What is a Hoare Frost ?

[Bootsy]

I have often heard the term 'Hoare Frost" - where does this originate and what constitutes a Hoare Frost ?

[Clanad]

Spelled Hoar Frost here in the U.S.,it happens when the air temperature is below freezing and the air is nearly saturated, that is, in high humidity conditions. Frost with large crystalline structure can occur in those conditions. The large crystals cause the frost to be "spiky"... that is it forms projections that can be quite large. It's much rougher in texture than "normal" frost... On sunny but cold days, the frost often evaporates from the sunny side of structures on whci it formed, but remain on the shaded side...

[gen2]

Spelled Hoar Frost here in the U.K. as well.