This morning's low on Black Cat was -17 F. This is the coldest reading since probably about 1994... though my records don't go back that far. I'm relying on memory.
Take a look at the station's graphs on
www.blackcatnh.com/weather (scroll the page down.) I've never seen this...
Look right around 7:30 this morning. All the sensors indicated a drastic change in just a few minutes.
All through the night it was getting colder gradually, reaching 3 degrees at midnight (yesterday's low was 2 at 1130 pm.) It wasn't getting cold quickly because we had a wind all night long, generally from the NW. In order for radiational cooling to take place you need to have calm wind so the heat near the earth can just rise up like heat does, and go bye-bye. Wind keeps it from doing that.
All of a sudden, right around sunrise this morning, the wind dropped off to calm. The wind speed graph doesn't show it as well as the wind direction graph... wind direction is clustered around NW all night, then suddenly becomes a fixed line on SE. In clear weather, SE is the 'default' wind direction on the open lake... the fair weather wind. That's what happens when it's calm everywhere else. The SE wind wasn't strong enough to push the wind vane back and forth like the NW wind had done all night long... hence the straight line on the 'wind direction' graph. The NW wind had pushed the vane back and forth, creating a clustering effect on the graph. So the wind was only strong enough to push the cups around but not the vane.
With the wind suddenly at calm, and an arctic air mass in place overhead, suddenly the heat took off skyward and changed places with the cold air at altitude (which is probably what caused what little wind we had, from the SE... it was probably the cold air from aloft falling into the lake's valley.)
You'll see the temperature graph suddenly drop about 17 degrees in just a few minutes.
At the same time, the barometric pressure rises equally fast, in response to cold, heavy air suddenly being thrown on it (in this way, the barometer is like a scale that weighs air.)
If just one graph had done this sudden change, I would've said I had a sensor to fix. But all of them did it, all at the same time... and you all verified the morning's extreme cold in your postings here. At midnight I thought for sure that -7 was about as low as we'd go here - it just wasn't falling fast enough and there was too much wind.
...factoid...
When an arctic air mass is in place but the weather is windy, the cold air is all at altitude (summits.) Calm conditions allow the cold and warm to trade places, like we know they want to (like when you open a window upstairs and feel the draft coming down the stairwell.)
This is what just happened on a large scale here. What's interesting is that it is a real-life illustration of the 'blast freezing' that took place in "The Day After Tomorrow." While what happened in the movie was so big that it's unlikely (Hollywood makes movies not documentaries), many scientists did argue that the science in that movie was theoretically possible... and what just happened here (assuming the weather station itself measured correctly) just illustrates it.