What makes winter roads icy?
Snow turns into ice
Deep snow on the road could be an obstacle that needs to be ploughed. But even if many of us have experienced spinning wheels in deep snow, most accidents occur in light snowfalls. This is probably because we’re more prepared and cautious in heavy snow and take it easier accordingly. But, in temperatures below freezing, how can roads with dry snow become icy roads? It’s a combination of two things: First, the weight of the vehicle compresses the fluffy snow into a compact layer of snow. Then, repeatedly, friction heat from the passing traffic actually melts the uppermost snow layer, which freezes again. Eventually a layer of ice is built up on the road. This is the most common cause for road icing.
Freezing rain and freezing drizzle
Even though rainwater makes roads slippery to some extent, rain covered roads are far from as slippery as roads covered with pure ice. Normally, when temperature is above freezing, it doesn’t snow – it rains. So how could there be ice when it rains? This is meteorology. Precipitation may pass several layers of air, and these air layers can have different temperatures. Suppose temperature in the clouds are well above freezing, then it will rain. But, if temperature at ground level is below freezing, then the raindrops will freeze to ice when they hit the ground. This will result in the road becoming entirely glazed by a layer of clear ice.
This is the most slippery type of road icing and it may come as an unpleasant surprise. Accordingly, it’s potentially the most dangerous ice on the road. This clear ice glaze is sometimes referred to as ‘black ice’ because the black road surface is clearly visible – but not the ice itself!
Do you want to know more about black ice? Read the blog post How to discover black ice.
Rain and snow mixed
From time to time it may appear snowing and raining at the same time, i.e. sleet. This could be due to snow from colder air layers partially melting when reaching lower, warmer layers. In this case the ground layer is above freezing temperature and there will be water on the road. But, it could also be the other way around; there could be rain falling from warmer layers, freezing when reaching lower, cold layers. This is a potentially dangerous situation since unfrozen raindrops will freeze on the ground. Sleet is a warning that black ice may appear!
Although dense fog is not so common at very cold temperatures, fog may glaze the road with ice. Actually, fog is water droplets that are so small that they virtually float in the air without falling to the ground. These droplets may slowly build up a thin layer of ice on the road, notably on bridges.
Hail are lumps of ice that build up from raindrops in subfreezing layers of air. If the frozen raindrops ascend by way of updrafts, humidity will add more and more ice to the frozen raindrops and they grow bigger and bigger until they eventually fall to the ground. Although rare, hails could be the size of a tennis ball – or even bigger – and cause severe damages when hitting vehicles, buildings or people.
Wind and snowdrift
Even if it doesn’t snow and the road is apparently dry on a clear and sunny day, winds may cause snowdrifts that potentially make the road icy. This is more common in flat regions where the wind isn’t slowed down by natural obstacles like hills and woods.
Rainwater or other water
After it’s rained – but before the road has dried – temperature could fall below freezing. If you’re not aware of the outside temperature when driving, you may think you’re driving on residual rainwater, and all of a sudden the road is covered with ice!
Also, there are other possible reasons for water on the road; flooded ditches, sprinklers, broken water mains etc. Whatever the reason for water on the road, at subfreezing temperatures the risk of ice remains the same.
A good driver is a wary driver
If you understand the causes for icy roads, you can be more attentive to warning signs like change of weather, falling temperature and precipitation character. And, being attentive makes the driver better prepared for driving in slippery road conditions.
Be prepared! Always check the weather forecast. Keep an eye on the outside temperature when driving. Make sure you can increase traction if road conditions turn out miserable. Drive safely out there!