While walking out to retrieve something from the car, I noticed the tree branches around me looked fuzzy even with my glasses on. Upon further inspection I saw a very spiky frost on each branch. When I told my mama what I saw, she said “I bet that’s hoar frost.” And after making the appropriate jokes I looked up what makes hoar frost form.
For normal ground frost to form, it needs to be a still night that drops below freezing. Water vapor condenses and freezes as the temperature drops. For hoar frost to form, however, the vapor needs to and on something that is already below freezing temperature-wise. The droplets form in spikes because they build up over time as various droplets condense and freeze immediately. It grows most frequently on tree branches where it is most likely to remain below freezing.
The term “hoar frost” comes from Old English meaning the old age appearance the frost makes. It is reminiscent of a white beard. If you ever find yourself outside on a cold and still morning, look around and you may well see it yourself!
