Shadow Halos

I looked out my office window on a recent very cold Wednesday and saw this.

All of the car shadows in the parking lot had halos. Something I had never seen before. Nor on the next couple of days as it was cloudy Thursday and Friday. And I don't go to the office on weekends.

And I had no idea of the cause. So I asked the go to guy for optics phenomena, Les Cowley of the Atmospheric Optics website. And his answer, in part, was:

"A puzzling sighting. The only thing that I can think of so far is this - The car shadows will be shortening and swinging clockwise as the earth rotates. The ground inside the shadow will have less heating than the sunlit area. ... "

And I think he was right, although he wasn't sure how it would have resulted in the bright halo. Read on ...

Then on Monday, this was the view. Note it wasn't that cold. Also note the wet blacktop; it had rained the evening before:

The normal view, no halos. And then, the next Thursday, it was again very cold, and it had rained the day before.

And the halos are back.

Now here is the the image that solved the puzzle for me.

Clouds had rolled in and the shadows and halos are gone. And in their place are the apparently dry areas where the shadows had been. But why would the shadow area be dry while the area in the sun is wet?

Solve that and you solve the puzzle.

Look at the red SUV towards the top of the images and the white vehicle above it. The red SUV has a halo and a shadow 'dry' zone while the white car does not.

The clouds passed and the shadows, and halos, returned. But as the sun rises the shadows shrink, the halos follow, and so does the dark wet areas. The next two shots were taken an hour apart.

You can see how the shadows are shrinking by looking at the white lines of the parking spaces.

And how the wet spot follows the shadows in. And that's the key.

The dry areas weren't dry. They were frozen. And as the shadow shrinks the ice is exposed to sunlight. And it melts, making the blacktop wet. And wet blacktop appears darker than the bit of ice exposed. And thus the halo is just that bit of ice that is now in sunlight but which has not yet melted.

🌞🌞🌞🌞🌞

This was not my first solution. I had thought that maybe there was a thin layer of warmer air above the cars, and this bent the light up and did so that it exited the warm layer and then was bent down again, not like a mirage. And thus light that would have ended up in the shadow area was redirected to the halo area, as shown in the (not to scale) diagram below.

            ☀️

           /    /

          / _ /

         /  🚗

halo

The light from the sun, the "/"'s,  either hits the car (the right "/"'s) and is bent to the left, the "-", when it hits the warm air layer over the car. And then it is bent down again, "/", when it exits the warm layer and hits the cold air, where it is combined with the light that goes straight from the sun to the halo.

An elegant solution, or so I thought, to the puzzle. Too bad it was not the correct one.

🌞❄️🌞❄️🌞

And the final word from Dr.Cowley:

"Yes, you have solved it.   The icy ground looks lighter - Its appearance presumably partly depends on the direction of the illumination and the sky colour.    As the shadow shrinks the ice at the exposed shadow border stays awhile thus generating the light border.

I liked your earlier mirage refraction explanation.   The direction of the refraction and the generation of a light shadow border made sense.  The only problem was the required magnitude of the refraction.   The relatively small air temperature differences from sun warmed car surfaces would not be enough."

😊😊😊