Hot House

"André, check out the roof, it's steaming!" says Dave.

"What?!"

"From the sun on the rainwater," he says. I go see.

Sure enough, when I pull open my shades, a gliding mist is rolling past my window, over the roof which is an arm's reach away, and off every other roof in the neighborhood, dissipating as it crosses over the edge. It's quietly a stunning sight, in no small part because it also provides my first look at my absolutely gorgeous surroundings since I put up that curtain at the beginning of winter.

But I realize as I take in the view that those vapors fringing the rooftops couldn't be caused by the evaporation of water in the sun. For that to be, the roof would have to be heated to 100 degrees Celcius. I assume that heating roofing tar that much could be dangerous, and a quick glance at the EPA Air Toxics Website yields the following:

Roofing tar is composed largely of polycyclic organic compounds (POCs, a subgroup of PAHs (never mind)) that have melting points between about 65 and 175 degrees Celcius, depending on the tar's hardness. The benzo(a)pyrene used in roofing tar is a very nasty chemical when airborne, with concentrations of 1.1 nanograms per cubic metre considered significant, and is found to have chronic, reproductive and carcinogenic effects in concentrations of 6 ppm (80 mg/m³) or higher. Heated enough to boil water on, even if the tar were still well below its melting point, it would pose a severe and long-term health risk to people living in the area. Also, our roof clearly can't possibly be boiling hot, or we'd have more immediate problems than tar-related cancer.

So while I look at the scene my brain riffles through my Physical Geography index and realizes that the mist is actually there because our roof is colder than the air passing over it. By cooling the sun-warmed air slightly, our damp roof causes some of the airborne gaseous water to condense into drops of suspended liquid, which make up the fog swirling by my window. My handy index further reminds me that the same thing causes mist beds over lakes and blanks out mountain passes when chilled alpine air sinks down into warmer valleys. Of course, the transfer of heat from the air to the water on our roof causes some of evaporation too, but that's invisible.

Cool. Also beautiful to see after it rains on a warm day when the air is saturated, the temperature just above the condensation mark.

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Edit: Well, I wrote that a little while ago and took it down because I wanted to edit it more. And I just did. So y'all know, I kicked the ass out of my Physical Geography exam. Finally done with school! 'Till summer school starts! Temporary yay!