Unique black ice causes chaos on Michiana roads
The morning low temperature was near 37 degrees, well above freezing, and liquid cannot freeze in that warm of air. However, the road surface temperatures in most of Michiana were still 32 degrees or colder, because of the long duration of cold temperatures last week.
Now, going back to the air temperature, since the overnight air temperatures stayed so warm, temperatures easily dropped to the dew point, which saturated the air and caused dew to form. When the dew settled on the freezing road, it froze and turned into ice. Ice coated many roadways, and there were reports that it turned Nimtz Parkway in South Bend into an ice skating rink.
This type of black ice is not very common, because variables like a quick warm up after a long period of freezing temperatures, and a liquid source need to be present.