(WBRE/WYOU) — Here across northeastern and central Pennsylvania, as we get into the winter season, we have to worry about when air temperatures, especially right at the surface, are at or below the freezing mark.
That concern is for black ice. So, what exactly is it? It’s a very thin layer of ice that accumulates on the pavement, say on a roadway or a sidewalk.
Now the transparency of black ice is a little difficult to see and this is one of the concerns that we have, especially during the morning time when the air temperatures are usually the coldest. Now there are three scenarios that we’re going to talk about.
The first one is when we have a major winter storm. We’ve had accumulating snowfall and temperatures warmed up the next day. The sun is out and the snow packs begin to melt. Those puddles eventually start to pile up on the roadways and the pavements are wet. Then at night, the air temperatures continue to fall right at or below the freezing mark, so any puddles of water can eventually freeze right onto contact.
Another concern is dealing with rain. This is often what we see with freezing rain situations. As the rain continues to fall, the air temperature is at or below freezing especially right on the contact with the pavement, so this will often create some very slick spots.
Freezing rain is one for most dangerous conditions to travel in during the winter months. Another scenario is when we also have fog. These are tiny little water droplets that will instantly freeze on contact. It will create that very thin layer of ice that we have to be careful when we’re driving, too.
Another thing we see are the road signs before you get to a bridge. It typically says, “bridge ices before road”. With this animation, you can see the temperature of the air continues to fall into the 20s and usually the bridge temperature falls quickly behind.
You can see here it’s right in the mid 20s. The road temperature still has some of the heat from the earth, so it takes a little longer time for the air temperature to meet with the road temperature.
That’s why when we have light rain falling and the road temperature is still above freezing, the pavement is wet. However, with the bridge temperature below the freezing mark, in the 20s, bridges start to ice over first. This is why we have to be careful when we’re driving here during the winter months!