Parts of Central Otago were blanketed in a hoar frost late last week.

Lauder residents woke to minus 6.6degC on Friday and minus 4degC in Omakau, where ice crystals formed along fences, paddocks and trees.

Niwa meteorologist Seth Carrier said the temperature of the objects on which the hoar frost forms must be below freezing – “ideally, well below freezing”.

“Water vapour within the air condenses into ice on those objects.”

Hoar frost differs from “regular” frost in that there is much more water vapour in the air during a hoar frost, allowing the ice crystals to form much more intricate patterns, he said.

“The frost forms similarly to dew, except that the air temperature and saturation point are both below the freezing point. Hence, water vapour in the air condenses directly as ice on objects during a frost, instead of as liquid water during warmer times of the year.”


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