Living 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 ground.
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 very tiny.
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 falling leaves.
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 looming.
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 spires.
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.