An MSNBC Week in Pics shot credited to one Jamie Roper. This one is selected because it's a damn good shot, but also because I had never heard this term "hoarfrost" before.
And also because, looking at this, I couldn't tell what it was without reading the caption.
It's a volleyball net covered with hoarfrost. Sometimes called white frost, this frost can only happen in the right circumstances -- when misty air at near freezing temperature hits a surface that's already freezing cold, such as a net.
How do you make a hoarfrost?