On the chilly Thursday morning, as I got on my bicycle, I noticed some frost on the grass even though my thermometer read 36F. How could this be? It turns out that such a situation happens all the time and understanding this effect can keep you safe while driving and biking.
The evening before the skies had been quite clear (see satellite photo)
Clear skies allows the surface to cool off rapidly be emitting infrared radiation (the atmosphere does the same thing but is not as effective). The long nights of the fall season helps, as does light winds...and we had them all that night. The result? The surface cooled more than the atmosphere above, producing a surface-based inversion (an "inversion" of the normal situation of temperature cooling with height)--see the figure below.
On such clear nights a large difference in temperature can occur in the lowest few feet of the atmosphere, with ground sometimes 1-5 F cooler than the air at the official height of temperature measurements, which is at 2 meters or roughly 6 feet. I have walked around my house at night with a thermometer to demonstrate the effect (my neighbors must think I am a touch crazy) and found cases when my shoes are 5F cooler than my chest! So when a TV weathercaster gives you the surface temperatures they ARE NOT at the surface, but rather at 2 meters! Thus, the temperatures at the surface can be substantially cooler than the official or other reports.
So if you hear the weather report and the temperature is 35F and we are in one of those low-wind, clear situations, frost or ice IS possible at the surface and you better be careful. Many cars have thermometers that provide the outside temperature on your dashboard. These sensors are a few feet up and can be warmer than the surface for the same reason as noted above. So if your car thermometer says 33 or 34, there could be frost at the surface.
Now there are other complications of course....after a warm period, heat from below the surface can keep the surface above freezing on these nights--but I would not depend on this. And bridges have no heat coming up from the ground below and they are generally the first places to have frost or ice.
And for those of you thinking that a little ice or frost at the surface is nothing to worry about, consider this. Roadway icing undoubtedly kills and injures more NW citizens than ANY other weather phenomena....windstorms, floods, hail, tornadoes, you name it!
If weather is going to take you out it will probably be some roadway ice somewhere!
We are now entering the core of the roadway icing season...so be careful! WSDOT is quite knowledgeable about this threat and pretreats some roadways to prevent ice formation, but they can't be everywhere, so you have to protect yourself.
If you want to learn more about this demonic threat, check out my website on roadway icing: http://www.atmos.washington.edu/~cliff/Roadway3.html
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