Roof Top Icing

March 17, 2022

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Photographer: Dale Hugo 

Summary Author: Dale Hugo; Jim Foster

My neighbor’s high-efficiency furnace produced an unusual growth of ice (top photo) one night earlier this winter. I’ve never seen anything like this! I think the factors involved in this structure are 1) a light wind from the south depositing the ice to the north, 2) temperatures that night that were around 0 F (-18 C), so the build-up occurred quickly, 3) when methane burns, like all hydrocarbons, water and carbon dioxide are produced, 4) another source of water from this stack is indoor humidity — perhaps enhanced by cooking and a humidifier on the furnace,  5) notice that this ice pillar is in the shade of the flue pipe.

By contrast, the ice from my furnace (about 80% as efficient and older than theirs) formed on the southeast corner of the chimney and has much less accumulation (bottom photo). Why the difference in direction? The mass on my roof has built up over many days. Also, prevailing winds from the northwest directed the vapor towards the southeast. It should be noted that my chimney flue is much wider than theirs -- older furnaces have wider flues. I suspect vapor deposition is what caused the odd ice pillar on my neighbor’s roof.

When water vapor is cooled below its freezing point in sub-freezing air (supercooled), desublimation by crystallization may occur on any available nucleus, such as a speck of dust or a piece of metal. In this case, it's likely that the cold flue pipe served as a nucleating surface, prompting desublimation of the super-cooled water vapor as it emerged. Compare this growth with the Earth Science Picture of the Day for March 9, 2022.

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