CSI-WX2: EvapoNose Cooling, Big Thumps

Yesterday I investigated three weird weather facts from Wednesday's powerhouse storm that affected the Midwest and Northeast, specifically us here in Central Pennsylvania. Today I'm investigating the "meteorological crime scenes" of two more.

A friend and coworker wrote on Facebook Wednesday morning:

I'll address the first part first. There was a big weather change at about 9 PM; based on the observations from Clearfield Airport which is closest to you (other stations showed something similar), there was a rapid increase in humidity (24%) and decrease in temperature (3.6 degrees), the latter within about 8 minutes. Here's a graph where I've highlighted in pink the time where I think it happened:

A rapid change like this is pretty unusual. What caused it? Looking at the observations, snow started following suddenly just prior to the change. I remember watching the snow coming in on radar and, though it may have been light at the surface, it was heavy higher up in the atmosphere and the air was very dry at ground level.

As the snow fell through the drier atmosphere and evaporated, it triggered something called "evaporative cooling" which rapidly raised the humidity in the air. In meteorology, when you increase humidity, the temperature must decrease. (By the way, evaporative cooling is what's responsible for the roads turning suddenly slick even if the temperature is steady in the mid-30s. Not only do you have frozen precip hitting the roadway, the temperature has suddenly fallen below freezing).

In your case though, Michelle, the rapid increase in humidity could have caused your sudden stuffiness, or (more likely if you have a well-sealed house) the rapid temperature drop (aggravated by the winds that I remember coming with the snow) cooled your house and triggered your furnace heat. The increase in temperature in your house or even dust particles in the air from the furnace could have caused your noses to go berserk.

As far as the next morning... well, going to bed with a stuffy head is one way to trigger a sinus headache in the morning but I would suspect that the pressure had something to do with it... Pennsylvania's proximity to the huge low pressure system that bottomed out at Category 2 Hurricane Strength in Wisconsin caused the pressure to fall at a fairly rapid rate all Tuesday and Wednesday morning (see graph below or data here).

But more importantly, it bottomed out at 29.08", a pressure so low that it hadn't been seen here in six years! (Feb. 23, 2003 had 29.06"; a storm on April 3, 2009 came in close at 29.09").

The second weird occurrence will not take such a lengthy explanation. This happened at my house Thursday night. Winds had been high all day (enough to have to rescue some Christmas decorations) but had calmed down when my family and I heard several loud thumps during dinner that emanated from our back porch. I looked out the window but couldn't find a cause - like I said the wind had calmed and there wasn't really anything that should be banging against the back of the house. What we did have, however, was a large amount of heavy slush on that porch (we had snow followed by sleet and rain Wednesday) and when I went out to scrape/shovel it off Friday I heard similar thumps while walking on the porch. Knowing that, I took a look at the graphs from my weather station that night:

I believe that the weight of the slush on the porch was unlike anything it had seen since we moved into the house in 2006 - and we had put wood protectant on it this Summer which meant that none of the liquid was soaking in. The sudden jump in Relative Humidity at the time on the graph above may have caused the wood to "pop", expanding or contracting, and the winds starting up again right after that may have caused the wood to shift. Hence the thumps.

Why did the humidity suddenly go up? Same reason as above: Evaporative Cooling. The observations from nearby KUNV airport confirm and the radar confirm that a snow shower came through at that time.

You can see the temperature did drop more during the time of higher humidity (thought not as much as in Michelle's case above probably because of the winds which more effectively mixed the air near the ground).

There were no real crimes here of course, but our Forensics Department investigates (and goes to court) all sorts of crimes and accidents affected by the weather; click on the video link at the top to see some of their recent cases. I'm no Forensics Meteorologist by far; I'm just doing some guesswork here based on the data; other meteorologists reading this can feel free to correct me or offer alternative theories. This is good "proof", however, that weather affects a lot more than you might think.

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