Saturday, November 13, 2010

A Weather Fact That Can Save Your Life

NOTE: I will be posting about snow and cold tonight...bottom line: no or very little snow in the lowlands. Media hype has begun, so beware!

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

16 comments:

lamont said...

Can evaporation/sublimation also cause cooling of water on the ground by carrying heat away and result in icing when the air is otherwise slightly warmer than 32F?

Karin Corbin said...

Under these conditions bridges often have the most ice of all the road areas.

Mark said...

Cliff - I ride a motorcycle so I face the same dangers as you on your bike, but at much higher speeds.

My home is at 850 feet with much of my riding is done closer to sea level.

So on those days where the ground is cooler than the air as you describe in this post, what is the thermal gradient over say 1000 feet?

I guess what I'm looking for is the total temp differential due to elevation and the ground proximity radiation cooling so I can establish a minimum temperature for riding. Your post seems to suggest that there can be a 5 degree differential between the ground and 6 feet. I'm wondering if there can be an additional 3-4 degrees of elevation difference which would suggest that 40 deg air near sea level could lead to freezing ground temp at elevation.

Brad said...

Different materials have different freezing points as well, cement roads seem to ice up faster than tar pavements, and steel, plastic and glass all frost over at different rates. Montlake Bridge is a good example where I lost control from ice on the bridge but there was none on the road.

Wx Enthusiast said...

Brad- it's not that different materials have different freezing points; the freezing point for the water on each surface is the same. The fact that ice forms more quickly on some surfaces than others is because of the conductivity of the surfaces. Montlake Bridge, for example, is a metal drawbridge. Metal has a much higher thermal conductivity than some other substances such as asphalt, so heat is transferred much more quickly to and from metal. Metal is a very poor insulator. You probably have experienced this before - especially if you've tried licking a metal pole when it's cold outside. ;)

Anyway, back to the metal bridge - the metal is already on the cold side, having had heat escape from it quickly to the air, much faster than asphalt road surfaces. Heat then flows quickly from any water on the metal's surface (or even water vapor in the air just above the surface) to the metal itself - because of its good conductivity - and when the water loses enough heat energy, it freezes. This happens much faster on metal than other surfaces precisely because of its much higher thermal conductivity. Other substances (glass, plastic, cement, as you mentioned) have varying conductivities as well that cause water to freeze on them at different rates.

So... as Karin mentioned, bridges freeze first, because there is no warmer ground below them to help insulate them, only the colder air that can surround the road surface on all sides. But extra care should be taken on metal bridges, such as the Montlake Bridge, as there are two things contributing to much quicker freezing of water than on most asphalt roads - the lack of insulating ground below the bridge, and the high thermal conductivity of the metal itself.

John Franklin said...

Cliff:
You state that a surface-based inversion is "an 'inversion' of the normal situation of temperature warming with height". I was taught (and recently heard a a TV weatherman say) that an inversion was when temperatures increased with increased altitude, which is unlike the normal condition of temperature decreasing with height. This would make a "surface-based inversion" simply an inversion caused by conditions at the earth's surface.
Your definition appears to be different than the one I learned but perhaps that is because I couldn't afford the tuition at UW.
Thanks for any clarification.

Joel Levin said...

Timely post. The long range forecast predicts cold, cold, cold.

Cliff Mass Weather Blog said...

John...sorry..that was a typo..fixed...normal situation is when the air cools with height. Need another cup of coffee!.thanks...cliff

Dan McShane said...

So far I have seen ice on the surface of the ground and other objects three times this fall with official temps that were on the order of 35. That was the case on Friday in Bellingham.
I have also observed this condition on snow fields at warmer temps. The air may be in the upper 40s, but the snow is ice hard.

Chicken Pilot said...

Demonic threat? Come on now, it is just mother nature. Learn to live with the planet and adverse weather does not seem so "demonic". I rode street bikes yr round. You just need to adjust.

linda said...

NWS.... says SNOW on thursday.

Gary said...

Regarding temp decrease with elevation:
I have hiked in the mountains for many years, and a useful number I learned early on is the "adiabatic lapse rate", or the rate at which temp of dry air decreases with elevation. This value is roughly 5.5F/1000' (9.8C/1000M). (Wikipedia has a good writeup on this.) So, at 850', the temp drop due to elevation would be about 4.5F. -- Gary

Gary said...

Re: Rate at which surfaces ice up: Two other factors to pay attention to are the color and texture of the surface. Both affect the rate at which a solid will give off (and absorb) radiation. Darker-colored substances radiate faster than lighter-colored ones, so that blacktop tends to form ice faster than lighter concrete (under the conditions Cliff is describing). And rough surfaces (I think) radiate faster than do smooth ones.

Check out "blackbody radiation" for another application of this phenomenon. -- Gary

mle_ii said...

Any thoughts about the weather that's supposedly coming the next week or so. Snow at lower elevations? Wondering if I should put on my snow tires yet (not studded) for traveling down to Oregon next week for Thanksgiving.

Thank you,
Mike

Corie said...

Wind gusts in Olympia tonight have power lines down along Littlerock Rd. Power went out here for a few seconds and has flickered off and on.
Officially, NOAA is saying gusts of 25 at the airport ( but who lives at the airport, as George Carlin used to say). It is always gustier along the foot of the Black Hills.

Christopher said...

Strength of Monday night winds in San Juan Islands was seriously underestimated. About 24 hours ago, max gusts were predicted at under 30. Then they bumped them up to 37, then on Mon afternoon to 47, and finally at 8:07 they issued a high wind warning of gusts to 60. How is it they were still so far off so close to the event?