Pull Over, Ma’am

Thankfully, those words—”Pull over, ma’am”—are not the reason I find myself on the shoulder of Interstate 35W at 7:00a.m. Nor is my action the result of car trouble. A different voice has caused me to stop the car on this Sunday morning-quiet stretch of highway. I have heard the call of beautiful scenery.

I resisted as long as I could. I’ve been driving through a pervasive layer of ghostly ground fog, which hovers like thin smoke over farm fields and wetlands. Streamers of water vapor dance slowly across lake surfaces. Thick white pockets pile up wherever the landscape forms a shallow basin. Now the sun has risen a few degrees above the horizon. Light is scattered through the layers of fog, filling the sky with peach-colored haze. I’m grateful for the lack of traffic so I can safely grab my camera from the back seat and shoot half a dozen frames.

FoggySunrise2009

Over the past week or more I have noticed the splotches of color touching maple trees in my neighborhood. Ground fog is the second clue I rely on to confirm that we are shifting from summer to autumn. This time of year the sun sets noticeably earlier and rises later. The days remain warmish, but in the prolonged hours of darkness Earth’s surface cools rapidly by radiation. As heat flows away from the land, air near the surface also begins to cool. When air temperature reaches the dew point, the particles of water vapor within it condense to form droplets of liquid water.

Yet only certain environmental and atmospheric conditions produce ground fog. Aside from radiative cooling, the critical factors are clear and windless skies. On cloudy nights, a larger percentage of outgoing longwave (infrared) radiation is held close to the surface. The air stays warm, so fog can’t form. A clear sky allows radiation to escape the atmosphere unimpeded. When breezes whip up, they push water droplets aloft to mix in the atmosphere. On still nights water droplets huddle into mini-clouds that remain close to earth. Ground fog usually evaporates as air warms in the hours after sunrise.

Back on the road, orange sunlight floods my car through the passenger window. On either side of the highway fingers of fog swirl upward, waking at the touch of the sun. A new day has begun. Autumn approaches.

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