Q: What is the difference between freezing rain and sleet? I always thought they were the same, but you talk as if there is a difference. Also, what is freezing fog?
A: This difference between sleet and freezing rain is one of the most common questions I get during the winter. Both occur when there is a layer of warm air that is above the surface with a layer of below-freezing air near or at the ground. Often during a freezing rain event, I will get emails saying, “Where’s the freezing rain? All I see is rain.”
Freezing rain is rain. There is only one difference, and that is the temperature at the surface near the ground is below freezing. The rain falls as water and then will freeze on untreated surfaces. This can cause a glaze on objects and can make roads slippery. The buildup of ice on tree branches can add enough weight to cause them to break and lead to power outages.
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Sleet occurs when the layer of cold air is deeper and the rain freezes before reaching the ground. These frozen raindrops will accumulate like snow but usually at a slower rate. And because the balls of ice are more visible to drivers, they often perceive it as more of a danger and treat the situation with sufficient respect.
Lastly, because sleet doesn’t stick to the tree branches, it often doesn’t cause as many power problems as freezing rain.
Of the three, freezing fog poses a big risk to drivers. Basically, this is fog that occurs when temperatures are below freezing. The fog is made up of drops of super-cooled water. These droplets of water will freeze on contact with objects. This forms a great view of icy tree branches when the fog lifts, but it also can leave a layer of ice on the roads. And like freezing drizzle, drivers often do not perceive this as a risk until it’s too late.
On occasion, freezing fog and freezing drizzle can leave an invisible layer of ice on the roads, which is called black ice. In true black ice, there is an invisible frost of ice on the road when the road actually looks dry. There’s often no way for drivers to perceive the risk until it is too late.
Black ice has become slang for whenever ice is on the road, visible as what seems as a wet road or not.
Q: I notice on your Pennsylvania maps that there is a diagonal line through Centre, Clinton and Lycoming counties. What is this line?
A: It’s not all of our maps, just the warning and advisory maps. The National Weather Service issues advisories often by county, but for these counties, they divide them by topography. The northwestern side of these lines is a higher plateau region and the south side of the line is more of a ridge and valley. Often it is just enough to cause a difference in the weather between the types of advisory.