Definition: Hoar frost: A deposit of ice produced by the deposition of water vapour from the surrounding air. It is generally crystalline in appearance.
There are two sorts of hoar frost: hoar frost proper and advection hoar frost.
The text enclosed in grey-shaded boxes, like this example, comprises Annex I to the Technical Regulations (WMO-No. 49) and has the legal status of standard practices and procedures.