We have a layer of frozen rain on our Japanese Red Maple.
I did a little research to find out how this happens and with the help of several webs sites that had lots of big words, I came up with the following explanation.
Snowflakes fall through a layer of warm air on their way to the ground, melt and become regular raindrops. These raindrops will, naturally, continue on their way to earth and as they do, they have to pass through another layer of air, but this time, the air is very cold.
This is where natures does its voodoo.
The cold air will cool the rain drops to a temperature well below their freezing point (0 °C). But, somehow, the raindrops remain in liquid form; a phenomenon called supercooling. And, as soon as these supercooled rain drops touch something, anything, like a tree branch, they freeze instantly on contact and form a film of ice on that surface.