What is Hoar Frost

Hoar frost is the ephemeral, delicate crystals that form on lawns, plants or glaciers.  They appear like diamonds sparking on the ground.  The crystals are fragile and if disturbed can cause natural disasters. Hoar frost has many names.  Some call it sugar snow or squares. They are crystals that grow when the evaporation from liquid drops to vapor occurs.  For the hoar frost to survive, the temperatures and conditions must be maintained.  The moment the temperature warms the hoar frost disappears. The hoar frost has different classifications.  The classifications are air hoar, surface hoar, crevasse hoar, or depth hoar.    

*Air hoar is when the crystals form above the surface.  It is air hoar that lays on our grass and disappears as temperatures warm up. 

*Depth hoar forms under the surface. The warmth of the ground and cold air form the crystals too.  When a thin layer of snow lays on top of the depth hoar, it sinks.  The depth hoar keeps growing as it warms up and refreezes.  It is not stable, and like glass, breaks easily.

*Surface hoar is essentially ice.  During the day, when temperatures rise, air holds more water vapor.  The water vapor condenses and you have surface hoar.  It is fragile.  It changes patterns while under the surface.  Oftentimes, people accidentally cause an avalanche by stepping on the frost.

*Crevasse hoar forms under the surface like depth hoar.  This  frost grows in the cavities and  fissures of glaciers.

Hoar frosts  are  “radiation” frosts.  The frost occurs on cold, cloudless nights. The moisture turns into ice crystals.  The ice crystals form when ground radiation heat escapes upward. Plants, rocks, fallen trees or other objects are colder than the air.   Hoar frost has several air holes in it.  As a result, when the wind starts to blow, the movement of the hoar frost causes the mass of snow to break up.  Hoar frost, if it lasts, starts to harden.  That is the first step to becoming glacial ice. Hoar frost is dew that lays upon the ground.  As the temperature drops, ice crystals form.  It is intriguing to note what a dew point is.  The air pressure in the atmosphere reaches saturation with water (humidity).   It combines with the air temperature until it reaches the dew point.  

The intrinsic value of hoar frost is in the complicated patterns they make.  The temperature and altitude decide how they will vary in forms.  They are beautiful from the needle shape, cups, to fern-like and feathery shapes.