After a (for my standards) cold fall and a very cold end of November, the winter stepped up its game in the beginning of December 2021. The first week of December, the temperature sunk even further than it did in November. The result: the coldest first week of December in Helsinki since 1937.
I was expecting to experience some proper winter weather when I moved to Finland for my studies. Already during my first winter, I counted myself lucky. I experienced some proper winter weather: Arctic temperatures and proper snow storms: some of the most beautiful winter days of my life. I couldn’t have dreamed of already experiencing extreme winter weather again directly at the start of the following winter season in November and December. But it happened! Especially the beginning of December 2021 was very cold, even for Finnish/Helsinki standards.
Due to a low pressure system passing south of Finland towards the northeast, northeasterly winds strengthened and even colder air moved over Scandinavia. An area of high pressure formed in the region as well, leading to perfect conditions for very low temperatures (see Wetterzentrale reanalysis figures above).
Ice skating in Nuuksio in December!
After the cold end of November, December continued in style. Two nights of minimum temperatures below -10 and -15 respectively, gave me the confidence that the ice was thick enough. After seeing people skate in Nuuksio in December in 2020, I decided it was time to go skating in that beautiful National Park myself. The ice growth conditions during the past 1.5 weeks were almost perfect and since there was relatively little snow, the quality of the ice was great too.
Coldest first week of December Helsinki
The coldest weather was however still to come. Around the 5th of December, the aforementioned high pressure system was located over Scandinavia. With the airmass being very cold already, this lead to perfect conditions for low temperatures at the surface. For 4 days in a row, even the maximum temperatures (!) dropped below -10 degrees Celsius. In the north of Helsinki, on the 8th of December the maximum temperature was around -15, and the minimum temperature below -20. In the city center (Kaisaniemi) it was slightly ‘warmer’.
In the photo gallery below you can see the temperature in Kaisaniemi and the Airport (Helsinki-Vantaa, in the north of Helsinki) compared to the previous year. Also, in the histograms, the maximum and minimum temperature distribution can be seen. It was clearly way, way colder than in December 2020, which was dominated by southerly winds.
The temperatures in December in Central-Helsinki. Red is maximum temperatures, blue minimum. Dashed line is 2020. 
The temperatures in December in the North of Helsinki. 
The frequency of the maximum temperatures that occurred in November in Kaisaniemi, compared to 2020. 
The frequency of the minimum temperatures that occurred in November in Helsinki-Vantaa Airport. 
Histogram of maximum temperatures. 
The frequency of the minimum temperatures that occurred in November in Helsinki-Vantaa Airport.
Coldest first week of December since 1937 in Helsinki
All in all, the first week of December 2021 was much colder than the year before and the climatological norm in most of Finland, and specifically in the south. In Helsinki it was even the coldest first week of December since 1937, according to atmospheric researcher Mika Rantanen. In Other parts of Finland it was the coldest start of December in several years or even decades.
The first week of December was the coldest in 84 years in Helsinki. pic.twitter.com/ElTq05v6VV
— Mika Rantanen (@mikarantane) December 8, 2021
It is noteworthy that during this cold period, no significant snow was measured in Kaisaniemi city center. Never before, such a cold period occurred without any measurable snow on the ground in Helsinki. However, in the north of Helsinki there was a little dusting of snow on several days.
Sea ice in Helsinki in December
The extremely cold weather led to sea ice formation along the coast of Helsinki. Ice formation in the Gulf of Finland in wintertime is quite common, although the sea ice extent on average becomes smaller and smaller as a consequence of climate change. And in the Gulf of Finland, sea ice formation generally starts way later in the winter season. So specifically considering the current climate, the sea ice formation in Helsinki started very early.
Statistics
After this cold first week of December, the weather became milder, with positive temperatures on several days. However, the end of the month was quite cold again. In Kaisaniemi, the maximum temperature in December 2021 was lower on 24 days compared to December 2020. The minimum temperature was lower on 23 days.
The maximum temperature was below 0 degrees Celsius on 21 days, which is 20 more than the year before. The minimum temperature was below -10 on 18 days and below -15 on 6 days. At the Helsinki-Vantaan airport, the minimum temperature dropped below -15 on 11 days and below -20 on 2 days. Quite noteworthy, specifically given the fact that on neither station the temperature dropped below -10 the year before.
