How Hoar Frost Forms

Most people have experienced when out walking on an early morning in winter hoar frost on shrubs and trees, and many people have wondered how hoar frost forms and what conditions are required for these phenomena to occur?

Hoar frost is a collection of many shaped soft ice crystals that form on the surface of vegetation usually on very cold clear frosty nights where the temperature falls below freezing point in the deepest part of the winter. Hoar frost is caused by moisture settling on an object that is chilled to below the point of freezing. The process is known as radiation cooling and the subsequent ice crystals that form are known as hoar frost.

The covering of hoar frost can be light or sometimes so thick one could be forgiven into thinking it is snow.  The moisture turns to ice crystals that interlock in many wonderful shapes and hang from tree and shrub branches. It can also form on hedgerows, grass and the fine feather like spines and needles can fix to solid objects like television antennas and fences that are exposed to air below freezing point. The sight of hoar frost can look very impressive and a picturesque feature of winters in the northern hemisphere.  Hoar frost has featured as a winter scene on many a Christmas card.

Hoar frost is very similar to dew except the air temperature needs to fall below freezing point (‘0’ centigrade) in order to form as hoar frost. The many shaped particles of hoar frost adhere to the tips of branches, leafs, conifer needles and other objects. Hoar frost is liquid dew that has frozen with a sudden drop in temperatures to that of below freezing. The dew does not freeze immediately but gradually become super cooled. Super cooled droplets finally freeze if the temperatures fall below freezing for any length of time.

Hoar frost is not rime. Rime is a completely different phenomenon and comes from freezing fog and it forms a continuous layer of ice as opposed to individual droplets.  Hoar frost can also be formed by sublimation. This is when water vapour forms directly on surfaces.

Hoar frost is also known as air hoar, crevasse hoar, surface hoar,  depth hoar and sometimes referred to as radiation frost. Hoar frost nearly always occurs on clear frosty nights and will linger on in the morning until the air temperature rises above freezing.