Last Monday the weather report included a patchy “freezing fog” alert. I wondered what the difference was between a freezing fog and an ice fog, so I did a little research. I thought that “Water freezes at 32oF” was an absolute truth, but it turns out it isn’t. Water will freeze at that temperature if it has something to freeze onto, but if it is suspended in the air, as in a cloud or fog, it can remain liquid down to 14o or lower. Droplets that are below freezing but still liquid are called supercooled. When the liquid touches an object it immediately freezes onto that object. If the temperature goes below 14 degrees,the liquid in the cloud or fog with turn to ice crystal and then be an ice fog, which is generally only seen in Arctic/Polar air.
Hoarfrost is similar to dew, except that it only forms when the temperature is below freezing. Also the air must be moist enough that the saturation point is reached so that the water droplets condense on objects. I’m still not sure I know the definitions. One source said hoarfrost is not derived from freezing fogs. That is called rime. But another source said hoarfrost often results from freezing fogs. Part of the confusion may be due to the language barrier: one source was from the US and one was from England. Yes, we speak the same language—sort of!
At any rate, what I saw was really beautiful, no matter what it was called.
As of February 5, Lake Michigan is 51% covered with ice. The entire Great Lakes system is 77% covered and Lake Superior is 92% according to the Great Lakes Environmental Research Laboratory. The ice is continuing to build so these percentages should increase in the next few days. There are good things about ice cover: it keeps the water from evaporating into the air which causes lake effect snow and clouds. I’m enjoying the bright blue sunny skies that we have had this winter. Of course, the open water also warms the air which means we don’t have that insulation that being east of the lake usually provides. Some scientists thought that ice cover would keep water from evaporating and thus be a good thing for the lake levels, but current thought is that lake effect snow usually falls within the watershed in which it was formed, so that moisture is usually returned to the lake system anyhow when it melts. But the latest research suggests that the greatest evaporation occurs in the fall when the water is warmer than the air, and the benefit of the ice cover is that it will keep the lake cooler in the following summer and delay the onset of the rapid cooling/high evaporation season. What we thought we knew and what new data suggests is often two very different things!
So I’ll leave you with a few pictures of the hoarfrost or rime which may or may not have been the direct result of freezing fog. Sometimes it’s best to leave the details to the experts and just enjoy the results. Stay warm!