By Chris Martz | December 20, 2019
Social media posts are well known for their exaggeration, and the weather is no exception. For the last week or so, I’ve been seeing Tweet after Tweet claiming that Christmas is going to be a “blowtorch” this year across the central and eastern half of the United States. By “blowtorch,” they mean warm.
There is no doubt in my mind that next week is going to be quite mild relative to average east of the Rocky Mountains given that a ridge of high pressure is currently building in with its center over the Great Lakes (Figure 1). The polar vortex feature is just northeast of Alaska over the Beaufort Sea where the green contours are. At the same time, a deep trough will be digging down into the North Pacific and the West Coast of North America. This trough will continue to allow storms to move into the West Coast, dumping lots of snow in the mountains as we progress towards the New Year.
For those of you who are not weather geeks, meteorologists, or aspiring meteorologists like myself, you might be asking yourself, “Why do upper level ridges result in warm weather?”
If you pay close attention to a TV weather forecast, you’ll likely be familiar with the concept that high pressure at the surface signals clear skies and quiet weather overall. Depending upon the origin of the air mass, the surface high can either bring in cold air (like we have seen for the past three days across the Midwest and Northeast) or warm air. The reason for why high pressure means clearer skies is because of the sinking motion of air in the atmosphere, which prevents convective cloud development.
Unlike surface highs, ridges of high pressure feature something that surface highs don’t always have, and that is warm air. While ridges are good indicators, they don’t always correlate well with warmer than average temperatures at the surface at the local level. For instance, if a surface low is underneath the ridge, then a localized area may be cooler than average. This can also occur when sea or lake breezes blow cool air onshore over a cold body of water. As a general rule, areas with overlying ridges can expect mild temperatures during the cool season and hot temperatures during the summer.
Whenever you’re looking at a weather map at the 500 millibar level in particular, ridges are located where the isobars are curving upwards and troughs are located where they dip downwards. On the map above from WeatherBELL, they make it easier to see where ridges and troughs are for the common person; areas that are orange or red indicate ridges and areas that are blue or purple indicate troughs. Keep in mind that they won’t always be color coded.
On a weather map, ridges indicate above normal “geopotential heights.” So, what is a geopotential height? Well, simply put, a geopotential height is the altitude at which you’ll find a given pressure level. Ridges and troughs are easily seen on the 500 millibar map because the altitude at which 500 millibars of air pressure can be found is where half of the atmosphere’s mass is located above and below that altitude. Remember, Earth’s atmosphere holds 1,000 millibars of air pressure. When all averaged out throughout the Earth’s atmosphere, air pressure decreases exponentially with height due to hydrostatic balance.¹
At an altitude of approximately 18,000 feet (5.5 kilometers), 500 millibars of air pressure can be found. This value (500 millibars, or any pressure level) will change depending upon whether or not there is above average or below average geopotential heights in the atmosphere (Figure 2).
Since warm air expands due to temperature’s direct relationship with volume (Figure 3), V₁/T₁ = V₂/T₂, where V is volume and T is temperature, areas in the atmosphere with such thermal characteristics are identified as having higher heights and are thus ridges.² Denser air masses are associated with cold air, and are commonly associated with upper level troughs.
Judging from recent model guidance, it’s likely that Christmas day will be unseasonably mild on Christmas day east of the Rockies, but south of the Midwest (Figure 4). Highs along the Rio Grande, across the Deep South, and down into Florida will be well into the 70s, maybe approaching 80°F (26.7°C) south of Orlando, Florida. From Northern Texas east to North Carolina, highs will easily top off in the 60s. There will be a sharp temperature gradient however stretching across the Plains, Midwest, and Mid-Atlantic dividing polar and tropical air masses.
The warm sector of the country on Christmas day is being portrayed by some people as an outright blowtorch. Of course, temperatures at that time will be above average on the order of 10°F (5.6°C) give or take a few degrees (Figure 5).
Given that climate activists tend to find the color orange appealing on temperature maps, I’m having a hunch that they’ll be having a field day with the upcoming weather next week; I’m sure all kinds of wonderful articles will pop up claiming that we’re having “Hottest Christmas Ever,” only to find out that this will not be the case.
Four years ago, as many of you may recall, the eastern United States had quite a warm Christmas. Over 44% of the Lower 48 was above 50°F (10°C), over 33% was above 60°F (15.6°C), and exactly 20% was over 70°F (21.1°C) (Figure 6). Temperatures across the southern tip of Texas and Florida were around 90°F (32.2°C), while 80°F (26.7°C) readings were recorded as far north as southeastern Virginia. Temperatures over 70°F (21.1°C) were reported well into New England.
Temperatures were some 15 to 30°F (8.3 to 16.7°C) above average from Texas to Maine (Figure 7).
However, despite this, 2015 was not the warmest Christmas day in the United States. That would be 1922 and 1955, where the average high temperature among all USHCN stations was around 54°F (12.2°C).³ In 1955, temperatures as high as 80°F (26.7°C) were recorded as far north as Farmville, Virginia and most stations across the Deep South and Southeastern United States were above 80°F (26.7°C) which were a bit higher than they were in 2015 (Figure 8). Temperatures as hot as 90°F (32.2°C) were recorded in southern Texas.
Temperatures in the 60s or higher occurred over a wide area stretching from Washington state (one location, Walla Walla) throughout the Southwest, Southern Plains, Southern Midwest, Southeast, and Mid-Atlantic (Figure 9).⁴
Since 1917, average maximum temperatures on Christmas day at all USHCN stations have been on a statistically insignificant decline (Figure 10).³
Given these facts, I see no reason for anybody to be concerned about Christmas temperatures. Anyone who attempts to attribute human-induced climate change on next week’s “mildness” will be skating on thin ice. In other words, Christmas hasn’t lost its seasonal chill, so don’t lose yours. 😉
 Augustyn, Adam. “Atmospheric Pressure.” Encyclopedia Britannica. Encyclopedia Britannica, inc., February 13, 2019. Accessed December 20, 2019. https://www.britannica.com/science/atmospheric-pressure.
 “Gas Laws.” Department of Chemistry [FSU]. Florida State University. Accessed December 20, 2019. https://www.chem.fsu.edu/chemlab/chm1045/gas_laws.html.
 Heller, Tony. “Declining Christmas Temperatures In The US.” Real Climate Science, December 17, 2017. Accessed December 20, 2019. https://realclimatescience.com/2017/12/declining-christmas-temperatures-in-the-us/.
 Heller, Tony. “Christmas Day, 1955” Real Climate Science, December 25, 2015. Accessed December 20, 2019. https://realclimatescience.com/2015/12/christmas-day-1955/.