During the summer months, clear, blue skies often mean warmer temperatures because more sunshine gets to us on the ground. However, any time of year a clear, open sky is actually a huge heat sink, a place in space with effective temperatures barely above absolute zero. That means all things terrestrial are radiating their heat energy to deep space leaving us colder. We can feel that even on a clear summer evening after the sun goes down. Or if you garden, think about first and last frosts which typically happen on a clear night; interestingly, air temperatures can be in the low 40°F range and frost can still form on plants because they lose heat faster to a clear sky than they gain heat from their surroundings.
When I see the weather forecast calling for clear skies and temperatures in the 20s and 30s, I am pretty sure frost will show up in places where there is moisture, like along open streams or in low-lying, swampy areas. Frost forms when water vapor—not liquid water—forms into a crystalline solid around some particle or surface. Where those cold temperatures are, at atleast here in Vermont, is also influenced by elevation, with cold air settling into valleys and typically resulting in greater relative humidities. Wind too can result in cold or moist air moving in ways that result in frost forming directionally.
The main thing I pay attention as a photographer of frost to is to be ready because it often doesn’t last long! Often the frost changes back to water vapor, a process called sublimation. I’m also always careful where I step and how close I am when exhaling to preserve the delicate feathers of frost.
If conditions stay cold and windless and there is a good deal of moisture available, frost can grow into “wings” or feathers or chains that are an inch or more long.
While I am usually able to predict when frost will occur, I’m also always ready for unimaginable delights like this hoar frost on a Red oak leaf.
Still Learning To See 
Stunning Photos!