On Sunday morning, the trees in southern Wisconsin were coated white with ice crystals. Given temperatures were below freezing and an abundant supply of moisture (you probably noticed the widespread fog), ice crystals formed on branches and other exposed surfaces. It was initially suggested that this was "hoar frost". However, given the areas of dense fog, there was likely a large amount of super-cooled liquid water droplets in the air. When super-cooled water droplets collect on surfaces to form ice crystals, the process is called "riming", which was likely the case Sunday and Monday morning. To say water droplets are "super-cooled" means that liquid water is actually below 32 degrees F. The droplets don't freeze until they encounter what's known as an "ice nuclei", i.e. trees, power poles, or anything left outside which cooled below freezing.
Hoar frost is similar to the process in which dew is formed, except the temperature of the object which accumulates the frost must be below freezing. Hoar frost is formed when water vapor is converted directly to ice crystals. So, the water molecules never actually change into the liquid state; they go directly from a gas to a solid. This process is known as "deposition". The opposite of deposition is sublimation, or when a substance goes directly from a solid to a gas. A phase change diagram is provided below.
Here are some photos taken outside the Weather Forecast Office in Sullivan, WI:
Some additional photos taken in the city of Pewaukee.
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