The Collaborative International Dictionary of English v.0.48:

Hoarfrost \Hoar"frost`\, n.
   The white particles formed by the congelation of dew; white
   frost. [Written also horefrost. See Hoar, a.]
   [1913 Webster]

         He scattereth the hoarfrost like ashes.  --Ps. cxlvii.
                                                  16.
   [1913 Webster]