Friday, December 27, 2013

White Christmas in Prague: 9 in 40 years, no trend

In recent weeks, the weather was often cold but we didn't have a white Christmas almost anywhere in Czechia (as has been correctly predicted since mid December). mentions some interesting statistics based on Centrum.CZ, NOVA.TV, PocasiCZ.CZ, and Infomet.CZ articles that make it clear that we shouldn't have expected a white Christmas.

Well, during the last 40 years i.e. my lifetime, Prague has only experienced 9 white Christmases.

December 24th snow cover in "cm" at six major Czech weather stations looks like this:

The black and grey curves near the bottom display the scores of our warm capital.

Purely visually, you will surely agree that there is no obvious trend in this graph. The snow cover on a particular date is pretty much a random number – the variations resemble the "white noise" because the figures for two years are uncorrelated even if the years are nearby.

This behavior is different from the annual mean temperatures that are "mostly continuous" functions of time and their behavior is closer to the red noise (or at least some "pink noise" which is demonstrably not white noise).

Prague experienced white Christmases in tbe following nine years:
1981, 1986, 1994, 1996, 1999, 2000, 2001, 2003, 2010
The average of these figures is 1996.67 – slightly later than the middle of the 40-year interval in late 1993. So it would suggest that the white Christmases were getting slightly more frequent in recent two decades. But the excess is statistically insignificant – in fact, the average is absurdly close to the middle of the interval.

But you might say that it's an accident and the snow cover will display a downward trend if you look at the extremes. An alarmist wishful thinking could say that the maximum snow cover was about 40 years ago but the minimum one took place recently.

The only problem with this claim is that the data refute it completely. There's no trend in the extremes, either. Well, if you look at the Christmas Eve temperature, the coldest one, –15.4 °C, was in 1996, again almost exactly in the middle of the interval. And what about the warmest Christmas Eve in Prague? It was one year earlier, in 1995, even closer to the middle of the interval, when it was +9.3 °C.

All these data look like "fine-tuned numbers" that want to look unnaturally (very) natural. But this is a coincidence, too. If we included the whole 1970s, a misbalance would materialize. The 1970s were warm and their Christmases were snow-free. There were no white Christmases in Prague between 1973 (and maybe the two earlier years) and 1980 included.

On the contrary, if you included the 1960s, the picture would get very different. A very large majority of the Christmases in the 1960s were white. It was a cool decade, the end of the "post-war cooling" we sometimes talk about.

If you predict that the Christmas in Prague won't be white on a given year, chances will be 78% that your guess will be right. You may be proved wrong, too. But most current citizens of Prague can't remember a period when the white Christmases were "more likely than not" and the white Christmases tended to be exceptions in the recent centuries and millennia, too. The "white Christmas as a standard" is clearly a myth but it's one that is being perpetrated for numerous reasons.

Just to be sure, Prague-Klementinum, not to far from the Prague Astronomical Clock, is Czechia's warmest weather station with the annual mean temperature slightly above 10 °C. The coldest one is at the top of Sněžka [The Snowy Female One], our highest peak (for 20 years in which we were stolen the High Tatras by our separatist brothers) that we share with Poland, where the annual mean temperature is almost exactly 0 °C.

Czechia's average annual mean temperature is about 7 °C, almost exactly equal to the annual mean value for Pilsen. As I have often emphasized, a 13 °C of global warming would be excellent news for us. Unfortunately, it is a news from the realm of pure fiction.

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