Freezing Fog Defined: The nature of water and freezing temperatures

Weather Blog

Featured image above courtesy of @TracieMariBella on Twitter

When supercooled water droplets found in fog come into contact with a surface that’s freezing, you get freezing fog!

An icy look can appear when temperatures get cold enough and with enough moisture. Take a look at what freezing fog can do right outside in your backyard in the photos below!

These water droplets can freeze on tree branches, stair railings, vehicles, roads, and in intense circumstances they can freeze to telephone poles and wires that can cause widespread power outages. Freezing fog on roadways can also create black ice and slick travel.

“A Freezing Fog Advisory is issued by your local National Weather Service office when fog develops and surface temperatures are at or below freezing. The tiny liquid droplets in the fog can freeze instantly to any surface, including vehicles and road surfaces.” – NWS

Did you know? Water does not always freeze at 32° as water can exist in a liquid state even at temperatures well below freezing. We refer to this liquid water as supercooled. This is why we tend to refer to 32° as the melting point and not the freezing point. Frozen water will always melt at that temperature, but it doesn’t always freeze.

Image courtesy: weather.gov

How does this happen? Ice needs something to bond to in order for liquid water to exist in a frozen state such as a nucleus or nuclei, which is hard to come by as liquid water is suspended in the air as fog. The rate at which water freezes is highly dependent on its structure and molecules, which is what makes freezing fog and the nature of water itself so fascinating. Under just the right conditions, when these supercooled water droplets come into contact with a frozen surface to bond to, the water will freeze right onto it.

Supercooled water droplets can exist within clouds too, and not just at the surface as fog. In fact, clouds are made up of a mix of both supercooled water droplets and ice crystals, since temperatures where clouds form are so cold! Some scientists have found water existing in a liquid state in temperatures as cold as -40°F within some clouds.

~Meteorologist Christine Gregory

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