Monday, December 7, 2015

Inversions A Wintertime Topsy Turvy Fog And Smog Creator

This visible satellite image taken Sunday morning
and provided by the National Weather Service in
Burlington, Vermont shows low clouds
and fog in the valleys, and clear skies
in the higher elevations. 
Here in Vermont on Sunday, the early morning forecast called for sunny skies during the day with unseasonably warm temperatures.

That forecast proved spot on, especially in higher elevations. Some of the state's valleys, however, stayed foggy, overcast and chilly most of the day.

What happened? It's called a temperature nversion, and they are generally most common this time of year, when the sun angle is low and the days are short.

Normally, the temperature drops as you go up in elevation.  Sometimes, though, there's a layer of warmer air above some chilly air trapped in lower spots, like valleys. The normal elevation temperature regime is inverted. You have an official inversion. Congratulations!

Inversions happen often at night, when cold air drains down into valleys on clear, calm nights. These inversions usually break up quickly in the morning, especially in the warmer times of the year.

That's because the sun comes up, heats the air right near the ground, which starts to rise, which mixes with the warm air above. The atmosphere in these cases sorts itself out, and you go back to the normal circumstances of it getting colder as you get higher up.

This time of year, though, the sun angle is pretty much at its lowest point. The sun isn't able to heat the ground easily, and it's not above the horizon for all that long. Which means it's sometimes too feeble to break the inversion.

That happened in much of the Northeast Sunday, and it was happening again in parts of eastern New England this morning.

For much of Sunday morning, the landscape was shrouded n many areas by dense fog or dense freezing fog.

Freezing fog is merely fog that occurs when the temperature is below freezing. Water droplets that make up the fog stay in liquid form despite the subfreezing tempertures. The droplets stay in liquid form, that is, until they touch something, when the instantly freeze.

The frozen droplets in dense fog cause beautiful rime ice on trees, but also black ice on roadways.  The beauty and the beast, I guess.

On Sunday, mid and high elevations in the Northeast in the warm air above the valley chill  enjoyed a sunny, mild day. The inversion also broke up in some valleys when enough sun broke through to disrupt the inversion.

Other areas stayed firmly in the foggy inversion. Montpelier, Vermont, for instance, had fog or freezing fog pretty much all day Sunday, and the temperature only barely rose above freezing to a high of 34 degrees.

However, Saranac Lake, New York, at a relatively high elevation and normally the coldest reporting station in the North Country, reached a sunny 45 degrees Sunday afternoon.

Another danger from temperature inversions is pollution, especially if the inversion lasts a long time.

I was in Rutland, Vermont on Sunday. The inversion sort of broke for a time, and the fog lifted. But the inversion re-established itself. It was dry enough by then that fog did not re-form.

But with so many people burning wood to heat their homes, and cars driving around and such, you could actually see a layer of brown smog hugging the populated valley floors around Rutland, West Rutland and Proctor.

Believe it or not, considering we're talking about "clean" Vermont, the area around Rutland in particular is prone to air pollution alerts in the winter because that valley is especially likely to get some persistent inversions during the cold season.

Elsewhere in the world, inversions can be more persistent, and last much longer. Salt Lake City, Utah often goes for days and days on end shrouded in fog and smog as inverstions take hold in the Salt Lake Valley.

And the type of dangerous, thick smog that has blanketed Beijing, China recently is also a product of wintertime temperature inversions.  I'm told the smog is back big time in today in Beijing, too.

This kind of thing used to happen in the United States, too. Most famously the industrial town of Denora, Pennsylvania suffered a smog inversion that killed perhaps 20 people and sickened thousands more in the late autumn of 1948.

Here in the Northeast, wintertime inversions usually don't last long. A storm system will come along with a bunch of wind, and the wind mixes up the atmosphere and breaks up the inversion. However, even then, inversions are a risk.

When a storm has a warm front approaching your area in the winter, sometimes onrushing mild air, which is lighter, goes up and over the dense cold air near the surface. The result is dangerous freezing rain.

Calm high pressure is likely to hold over much of the Northeast for at least the first half of this week, so we're likely to see more inversions, more fog or freezing fog, and maybe some smog in spots.

It's one of those cases that if you like sunny, mild weather this time of year, head for the hills.

except in  of the state's all

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