Answer Man: Why the warnings about bridges and overpasses?

News-DemocratMarch 18, 2014 

How does pavement temperature correlate to air temperature? I mean, if it's been above freezing and drops below, how much time does it take for the pavement to also drop below? They're always warning us about bridges and overpasses so that pavement must correspond to the air temperature fairly accurately. Just curious.

-- Mary Heeren, of Breese

Trying to correlate the temperature of the air with that of the road surface can drive you to exasperation because it depends on so many things: recent and current weather conditions, surroundings, pavement material, traffic, etc.

I can give you a perfect example. My neighbor and I share what I call a Mount Everest of a driveway. His third is black asphalt; my part is concrete. Despite the same temperature, snowfall, shoveling and subsurface ground, guess whose driveway always melts first while I leave my car parked at the bottom and cross-country ski up to my house?

It's those same variables that makes nailing down a precise answer to your question impossible. Yet it's an important consideration that numerous state departments of transportation call to drivers' attention on their websites. Here's about the best they can offer:

In the fall, the subsurface likely will remain above freezing for a time even when the air temperature initially drops below 32 degrees. Often during the autumn, the cooler air cannot offset the warmth being radiated from the subsurface so snow may melt when it hits the road.

This is where drivers often get into trouble. They'll forget that bridges and overpasses don't enjoy this subsurface warmth. The below-freezing air is blowing over, under, around and through. As a result, the temperature on these surfaces drop rapidly to match the air temperature. So drivers will be zipping along safely on the road only to crash as soon as they hit the frozen bridge deck.

It may be just the opposite in the spring. Temperatures may suddenly bounce into the 40s and 50s, but it still cannot transfer enough heat to the road surface to offset the colder temperatures underneath. Now, the road may be more dangerous than the bridges because they react more quickly to the temperature climb.

So, as you probably guessed, there is a time lag both ways before road surfaces drop or climb to meet changing air temperatures. But, as I said, that's only a small part of the story.

For example, several days of sunshine could heat the surface significantly above the ambient air temperature as opposed to a week of thick cloud cover. Typically, the darker the pavement (like my neighbor's driveway) the warmer the surface temperature will become. Heavy traffic, the sun's angle and nearby shade trees or buildings can influence surface temperature, too.

"It is nearly impossible to predict what maximum or minimum surface temperatures will be reached without using some very expensive equipment and very complex algorithms," Dale Keep wrote in the September 2011 issue of Ice Management.

That's why, as state road officials say, it's best simply to keep track of surface temperatures if you can because air temperature is usually a poor indicator of what the surface temperature is. Iowa, for example, has an online map that shows both road and air temperatures at dozens of sites. At 2:30 p.m. Tuesday, I-280 near Bettendorf had a surface reading of 65 degrees while the air temperature was 48.

One final interesting statistic from Keep, who owns Ice & Snow Technologies in Walla Walla, Wash: At 30 degrees, a pound of salt will melt 46.3 pounds of ice; at zero, it's only good for 3.7 pounds.

I know many burning candles can leave carbon footprints in a room, so I hesitate burning them. Over Christmas, I bought a package of Air Wick Cinnamon Roast Chestnut Oil. But calling the company for an answer to my concern of where the oil goes, I got no answer.

-- Bonny Litteken, of Highland

I hope you don't think me a company shill, but theirs is the only information I have to go on and I found nothing to the contrary on the Internet.

"When used in accordance with label directions our candles do not leave behind a residue," Nora Flynn, of Reckitt Benckiser, wrote me.

Today's trivia

Which retired pro football player do you suppose pulled in the highest income last year?

Answer to Tuesday's trivia: Like Crest and Colgate, President Lyndon Johnson's gift-giving often would have received the American Dental Association's Seal of Acceptance. According to the LBJ Library, Johnson kept giveaway items in the Oval Office -- including electric toothbrushes with the presidential seal stamped on them. Rumor has it that Pulitzer Prize-winning historian Doris Kearns Goodwin received five of them during her tenure as a White House intern. When she finally asked the reason for them, LBJ reportedly told her, "I want people to think of me right away when they wake up and right before they go to bed."

Send your questions to Roger Schlueter, Belleville News-Democrat, 120 S. Illinois St., P.O. Box 427, Belleville, IL 62222-0427 or rschlueter@bnd.com or call 618-239-2465.

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