hoar (hōr), a. & n. 1. Grey-haired with age; greyish white; ~-frost, frozen water vapour deposited in clear still weather on lawns etc.; (of things) grey with age; ~stone, ancient boundary stone. 2. n. Hoariness, ~-frost.
The above is a discription from the Oxford Dictionary. But of course, a picture is worth a thousand words. This morning as the sun rose, the world was white and dazzling!
The temperature was -12 c and the humidity high, around 77%. And of course the most critical element for hoar frost to form is lack of air movement. It had been a very calm night, not a breath of wind. As the morning wore on, there were small gusts of wind. Each one sent a cloudy shower of ice crystals drifting through the trees to the ground.