Freezing Rain

Freezing rain occurs when frozen precipitation falls through a warm layer of air, causing the precipitation to melt and change from solid to liquid. However, because the surface where it lands is below freezing, the liquid precipitation freezes on contact, creating a dangerous icy layer. Typically, the severity of freezing rain events is related to the duration of the event.

Freezing rain events can be caused by multiple atmospheric patterns that have specific sets of conditions:

• Low-lying valleys typically contain cold air, and when warm, moist air flows over the cold air, freezing rain events often occur. 
• Frontal occlusion, or when a cold front overtakes a warm front, can trap cold air near the Appalachian Mountains. 
• Easterly flow from the Atlantic Ocean holds cold air masses against the Appalachian mountains while warm, moist air masses flow over it. This is also called cold air damming.

As the climate warms, winters in the Great Lakes region are warming faster than other seasons. We already observe a decrease in snow in the southern part of the Great Lakes Basin, as precipitation that would have, historically, been snow is falling as rain. The rain-snow boundary is migrating north. Because freezing rain occurs at temperatures near freezing, it is plausible to expect freezing rain events to be more likely in regions that previously experienced snow. Other changes in the location of winter storms would also have an effect on freezing rain events.

The top figure from Delpizzo et al. (2025) shows the annual average number of hours of freezing rain in the region. The largest values are in the eastern part, where there is more orography and greater amounts of moisture because many winter storms transport water from the Atlantic. Unlike snowfall, there is no obvious lake-effect for freezing rain.

Changes in freezing rain as the climate warms are not easily described. Events are uncommon, and we are searching for changes in events that are, by their nature, difficult to analyze. Analysis of the observations shows freezing rain events are becoming more common but shorter-lived.⁶ This is consistent with regional warming.  Geographically, there has been an increase in freezing rain along an east-west line just north of Lake Huron. In the more southern part of the region, 42°N – 46°N, e.g., southern Michigan and Wisconsin, trends vary from west to east. West of Lake Michigan has seen a decrease, and east of Lake Michigan an increase. We note an exception to this in Rochester, Minnesota, that shows an increase and has strong orographic forcing.⁷

There have generally been decreases in the Appalachian Mountains.

Because of the large influence of orography and lakes, the changes in freezing rain are not spatially homogeneous. Because of this spatial variability, planners need to be aware of local conditions that may not be consistent with overall trends in the region.

The bottom figure shows the changes in freezing rain by month. There have been increases in January, likely because snow events have been replaced by freezing rain events as the region has warmed. The largest statistical changes, however, are in March and April. March shows a decrease, and April shows an increase.⁸ Further analysis would be required to describe the changes in March and April more completely.