Weekly Weather Event -Week of January 4thJanuary 8, 2021
This past week, much of southern Wisconsin has turned into a winter wonderland as trees and buildings alike were blanketed in frost. Several continuous days of freezing fog made the conditions right for frost to form, which resulted in some stunning pictures, including some from our very own Dr. Angela Row. While freezing fog is by no means a rare occurrence during winter months, it is more unusual for freezing fog conditions to persist for several days in a row, creating perfect conditions for the persistent frost earlier this week.
Hoar frost is the best-known type of winter frost, which occurs when water vapor immediately turns into ice upon contact with a surface, bypassing the liquid phase all together. This tends to occur on cold, clear nights with little wind. In contrast, the type of frost seen all throughout southern Wisconsin was likely rime ice. While the two phenomena look similar, rime ice is formed when supercooled water freezes on contact with a surface. This is easiest to achieve in high humidity environments, which include dense freezing fog.
While rime ice is beautiful on the ground, it is quite the logistical disaster for aviation. Freezing fog mimics conditions more prevalent high up in the atmosphere, which can cause a buildup of ice on aircraft even before take-off.