Kevin Myatt is The Roanoke Times' weather columnist.

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The last couple of days have reminded us of something familiar that's been missing much of this summer: haze.

You probably didn't miss it at all because it's something to be forgotten: Milky gunk blotting out our glorious mountain views. Haze has little or no redeeming value. From the top of Roanoke Mountain at mid-morning on Wednesday, I could not make out a single mountain ridge. Usually, you can see all the way to Apple Orchard Mountain more than 20 miles away.

We tend to think that haze is something mankind invented in the 20th century, but that's not really true.

Haze forms naturally in summer sunlight and stagnant air. The Blue Ridge and the Smoky Mountains gained their names from the effects of haze long before there were power plants and sports-utility vehicles. Animals and plants give off various gases and particles that drift into the air and can be acted upon chemically by sunlight. Throw in dust, moisture, smoke, maybe even a little volcanic ash now and then, and you have a recipe for haze.

But, as with many things, the pollution we create in our modern lives has compounded what nature is already doing.

Haze is not synonymous with ozone, though ozone is often a major component of haze. Ozone forms from a complicated chemical process beginning with nitrogen monoxide, created in large quantities by car exhaust. The nitrogen oxide takes on another oxygen atom and become nitrogen dioxide, which in turn has an oxygen atom split off by the sun's ultraviolet radiation. These loose oxygen atoms are then attached to standard oxygen gas molecules (composed of two oxygen atoms). The triple-oxygen molecules that result are ozone.

Ozone has been linked to breathing problems, so that's why many metropolitan areas -- including Roanoke -- are making a concerted effort to reduce ozone by urging citizens to take various measures to limit car exhaust during "alert" periods. Roanoke, though not a producer of ozone on the magnitude of Washington, D.C., Norfolk or Richmond, is in a bowl-shaped valley that traps what ozone is formed, so our problems can magnify more quickly than they would if we were in open plains.

Ironically, we owe our very ability to live to ozone, but not what gets made at the surface. The ozone layer high in the stratosphere blocks out harmful radiation. Different processes are thinning that layer, a topic for another day.

Our surface haze also contains smoke, industrial exhaust and power plant by-products from upwind areas like the Ohio and Tennessee valleys.

All of this stew coagulates at the surface when high pressure takes hold of our weather at the surface and aloft. High pressure areas feature large zones of sinking, stable air. This allows little wind to stir the air, little rain to wash it out, and lots of sunshine to cook the chemicals. All of that mess drifts to the surface, wrapping around our hills and dales.

For most of this summer, we have been missing the big high pressure dome overhead, so we have not had those long hot, hazy stretches. Instead, we've had frequent storm systems and fronts moving through, a good amount of rain, and strong winds aloft to keep things stirred up. But now, in late August, we finally do have a modest high pressure at the surface and overhead, so haze finally has its hour to strut upon the stage.

A couple of notes

>Keep an eye on Hurricane Frances, far out in the Atlantic now. With high pressure developing north of its track, this storm could be shoved west toward the U.S. in about a week. But upper-level winds at the edge of this high may also be too strong for a lot of intensification. We have plenty of time to watch it. Don't cancel any coastal plans yet.

>Reading John Goss' astronomy column on Wednesday, I realized that I had made a mistake in my last column. Astronomical autumn begins on Sept. 22, which is a day later than I had written. But meteorological autumn still begins Sept. 1, which is Wednesday.