Understanding Winter Precipitation: Rain After Snow Explained

When rain falls in winter shortly after snow, it can be a bit confusing and occasionally hazardous. This phenomenon, often referred to as freezing rain, is a fascinating interaction between atmospheric conditions and the ground temperature.

Freezing Rain: A Hazardous Condition

To understand what happens during freezing rain, it’s important to know the basic atmospheric conditions. When rain falls on a surface that is below freezing (32°F or 0°C), it can take various shapes and forms. For instance, sleet forms when raindrops pass through a layer of sub-freezing air just before hitting the ground, freezing into small ice pellets. But when rain falls on already frozen ground and the temperature is just above freezing, it results in freezing rain. This occurs when supercooled raindrops (rain that has cooled to below freezing but remains liquid) hit the surface and freeze immediately upon contact, creating a layer of ice.

I experienced freezing rain a couple of times this year in my area. These events can be quite treacherous, especially because there’s nothing quite as slippery as water ice. Regularly, we call such conditions a ‘wintry mix’ in Massachusetts, where weather forecasters often predict the unpredictable transition between snow, sleet, and rain. The transition between these conditions can be influenced by factors such as coastal vs. inland temperatures. For example, near the coast, you might get 3 inches of snow followed by rain, while inland you could see 8 inches of snow.

Wintry Mix and Its Variations

The term ‘wintry mix’ is used to describe the unpredictable mixture of winter precipitation. This can include snow, sleet, and freezing rain. In my experience, the transition from snow to rain can cause a variety of conditions, such as slush, which forms when partially thawed snow mixes with rain. Regular people often refer to these conditions as a ‘mess’ since these mixtures can be quite slippery and messy.

Ice Storms: A Spectacular Yet Disruptive Precipitation

A particularly spectacular but disruptive form of winter precipitation is the ice storm. This occurs when the temperature at ground level is well below freezing, but higher up, the temperature is slightly warmer. As a result, rain falls and immediately freezes on contact, forming a hard, ice-like layer on everything, sometimes as thick as 3/4 inch. Ice storms can be beautiful, but they are also destructive, often bringing down tree branches and electricity cables due to the added weight of the ice.

Such ice storms are common in my part of Canada. If the temperature is cold enough, the ice can be quite treacherous but if it’s not, the conditions become more 'sloppy.' In these cases, the mess often freezes later that night, making surfaces slick and uneven. Additionally, car doors can sometimes freeze shut, posing a real challenge for drivers.

The UK and Other Countries

The premise of rain after snow or snow after rain is a common occurrence in the UK and many other countries. The weather conditions are often at the edge of creating either snow or rain, making it common to see transitions between these forms. In the UK, the conditions are often just at the edge of creating snow or rain, leading to frequent occurrences of rain that turns into snow, snow that turns into rain, or a mix of both, sleet.

During such conditions, rain followed by snow often creates the most dangerous driving conditions. The combination of wet, icy surfaces can lead to significant impairment of visibility and poor traction. This is especially true when the transition occurs rapidly, such as what happened a few days ago. After a frosty night, it snowed lightly throughout the day, accumulating 2cm. By the time we got home, the conditions had changed drastically, transforming from gentle snowfall to heavy rain soaked with pockmarks on the surface.

Conclusion

In summary, the variety of winter precipitations can be quite fascinating and sometimes confusing. From freezing rain and sleet to slush and ice storms, each type of precipitation presents unique challenges. Understanding these phenomena can help you better prepare for and handle the weather conditions you may face during the winter months. Whether it's the mess created by a wintry mix or the spectacular but treacherous ice storm, these natural occurrences are an integral part of the winter experience.