Head in the Clouds

Harvest Moon, copyright 2009 Christine Petersen

Autumn weather is as changeable as a teenager’s moods. On Sunday night a heavy bank of clouds rolled in just as the Harvest Moon began to peek over the eastern horizon. The low pressure system parked over our heads and remained in place for two days, bringing two inches of much-needed rain.

Wednesday dawned fresh and golden. By early afternoon warmer air had caused unusual cloud formations to build up. Altoculumulus undulatus clouds are named for their resemblance to rippling wave trains along the surface of the water. Also known as billows, these clouds form as the result of wind shear. Localized differences in wind speed and direction break up larger altocumulus clouds, reshaping them as narrow cloud-rolls. Billows may be ramrod-straight or form gentle curves, but are always evenly spaced in parallel rows.

I recall seeing billow clouds a few times in my childhood. Back then I found their symmetrical design a little frightening, reminiscent of the ribs of some skeletal sky-giant. As an adult I observe natural phenomena through the rational lens of science. Yet plenty of youthful imagination still tinges my view. Standing in the yard my eye was drawn to the straight line of those clouds, and for a moment I imagined having the ability to fly along that sky-path. But even as I watched, the clouds began to lose their structure—joining then stretching, thinning and breaking apart. Within 20 minutes, the sky was completely clear and blue. And I remained affixed to the ground.

Tentative sunlight at the terminus of the week brought local maple trees to the peak of color. While our neighbors began to spend evening hours raking, trees in our meadow and yard have steadfastly held on to their leaves. A golden glow fills the air at all hours, seeping through the windows and reaching my office as I work. As if I needed another distraction!

But this too shall pass. As I write this in the late hours of Friday night, the drying leaves of a pin oak outside my office window begin to rattle. The wind has picked up from the west. With it, capricious Autumn may bring the first snow of the season.

Announced by all the trumpets of the sky,
Arrives the snow
—Ralph Waldo Emerson, from “The Snowstorm”

Snowy meadow after October snow, copyright 2009 Christine Petersen

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.