Re-Tracing February’s Arctic Air Mass and Record Cold
It took 89 years, but as the headlines have reported, on Feb. 4 Mount Washington Observatory managed to tie its all-time record low air temperature of -47 °F, originally set in January 1934. Observer Karl Philipoff documented the harrowing and historic day in a previous observer comment, and while I will probably think about my experiences that day for the rest of my life, my aim with this post is to take a look at the meteorology behind this event.
To start, we need to look up, way up, to the top of the troposphere, or more specifically, the tropopause. The tropopause is the boundary between the stable stratosphere and the lower, more turbulent troposphere. During the warmer months, and in tropical latitudes, the tropopause is much higher due to the atmosphere being much warmer. In the tropics, the tropopause may be as high as 11 miles.
Meanwhile, at the frigid poles, the tropopause is much closer to the surface at a height of around six miles. I mention all of this because it helps to establish some context. Below is a map of the continental U.S. with an overlay of something called the 2 PVU surface. In short, the height of the 2 PVU surface is more or less the height of the tropopause.
Meteorology is a bit like golf, where the lower your pressure value is, the higher in the atmosphere you are. In this image you can see tropopause height values that range from 30 milibars (mb) in the tropics to 690 mb over the White Mountain region. For reference, the summit is located at 6,288 feet or around 800 mb. In essence, this map shows us that the tropopause was located 110 mb or about 4,000 feet above the summit on Feb. 4. To get the troposphere that low, it has to be very cold.
The next image shows just how cold temperatures were around 850 mb (5,000 feet). Those fuchsia colors may look beautiful, but they represent temperatures lower than -40 °C. Cold of that magnitude is typically found high in the Arctic where the polar vortex usually spends its winters before breaking down in the spring.
Feb. 3 and 4, however, were not ordinary days for both the summit staff and the polar vortex. On this day, the vortex was a long way from its typical home and was displaced southward into the middle latitudes. More accurately, this cold air outbreak was the result of a piece of a much larger vortex breaking off and being pushed south due to a warmer Arctic and weaker jet stream.
Fortunately, we are able to trace the journey of this exceptional airmass. By using what’s known as a reverse trajectory, we can see where air masses originate. The following map tells an interesting origin story for the air that brought us our record cold.
By running the model back four days, we can see that the nursery for the airmass was none other than the high Canadian Arctic. Over the course of four days, the airmass traveled from Salisbury Island to New England. This translated to a 1,300-mile cross country journey. The half-loop path that the air took was the result of a powerful storm system that helped to dislodge cold air to the south, and eventually over the Northeast and the White Mountain higher summits.
While we can definitely confirm that we tied our record low temperature, there is still a lingering question that we have gotten from hundreds of people. Did the summit make it into the stratosphere during the Feb. 3-4 storm? Did we as observers smell Ozone when we went outside in the grueling conditions?
The short answer is no. For starters, the stratosphere contains the highest ozone concentrations of any layer of the atmosphere. If we were indeed in the stratosphere, then we would have noted an increase in the gas. Fortunately for us, the New Hampshire Department of Environmental Services (DES) maintains an ozone monitoring site on the summit. Their data for the time period in question shows something surprising.
During the coldest part of the two-day cold snap, Ozone levels actually decreased. This dealt a critical blow to hopes that we were briefly in the second layer of Earth’s atmosphere.
A weather balloon launched by the National Weather Service in Gray, ME (about 80 miles southeast of the summit) showed that the tropopause was located around 795 mb while the summit was at 777 mb. So, maybe there was a chance after all, right?
Well the story is a bit more complicated than that. The higher summits of the White Mountains do a lot to stretch and uplift air as it flows up and over the terrain. On this day, winds were blowing from the northwest, and up and over the summits. We suspect that this lift may have pushed the tropopause higher over the immediate summit area. From here, it quickly fell in elevation with air sinking on the leeward side of the Whites.
From record-setting cold to exceptionally low tropopause heights, this event has reshaped me as a forecaster and a person. I will never forget the dedication of my humble team and their willingness to go above and beyond when weather was at its most extreme. Our work up here is humbling, rewarding, and rarely easy, but I feel lucky to have a front row seat to nature’s extremes. If you enjoy sharing this ride with us, please consider supporting the Observatory.
Francis Tarasiewicz, Weather Observer & Education Specialist