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Weather picture of the month January 2005 

 

 
Picture taken at Kampenhout on December 27 2004.

 

Hoar frost and rime are typical hydrometeors of the wintry season.

Hoar frost  forms in the same way as dew but with temperatures below the freezing point. A deposit of ice on objects at or near the ground is produced by the sublimation of water vapour from the surrounding clear air. It has a crystalline appearance, generally assuming the forms of scales, needles, feathers or fans.

Like dew, hoar frost is formed in two ways:
  • -Exposed surfaces are cooled below the dew point of the ambient air; such cooling is usually  due to nocturnal radiation and the hoar frost is deposited mainly on objects at or near the ground.
  • -Warm moist air comes in contact with a colder surface, the temperature of which is lower than the dew point of the air; this is generally happens as the result of advection of air.
Rime forms when the deposit of ice is the result of rapid freezing of very small supercooled water droplets in fog or clouds. It is composed of grains more or less separated by trapped air, sometimes adorned with crystalline branches. 

 

The picture shows a combination of both hydrometeors as hoar frost was formed during a long clear frosty night, followed in the forenoon by freezing fog producing rime. 


List of pictures of the month