Rimfrost is ’hoar-frost’, ’rime’, ’white frost’. Rim, Icelandic hrim and English rime is a common Germanic word *hrima- made from the root *hri-, from which also comes German Reif. The Indoeuropean root is *kri-, meaning approximately ‘to foam’, ‘to froth’. Related is Swedish rimma, rimsalta meaning ‘to salt (pork)’.
English hoar, hoary means ‘gray or white with age’, ‘venerable’, ‘old’; the meaning ‘rime’ is probably because frost can grow out like white hair (see picture below). The Old High German and Old Saxon version is hér ‘distinguished’, ‘noble’, ‘glorious’. The comparative form of hér is hèriro, herro, Modern German Herr. The Germanic stem is *haira-, probably meaning ‘gray’, ‘white’, ‘old’ but I don’t dare to go further back now.
Swedish herre is borrowed from Old Saxon in early medieval times.
I took this picture on December 31. We have had a couple of thin snows since, but happily they have melted. There was one this morning too but it is raining away now.