Wednesday, January 29, 2014

Illusion of open water

What: When we first arrived at the Thrust Fault, looking north to Apple Tree Bay it appeared as though the bay hadn't frozen entirely on the other side of the bay. In the picture above you can notice the thin strip of darker area that's reflecting the houses and trees in it. I've often noticed this effect when looking across the lake at NY, where the thin strip of land closest to the lake often appears twice as thick, as though in reflection. I'd always kind of unthinkingly attributed this to simply being a reflection of the horizon in the water.

But when we climbed up to the top of the point and changed our perspective, it became quite clear that the bay was indeed completely frozen over and the "reflection" wasn't simply a reflection (the surface of the ice is super rough and not reflective. What was it then?

I had also noticed a thin rippling effect in the air while looking out across the lake. It reminded me of my childhood in Southern California. I loved watching those heat waves dance up off the pavement - the phenomenon of a distorted horizon when looking down a long road in the middle of the desert. As a child I remember seeing the ripples on the road, but I don't remember the reflective quality. I was more than surprised to see those with mid-day temperatures hovering around 0.

I remembered looking into this in regards to the lake a couple of summers ago, but had forgotten the details so wanted a good refresher. The cause of the effect is the same in the winter as in the summer, as the ripples are indeed caused by heat, but it's actually relative heat. The effect, known as an inferior mirage occur when the surface is much warmer than air directly above it. The gradient needs to be about 5degrees per meter (normal atmospheric change is about 1degree of cooling for each 100m you travel up in elevation). While the lake is frozen, it's not super thick ice in parts so the warm water temperature (about 35) heats up the air right above the ice while the ambient temperature was frigid (about 8 on that day). So sufficient temperature gradient to cause the optical illusion!

The refraction occurs only at shallow angles, so as soon as we changed our vantage by going up to the point (as in the photo above), the mirage disappeared and we were left wondering when the rest of the lake would freeze over already!!

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