
than the actual object's position, the condition is termed
When an image appears much taller, the condition is termed
towering. When an image appears much shorter, the condition
is termed stooping
In a Fata Morgana mirage, distant objects and features at
the horizon appear as spikes, turrets or towers, objects with
great vertical exaggeration rising from the surface. Charles
Funk of Funk & Wagnell's Dictionary fame traced
the origin of the name Fata Morgana to Italian poets who named
what they saw rising up across the Strait of Messina after
the fairy castles of Morgana. Literally, Fata Morgana means
the Fairy Morgana, a reference to the English legends of King
Arthur's enchanted sister Morgana, who dwelled in a crystal
castle beneath the sea.
Alistair Fraser, an expert on atmospheric optics at Pennsylvania
State University, attributes the Fata Morgana to a situation
where the temperature increases slowly with height from the
surface until it reaches a shallow air layer where the increase
in temperature becomes quite rapid. This layer is then topped
with another layer of slowly increasing temperature. This
atmospheric temperature structure will magnify objects whose
light rays pass through the middle layer. Minor spatial variations
in the inversion pattern can project a complex image pattern
toward the observer. Variations in the degree, thickness or
location of the layer may even cause relatively smooth surfaces
of water or snow to appear as a line of irregular towers and
In a nutshell, we see mirages everyday, at sunrise and sunset—ever
notice how much larger the sun appears at that time of the
day? Same thing happens when our full moon rises and sets.
These are yet another kind of superior mirage, just a lot
more common than a Fata Morgana.
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