What Is Hoar Frost?

We awoke yesterday morning to a true winter wonderland. Hoar frost everywhere, sun shining, perfect deep blue sky, and a temperature of 5° F. Brrr! I was conflicted. The beauty outside was calling loudly for me to grab my camera and join in on the fun, but I hate being cold - HATE IT - so my warm house won the argument.

But I did take a couple of photos from inside the house before the breezes started blowing the hoar frost off. I wish I could have gotten an image of THAT. It was like diamond dust floating down.

What is hoar frost anyway? I've been asked that before and it's easy to understand, really. When heavy, humid air is warmer than the surfaces it comes into contact with, and the temperature of those objects is below freezing, the moisture freezes on contact.

The humidity Sunday morning was 80% and with the temperature being as cold as it was, surfaces were pretty cold. Usually, there's fog along with the hoar frost, but this time there was only a slight amount of fog.

There is a similar frost, called rime, which is formed only on the edges of objects, including leaves, when a moist wind blows across objects and freezes to them. This only occurs when the temperature is very low. If you look at this close-up photo of our burning bush (Euonymus alatus), it looks like this might be rime, because the frost has formed in a single direction, but at the time the frost formed, there was no wind.

From a distance, it would be difficult to tell the difference between the two, and it doesn't really matter much - they're both beautiful.