Between late autumn and pre-spring

Hard to write a winter timeline, when traces of winter are sparse. The weather since many weeks has been dull and seemingly the time has stopped in late November. For me November has lasted at least three months this… autumn?

First snow was recorded in Warsaw on Wednesday, 29 January 2020, thus breaking by 5 days the previous record of the latest first snow set in 2007. But thirteen years ago the first snow melted over more than a week later and in the meantime temperature dropped to –12C. This year the snow lingered on the ground for a few hours (despite positive temperature) and melted overnight. First snow is usually a shy snow… but in January?

I took delight in watching snowflakes whirling in the air, but having walked out of the office, I found myself in soaked shoes in a slippery slush, which diminished my spurt of winter-joy. For some reason nobody bothered to clear pavements out of slush. Pedestrians again appeared to be second-class citizens in the capital of Poland.

I cherished the snow for… its colour. White surfaces reflect light thanks to which days when snow lingers are brighter. The brightness was definitely missing in the time of year when days are the shortest. The brightness was what I have been yearning for.

Temperature-wise, the current winter (defined as December, January and February) stands the chance to be the warmest ever. Combined average temperature of December 2019 and January 2020 was +2.9C. If February’s mean temperature exceeds +1.1C in 2020 (which judging by long-term forecasts is likely), the 1989/90 winter with average temperature of +2.3C will become the second warmest.

The foregone conclusion is that current weather is the best proof of the global warming. Indeed, it is, but I would argue whether is can be named the aftermath thereof. If you dissect weather pattern day after day, you will see that unlike in previous anomalously warm winters there were periods of extraordinary warmth interspersed with episodes of regular, though not harsh winter. This winter, except for 3 spells of high, yet not record-breaking temperatures (15 December 2019 – 22 December 2019, 7 January 2020 – 15 January 2020, 31 January 2020 – ???) has seen fairly stable weather with temperatures somewhat above long-term average and never dropping below it.

The lack of even temporary negative deviations from long-term mean cannot be put down to global warming, but there are two other causes of it.

Firstly, the strong positive North Atlantic Oscillation (which sadly has not brought as much precipitation as it usually does). Most of the times, air from the west was blown in over Poland and further east and north. Positive deviations from mean temperature, far bigger than in Poland have been observed in southern Scandinavia, in Baltic countries and in western Russia. Masses of arctic or Siberian air have not had a chance to trespass onto Poland.

Secondly, the polar vortex, which since November 2019 has been holding up well, with strength the biggest since 40 years. This means cold air is kept in the North Pole, where temperatures for several weeks have been below average and beyond the polar circle (except for sea shores of course) – keeping track of weather measurements in northern Scandinavia I saw repeatedly temperature dropping below –30C in North Sweden and Finland. Historically, when the polar vortex was weak and winters in Europe were harsh, temperature in Greenland, Iceland or far beyond the polar circle was well above average. The polar vortex is slowly disintegrating, but effects of the process might be visible in late February or early March.

The two phenomena combined are responsible for the weather pattern this winter. I lack knowledge to determine to what extent they are impacted by the ongoing climate change, but holding the accursed global warming responsible for lack of winter in 2019/20 in Warsaw is an example of jumping to conclusions. NAO+ and polar vortex are the actual culprits.

No major change in the weather is expected in mid-term though. Yesterday saw the climax of the mid-winter short heat wave. Day-time high was +11.6C. The situation is not unprecedented, since it was already warmer in early February. Record-highest temperatures measured in Warsaw in the first decade on February were: +13.4C on 3 February 2002, +13.1C on 5 February 2004, +11.8C on 6 February 2000.

Next week another episode of winter with some snow is expected, then temperatures are to revert to above-average values. Most models estimate average temperature in February should run between +2.5C and +3.5C, but some indicate second half of February and first half of March will be the only period this winter with temperatures close to long-term means.

Still, three records of mild winter stand a chance to be broken:

1. the lowest number of days with snow cover: 15 days set during 1988/89 winter (22 November 1988 – 24 November 1988, 4 December 1988, 8 December 1988 – 11 December 1988, 15 December 1988 – 18 December 1988, 21 December 1988, 8 January 1989, 2 April 1989),

2. the lowest maximum snow cover measured over any entire winter: 4 centimetres accumulated on any day during 2007/08 winter (measured on 7 January 2008, on 18 February 2008 and on 27 March 2008),

3. the highest minimum temperature measured over any entire winter: –8.0C measured on 17 February 1975.