Rare light pillars seen Dec. 9 in Beloit, Wis. (Tom Purdy)

Looks as if Mother Nature’s in the holiday spirit! The atmosphere decided to put up lights of its own Sunday evening in Beloit, Wis., where colorful columns of light shone in the night sky.

Tom Purdy snapped this shot of the luminous pylons a little before midnight Sunday. These “light pillars” are particularly rare. Purdy reports that the apparitions were accompanied by an equally curious phenomenon: freezing fog.

Indeed, the National Weather Service reported over five hours of freezing fog between 10 p.m. and 3 a.m. in nearby Janesville — the seat of Rock County. At the time, the air temperature was between 14 and 18 degrees, with 100 percent relative humidity.

Freezing fog seems a bit counterintuitive. After all, how does fog form below 32 degrees without chunks of ice falling from the air? The answer lays in supercooled water droplets.

When precipitation forms, it requires a nucleus. That’s something for the water to hold onto. It can take the form of pollen, dust or small organic aerosols. When it’s below freezing, the water falls as snow. But if water droplets have nothing to latch onto, they can’t freeze. That leaves them suspended in the air as supercooled water droplets. When they strike a subfreezing object on the surface, they immediately ice up on contact — coating the ground in a dangerous, slick glaze.

However, that’s just half the story. The formation of light pillars requires hexagonally shaped ice crystals in the air to reflect the light. Moreover, they have to be oriented horizontally, like flying discs or plates to bounce light back toward the observer. Less-organized orientations can produce sun pillars and halos.

And for the pillars to form, the ice would require a nucleus — contrary to the requirements of freezing fog.

So how could both have been present simultaneously? Doing a little detective work can attempt to explain.


Atmospheric sounding Dec. 10 from Green Bay, Wis. (University of Wyoming)

A vertical profile of temperatures and moisture in the atmosphere, known as a sounding, provides key clues. The sounding from nearby Green Bay shows a very shallow layer near the surface where the atmosphere was at saturation and well below freezing. In fact, this layer was only about 100 yards thick. Weak southerly winds carried in a source of moisture. That would account for the freezing fog.

Now we have to figure out how some of the water was nucleated to form the ice crystals needed for the pillars.

Returning to the sounding, we notice that the temperature increases with height in this narrow layer hanging just above the ground. That’s unusual in the atmosphere. It’s called an inversion. Inversions trap pollutants at the surface, preventing them from rising.

It is likely that particulates from vehicles and other fine elements from the ground got penned up within the surface layer on a hyperlocal level. It’s possible that there was just enough nucleation for sufficient crystallization to create the pillars, while still leaving some supercooled droplets to trigger the freezing fog.

Above the inversion, the air dries out and warms up. That may explain why the top of Purdy’s light pillars appear to fan out and become more diffuse. It’s probably the changing air above the thin surface layer. The pillars Purdy captured are short, compared with past examples. According to atoptics.co.uk, “the higher the crystals in the atmosphere, the taller the pillar.”

Here’s one more view of Sunday night’s stunning pillars in Beloit: