An explanation about the ice on the road on days like today!

[Paula]

HI All,

What a crazy morning. I think most of us were surprised by the state of the roads. I always like to try and understand WHY something is the way it is.. althought I promptly forget once told. SO, I asked a friend of mine who I knew would know.. and he did. I suspect a number of you might be curious (or not).. so here is a quote from Markus Kellerhals about what the heck happened this morning.

"Last night was actually a really good set up for ice to form with an ample supply of water from the rain last night. Then turning clear and cold and the wind dying. Most clear and cold nights lack the wet ground to freeze so you have at most a bit of frost which isn’t as slippery as a real ice layer.

On a clear night the way things cool down is by the surface radiating away energy (in the form of infrared “light”) to the sky (and ultimately space). The ground is a very effective radiator, while the air is much less effective a radiator. So the ground actually cools much faster than the air. In fact most of the cooling of the air takes place by the ground cooling the air it comes in contact with. So on a cold clear night there is actually a temperature inversion with the ground at 0 or below. The air just above the ground might be at 1 degree and the air at face level might be 4 to 5 degrees. And if you could measure the temperature 100 metres up it might be warmer still. Environment Canada and other organizations measure temperature at around 1.5 metres above ground (this is by international convention). So it is quite possible for ice to form at the ground (0 degrees) and still have a few degrees positive temperature reported.

The surface freezing only happens if it is clear (clouds slow down the radiation of heat), calm (wind will cause turbulence that keeps mixing down non freezing air from above, and an open view of the sky (trees, houses and other sky obstructions also slow the cooling down).

The one other relevant thing is that certain areas cool off more than normal. Often little hollows (i.e frost pockets) where cooler (denser) air collects over the course of the night. These are prime spots for icing as they tend to be both cooler and wetter than average.

Here’s a neat blog post on roadway icing (a related phenomenon) http://cliffmass.blogspot.com/2011/10/m ... on-in.html from a prof at UW"

Your eye on the streets intrepid reporter, who doesn't stop until she gets the answers to her questions (which drives MANY people crazy),

Paula

[Greg Miller]

Thanks for posting this Paula!

The conditions today were a big surprise for sure. The school temperature map showed 4-5c throughout and it looked consistent. I checked it again around 7, and everywhere was still 3c or warmer. Sections near my house that were wet at 6am became very icy by 7am.

I guess one thing to keep in mind is the school sensors are on the roof tops, so during clear nights with light winds the bubbles of cool air caused by radiative cooling that Paula described can be much cooler than on the roof tops.

Additionally, I think that the added evaporation of a heavy rain over the night would add to the road surface cooling. This is just a guess and I would be interested to hear if anyone knows more about this. Luke?

The biggest lesson of the day is to head home when the road starts to sparkle. In hindsight, the conditions we saw on Shelbourne were an obvious omen.

I ended up in the ditch just past Mt. Doug in trying to miss Marcus, who went down in front of me. Nice soft landing for me, but the smell wasn't so good. How are you Marcus? Did anyone else go down?

I got out at lunch for a spin around the waterfront. Sunny, mild, and on my summer bike. What a difference a few hours can make!

Greg