Hoarfrost is a weird thing. It appears in the morning and can disappear as it thaws in exposed areas. Shaded areas can maintain throughout the day. The season for hoarfrost on trails seems to be short. It never really hangs around for long. But it can be a real biatch to ride! My advice for tackling hoarfrost? Avoid it. By that, I mean look for the ice crystals and broken ground in the trail and steer for hardpack. Usually, although not always, there is a path through or around the hoarfrost in the tread. Hoarfrost tends to cluster near the upside of the trail, as moisture runs down the hillside. That means, often times, your best path is a narrow strip of tread on the downhill side of the trail.
One last note, it is "hoarfrost" not "whorefrost." Although it has been said the the term originated from scantily dressed prostitutes who died from exposure to the cold. Or at least according to what I read on google.