Our yard was covered with Hoar Frost, frozen dew, or white frost, this week and the photo opportunity was irresistible.
It is basically explained as moist air and fog in our case, hitting freezing temperatures.
If you want an interesting video explanation, the Weather Channel has one at http://www.weather.com/video/what-is-hoar-frost-32366
One helpful weather person defined other types of frost for us.
Rime frost looks like icing around the edges of petals and leaves and only occurs when the temperatures are very low.
Fern frost is what we see on windows in really cold weather - I saw this a lot as a child growing up in Ohio.
Homelandscapes author Cathy Bell writes about winter life at Old Faithful in Yellowstone National Park, 2013-2014.
Here's one paragraph from her post about Hoar Frost (click on the link to read the rest).
"Surface hoar is a kind of frost that forms when humid air comes into contact with very cold snow on the ground. It tends to form on cold, clear nights with little wind. Why? The lack of cloud cover means that there’s no insulating atmospheric blanket to slow heat loss, so snow on the ground gets very cold very fast. (Trees and shrubs provide a similar blanketing effect, so surface hoar is much less common in forests, say, than in open areas.) If you have air with high moisture content, the molecules of water vapor that bump into the cold snow surface can suddenly have so much heat sucked out of them that they freeze in place—without ever going through the liquid phase."