It’s been two weeks since our last frost, and judging from my first dozen shots, it seemed like my photography skills dropped from mediocre to downright pathetic. But then I got a few good shots of frost on windows. The fact that I shoot windows on cars means that I see some special forms that would not normally appear on house windows. The most common type I call ‘cactus frost’ because of its resemblance to saguaro cacti.



The resemblance may be a bit more obvious in the following shot, which I took two years ago on seeing this form for the first time.



It took me a little while to figure out a likely cause. The dark regions are places with frost. The reason for the parallel and nearly straight frost regions is probably a preferential freezing in the tracks left behind by drops as they run down the window during rainfall. Notice in the middle picture how some of the dark regions merge just like drop streaks merge. In cold weather when dew starts to deposit on the window, these streaks may have slightly thicker water films due to residue left behind by the drops. I doubt though that such a film would be uniform; more likely, small droplets condense on the larger residue particles. After these droplets freeze, ice starts growing into a very thin film between the droplet regions. Such growth would produce the spiky barbs in the cactus.

-JN