Hoar frost.
In Boulder CO, there are a lot of cultivated non-native hardwood trees along the streets and sidewalks. In fall, they of course show the enormous variations of leaf colors typical of hardwood forests.
The hardwood fall color changes back east were something I missed when I came out west, where there's generally just aspen, cottonwood, and evergreens and the "fall colors" are just yellow from the aspen and green from the evergreens. Big deal. People take special trips up in the mountains to "see the Aspen turn." But it is just patches of admittedly brilliant yellow interspersed in the greenery of the pines. Pretty, but not like the tremendous color variations of a true hardwood forest.
So one of my most vivid visual memories was one glorious autumn morning like that up around 6th and Cascade in Boulder, where the frost, not unlike Larry's first pics, formed on the still leaf-bearing but "turning" hardwood trees and I took a walk.
The early morning light blazing through the variegated colors of the trees, and scattered glintingly through their frosty ice crystals above was totally mesmerizing.
The effect of those red, blue, green, purple, white sparkles of light bouncing all over, enveloping me, engraved itself on my visual memory forever.
Dazzling. Like walking through a giant kaleidoscope.
I still call that my Kodachrome morning.
Terry