You may remember the bitter cold snap that struck North America just a short time ago. Here is a quote from my letter on Oct. 27 (CL#1796): “I can see anomalies of minus 20C (-36F) in six different states, from Texas to Minnesota. For any piece of land in a temperature zone that number is practically unheard of, at any time of year.” I showed an image of this very rare event, which unfortunately has not been preserved because of technical difficulties that are interfering. (You can still see the right map, but only one that is constantly updated to the present day.) In addition to that, I failed to do as much analysis as I could have, but vaguely do remember seeing total precipitable water (TPW) readings as low as 3kg for the coldest locations on that day. I have vowed never to miss another chance to fully analyze an extreme cold situation like that one, especially one so far away from either polar region, were it to come about. Well, today we’ve got one, at about the same latitude as Minnesota, though not quite as deeply cold. You can see it on this map in Kazahkstan, best doing so by adding magnification.and getting the right angle of sight, where I am getting a reading from shades that look like -16C in spots:
Now we’ll do the same thing with the (Total) Precipitable Water (TPW) map, where the readings that match the location of the cold anomaly are truly unusual lows, relative to the geography, of just 3 and 2kg. On the straight temperature chart (not shown) I can see a daily average reading of -22C, or -8F. This can be compared with a current average for the whole country, found elsewhere, which is close to 0C for this day. The area under scrutiny is not especially elevated, and the small amount of snow on the ground is probably near normal. The cloud map, interestingly enough, shows a clear sky over the coldest spots and light clouds close by where things are a bit warmer. (This map can be seen at the bottom of the letter.)
What would the average TPW reading for this area have been during a baseline period three decades old? Or, for that matter, even the current average would do just fine since the average temperature is only up a little more than one degree in that time span. There is no data available, but I think 5kg, maybe 6, would be a reasonable number, arrived at by making comparisons with other locations that are roughly similar in geography but have different current readings for both anomaly and TPW. Using the 10C per double rule, but now in reverse, a move from 5-6kg down to 2kg, or possibly even a bit less, is fully enough to impose a temperature difference of -16C. How many other ways—beside an absence of TPW—can you think of that would remove so much normal heating power from a sizeable area with just one swift kick? A few days from now all that heating energy could very well be back in place, and perhaps even more. I think all it would take would be a considerably larger dose of high-altitude PW cruising by in an overhead stream. This stuff is constantly shifting gears, higher and lower, day after day, everywhere in the mid to upper latitudes. You can watch it happen at the http://tropic.ssec.wisc.edu/real-time/mtpw2/product.php animated website.
What would 10kg of TPW do for Kazakhstan temperatures right now? The answer is surprisingly close. Looking at the map again, you will see an area immediately south of the cold anomaly, still in Kazakhstan, where a stream of PW has broken through and raised the local TPW to as much as 11kg in spots. On the anomaly map, when you match things up, you can see temperature anomalies in the area ranging from zero to plus one. I think the number could be even higher were it not for the fact that this spot has an overcast of clouds heavy enough to produce snowfall, implying a sharp dropoff in solar energy during daylight hours. Here is the map that has the story:
Carl