|This photo shows an inversion trapping pollutants|
in the Salt Lake City valley, while the air is
clear above the inversion. Photo from the
National Center for Atmospheric Research.
Very frigid. Among the coldest mornings of the winter.
Meanwhile, up at the summits of those mountains those freezing skiers wished to go to, temperatures were in the upper single numbers and teens above zero.
In Whiteville, NH. this morning, it was 21 below. The 6,000-foot summit elevation of Mt. Washington was 12 above.
In the valley at Morrisville, Vermont, the temperature at 7 a.m. was 21 below. A dozen or so miles away within easy sight of Morrisville, the summit of Mount Mansfield was a full 30 degrees warmer at 9 above.
Usually the opposite is true. The higher in elevation you go, the colder it gets. Typically, it's roughly six or seven degrees colder for every 1,000 feet or so you go up in elevation.
That's why this morning's weather phenomenon is called an "inversion." The normal temperature pattern in the atmosphere was inverted.
So why were things so topsy-turvy in the Great White North this morning?
The short answer: An area of Arctic high pressure that had settled directly overhead in northern New York and northern New England last night.
Cold air is heavier than warm air. When the Arctic air moved in Monday night, it undercut and displaced warmer air, so that the heavy cold air lingered in the valleys while the warm, or at least warm-ish air stayed above.
At night, air in the valleys gets even colder at night without the weak heat from the winter sun, so the inversion intensifies.
That's why it was so cold in the valleys compared to the mountain summits.
The inversion has been in place since early Tuesday morning. In the summer the heat of the sun usually breaks up inversions. The sun heats the ground, the air near the ground warms, starts to rise, causing air up higher to start mixing down toward the valleys, and pretty soon things go back to normal, with colder air up high.
In the winter, the sun's heat is weak, so it doesn't heat the ground much. Plus, snow cover reflects what little heat comes in from the sun back up to space, so that keeps the sun from working well. The inversion holds firm.
This is why inversions are more often than not a winter type thing.
Inversions also trap pollutants. Smoke or particles or anything else get blocked from blowing away by that warm air up above. The warm air layer acts like a lid. So the pollution sits and gets thicker and thicker, especially in valleys.
You don't think of Vermont as the Capital of Air Pollution, but there was an air pollution alert for the area around Rutland, Vermont last night and this morning. Rutland basically sits in a bowl, surrounded by mountains.
Wood heaters, cars, trucks and whatnot released particulates into the air, which got trapped in Rutland's valley.
Pollution from inversions can be a big wintertime problem in bigger cities in deeper valleys surrounnded by bigger mountains than those in the Northeast.
Salt Lake City, Utah has problems like this frequently in the winter.
If the air is moist, dense fog will often form in the valleys while it's sunny in the mountains. That happened across northern New England in mid-December during a period of calm weather after a snowstorm.
An inversion usually breaks up when any kind of storm system starts to move air around. Another cold front is approaching the Northeast, and that will start to break up the inversion later Thursday and Friday.
We'll at least temporarily go back to frigid mountaintops and slightly warmer winter valleys.