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Because the cold air drains into the river valleys, the almost daily morning fog sits as this misty, foamy sea along the rivers, while the hills and mountains rise above it. 

“The fog was where I wanted to be. Halfway down the path, you can’t see this house. You’d never know it was here. Or any of the other places down the avenue. I couldn’t see but a few feet ahead. I didn’t meet a soul. Everything looked and sounded unreal. Nothing was what it is…as if I was the ghost belonging to the fog, and the fog was the ghost of the sea. It felt damned peaceful to be nothing more than a ghost within a ghost.”

- Eugene O’Neill, Long Day’s Journey into Night

As September rolls around, its multitude of characteristics become wonderfully evident.

The leaves on the trees, perhaps giving us a hint in August, lose their summer green, revealing the autumnal hues of yellow, orange, red, and gold, accented by a few paler pinks and purples, to contrast the ruddier crimsons and warmer browns. The gardens, already rewarding us through the summer with greens, peas, summer squash and the like, now climax with their bounty of longer-season tomatoes, corn, and winter squash. School might have opened in August, but the daily routine settles in during September, from the morning challenge of lunches and homework to the sounds that ring out at the close of the school day.

And where is the weather in all of this? You’ll find it, perhaps hidden at times, in the fog. Fog begins the morning more often in September than any other month, at least on the records at the Fairbanks Museum. Using data for the past six years (when our records became digitized, allowing for greater analysis), Chart 1 shows the monthly fluctuation of relative humidity, as well as cloud cover, for observations done at 7 a.m. Fog produces relative humidities near or at 100 percent, which goes for the cloud cover as well. It spikes each year on, or very close to September. The neighboring months of August and October share many of these foggy mornings, occasionally being September’s equal, all related to why this time of year brings fog nearly every morning.

While I know you recognize fog, it is helpful to know a couple of things about it. First, fog consists of tiny, microscopic droplets of water, averaging 0.0004 inches in diameter. They are small enough to remain suspended in the air and are numerous enough to reduce visibility. By definition, fog technically results in visibility of a half-mile or less, though we often describe conditions as “light fog” when you can see more than a half-mile. Secondly, fog is a cloud, with its base near or at the ground. If you were standing on the summits of mountains while they are lost in the clouds, it would appear as fog.

Fog forms in two basic ways, and thus there are two primary types of fog. The fog that we’ve described as part of September’s weather, is called radiation fog. No, it has nothing to do with nuclear radiation. The term refers to the thermal energy, or heat, of the atmosphere, radiating out into the atmosphere above, and eventually out into space, something that happens nearly every night after the sun sets. The cooling process is affected by several factors, most importantly wind and cloud cover. Wind helps the atmosphere to remain mixed and decreases the amount of radiational cooling. Clouds act like a blanket, slowing or stopping the heat energy from escaping to the upper atmosphere. Both tend to keep temperatures warmer, reducing or eliminating the formation of fog. The other common type of fog occurs when warm, moist air travels over a cold surface, most often ice and snow, though cold water will work as well. This goes by the name advection fog, perhaps most common locally during the late winter and spring, as the old snow lingers, while warmer spring air makes a few visits.

Radiational cooling is most effective, on clear, calm nights. As the cooling takes place, the colder air becomes denser and thus heavier than the warmer air that surrounds it. This heavier air follows the natural terrain, flowing downhill, and through ravines and along the drainage of smaller brooks and streams, settling into the deeper river valleys, and broader basins where lakes and ponds are found. The other ingredient that goes into the formation of fog is water. The water exists as invisible water vapor, a part of our atmosphere that varies considerably over time, as well as from place to place. As the air gets colder, the invisible water vapor in the air begins to condense on microscopic particles in the air, tiny fragments of materials like dust, soil, smoke, and sea salt. This forms countless microscopic water droplets, or specifically cloud droplets, a process that continues until the sun finally rises, and stops the cooling process.

September finds this combination of cooling and moisture at its annual maximum, though it extends from early to mid-August, through mid to late October. Because the cold air drains into the river valleys, the almost daily morning fog sits as this misty, foamy sea along the rivers, while the hills and mountains rise above it. We can see this both from local views looking from the hills down into the valleys, as well as from satellite images, where the drainage systems can be quite evident (see figures 3 and 4) As far as cooling is concerned, September’s longer nights increase the amount of time for the air to cool, and thus increases the potential for fog. Naturally, that continues into October, however, water vapor experiences a specific change through October. The leaves fall. Leaves actually produce a significant amount of water vapor through part of their process of turning sunlight into energy, a phenomenon called transpiration. As the leaves shut down in October, briefly offering a radiant display of our fantastic autumn colors, the moisture decreases significantly. One other factor in October comes from the increased cloud cover, which can limit radiational cooling. Foggy mornings become less common during the middle and end of October.

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Foggy September mornings have any number of consequences. I remember long-time West Burke observer Gil Ford mentioning to me that we sometimes lost half a nice day in St. Johnsbury during the foggy starts. For those driving to school or work, extra caution is a must, with young school kids walking to school, or just waiting near the side of the road for the school bus. The moisture really slows down the drying process, becoming a challenge for those last cuttings of hay, and any painting projects you would like to wrap up before we get deeper into Autumn. Curiously, the formation of fog brings the cooling of the night to a halt, often preventing early frosts in some of the larger river valleys. And for drivers, the reduced visibility impacts the time it takes to get to your destination.

The fog, of course, is patient and will take its time to gradually burn off, welcoming a sunny rest of the day.