Q: Does it ever really get hot enough to fry an egg on the sidewalk?

A: It's highly unlikely. But scientific opinion has never been known to discourage people from trying to prove an age-old urban legend.

When I was a kid in Brooklyn before air conditioning (imagine!), at least one city newspaper would cook up an egg-on-the-sidewalk story sometime during the "silly season"--the dog days of summer, when even bank robbers were too lazy to make news and reporters had little to do. But to my recollection nobody ever claimed to have actually pulled off the egg trick.

Every Fourth of July, the old Mojave Desert mining town of Oatman, Ariz. (pop. 150), holds a solar egg-frying contest by the side of the fabled Route 66. According to Oatman's exalted Egg Fry Coordinator, Fred Eck (note to editor: that's Fred Eck, not Fried Egg), the person who comes closest to cooking an egg in 15 minutes by sun power alone wins.

An occasional egg has indeed been cooked in Oatman, but the rules allow such gimmicks as magnifying glasses, mirrors, aluminum reflectors and the like. No fair, I say. We're talking here about breaking an egg directly onto the ground and leaving it alone.

During the recent heat wave, I determined to find out whether it was possible to fry an egg on a sidewalk without any optical or mechanical aids. I took both an experimental and a theoretical approach.

The experiment

I own a wonderful gadget called a non-contact thermometer. It's a little gun that you point at a surface and pull its trigger, whereupon it instantly reads out the temperature of that surface, anywhere from 0 to 500 degrees Fahrenheit. The so-called MiniTemp, manufactured by Raytek in Santa Cruz, Calif., (call 800-866-5478), works by analyzing the amount of infrared radiation being transmitted, emitted and/or reflected from the surface. Hotter molecules emit more infrared radiation.

Ever since I bought my MiniTemp I've been like a kid with a new toy, measuring the temperatures of everything from my refrigerator, freezer, oven and grill to my wife and cat. (Wife: 97; cat: 88.) I thought it would be an ideal tool for my sidewalk cooking experiment, because I already knew how hot it has to be to cook an egg, and if you keep on reading, so will you.

On several scorching days in Pittsburgh and in Austin, where I was visiting my daughter (my research budget wouldn't support a side trip to the Mojave Desert), I went around measuring the mid-afternoon temperatures of a wide variety of sidewalks, driveways and parking lots, trying not to upset any Texans by looking as if I were pointing a gun.

The ground temperatures varied quite a bit depending, not unexpectedly, on the darkness of the surface. Blacktop paving was much hotter than concrete, because dark objects absorb more light and therefore more energy. So there goes one cherished notion about outdoor egg-fries; you'd have a better chance in the middle of a blacktop street than on the sidewalk.

Although the air temperatures hovered around 100 degrees, I never found a surface hotter than about 125 degrees on concrete or 145 degrees on blacktop (remember that number). In either case, the temperatures plunged almost immediately when the sun went behind a cloud, because much of the infrared radiation coming from the surfaces is simply solar radiation that is being reflected. Bright, shiny metal surfaces, in fact, reflect so much solar radiation that the MiniTemp won't give accurate readings of their temperatures.

Now it was time for the crucial experiment. I had previously taken an egg from the refrigerator and warmed it to room temperature. I cracked it directly onto the 145-degree surface of an asphalt-paved parking lot at noon on a 100-degree day. I didn't use cooking oil, which might have cooled the surface too much. Then, I waited.

And waited.

If you don't count the odd glances I received from passersby, nothing whatsoever happened. Well, maybe the egg white became slightly thicker at the edges, but there wasn't anything remotely resembling cooking. The surface just wasn't hot enough to cook an egg. But why not?

The theory

First of all, only the white of the egg, or albumen, was in contact with the hot surface--the yolk floats on the white--so it's a matter of what temperature might be required to cook the albumen. And what do we mean by "cook," anyway? Egg white is a mixture of several kinds of protein, each of which is affected differently by heat and coagulates at a different temperature. (You expected a simple answer?)

But in an eggshell, it boils down to this: Egg white begins to thicken at 144 degrees, it ceases to flow at 149 degrees, and it becomes fairly firm at 158 degrees. Meanwhile, a yolk will begin to thicken at 149 degrees and lose its fluidity at 158 degrees. So to "cook" an entire egg to a non-runny, sunny-side-up condition, you'd want both the white and the yolk to reach 158 degrees and to stay there long enough for the rather slow coagulation reactions to take place.

Unfortunately, that's hotter than any reasonably attainable ground temperatures. But more important, when you break a 70-degree egg onto the 145-degree ground it cools the surface down considerably, and there is no continual replenishment of heat from below as there is in a frying pan over a fire. Also, pavement is a very poor conductor of heat, so none can flow in from the surroundings. Thus, even though a parking lot's black surface might get close to the coagulation temperature of 158 degrees on a really, really hot day, I'm afraid that actually cooking an egg on a sidewalk must forever remain but a midsummer night's dream.

But wait! In Austin I measured the roof of a sun-baked, dark blue, 1994 Ford Taurus station wagon at 178 degrees, which is more than hot enough to coagulate both white and yolk. And because steel is a good conductor of heat, that temperature could be maintained by heat feeding into the egg from other parts of the roof.

Do you suppose . . . ?

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh and the author of "What Einstein Didn't Know--Scientific Answers to Everyday Questions." Send your food or cooking questions to wolke@pop.pitt.edu.