Most of Saturday was cloudier than expected, especially over northern Vermont. While the view of the clouds from the ground was kind of dull and gray, fascinating stuff became apparent on satellite imagery, especially for those of us who are weather geeks.
The satellite photo of the clouds is in this post for your reference. It's a little hard to see what I'm talking about, so click on the photo to make it bigger and easier to discern.
So here's the set up: An incoming large high pressure system trapped moisture in some of the lower layers of the atmosphere. Usually, in the warmer seasons at least, uneven heating helps mix drier air downward from up above, and the clouds dissipate.
The high pressure was too strong to allow much of that mixing to happen during most of the day, so we were stuck under the clouds that were only about 1,000 feet thick. But thick enough to block out the sun.
In a way, this could be a sign of the season. In the winter, when the sun is much weaker and lower, the heat doesn't make it through the clouds to mix the atmosphere to ultimate chew up the clouds.
That's a big reason why it's so often overcast in Vermont during the winter, especially between November and January, when the sun is lowest.
Perhaps the sun would have broken through earlier Saturday afternoon if it were June or July, not August, when the sun is beginning to wane.
Now, the really cool part of this satellite image is that very narrow band of clear skies, stretching southeastward in the satellite photo from around Montreal into north central Vermont near Jay Peak and on into the Northeast Kingdom. After this photo was taken, the narrow clear line continued moving southwestward and broadened a little bit.
Also note the clouds seem a little thicker than elsewhere just to the west of that clearing band.
I asked for help from the National Weather Service in South Burlington and as always, they got back to me right away with an explanation. I'll do my best here to relay what they told me.
At the time the satellite photo was taken, a weak weather boundary was moving southwestward from eastern Quebec. It was essentially a mini, weak cold front.
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| A "dry slot" of mostly clear skies behind a cold front is seen in eastern Nebraska, Kansas and Oklahoma in this old satellite photo. |
As the cold front approached, it provided a little lift into the atmosphere. The lift encouraged more cloudiness, which is why those clouds were thicker just to the southwest of that skinny clear line.
Behind this "front," there air sinks. The boundary was basically like a wave coming ashore at a beach. As the wave comes in, the water rises, than falls again as the wave passes. Behind this boundary in northern Vermont, the air was sinking.
Sinking air can warm the atmosphere and that is often death to clouds. That's probably why there was that narrow band of clear skies. Once that narrow air of sinking air passed, the moisture in the atmosphere was able to reassert itself, and the clouds once again formed in the far Northeast Kingdom and southeastern Quebec.
You see this kind of thing on a much larger scale with strong cold fronts, especially in the late autumn, winter and early spring. These much bigger versions of what we saw in that Vermont satellite photo are usually associated with strong storm systems, not weak, tiny little things that we had affecting us.
Lift ahead of these big cold fronts can often create a solid band of showers and thunderstorms. Behind the front, the air sinks, and you get a band of mostly clear skies known as a "dry slot."
As the narrow line of clear skies continued southwestward across Vermont during the mid-afternoon, it gradually widened, then the clouds over the entire region broke apart. The August sun finally did it's job, mixing the air and melting away that overcast. It was a gorgeous late afternoon and evening.
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