|The Hills in Frost|
|Frost on needles|
|Frost on branches|
Hoar frost forms on clear cold nights,and forms on objects that have been chilled below freezing by radiation cooling. It occurs when the relative humidity in supersaturated air is greater than 100%. The cold temperature and the cold object create a situation that the water vapor forms ice on the object directly. There is no in between solid water stage. If you look at the top photo, you'll see trees in the forefront that are green and have no frost on them. Likely the conditions were not right to take the vapour directly to ice.
Rime frost is not hoar frost. It is dew or liquid water that has frozen.It did not pass directly from vapour to ice.
I think we, in North America, tend to use an imprecise English, especially since the social sciences have become so strong. We would normally just call this frost on the trees. An other example would be the substitution of issues for problems. This has been picked up by politicians so that now issues don't need to be solved, and the term problem which requires a solution is never used.
For more information the best site was http://www.weatheronline.co.uk/
A post to Watery Wednesday @ http://waterywednesday.blogspot.com/