In August 2020, I arrived in Helsinki for my master studies at the University of Helsinki. I remembered the end of summer and the first months of the fall in that year to be way warmer than in the following autumn. I remember in the months following my arrival to Finland, I was able to enjoy some nice, warm, late summer weather several times, whereas in the late summer and fall of 2021, it felt cold, grey and rainy the majority of the time. I decided to check the data to see if my feelings were correct and if the fall of 2021 was indeed colder, or that it was just a matter of selective memory.
I did some data-analysis on the observations of the weather stations in the center of Helsinki (Kaisaniemi) and the Helsinki-Vantaa Airport (Helsinki-Vantaan Lentoasema) for the months August, September and October in 2020 and 2021. The observation data is accessible through the Finnish Meteorological Insitute (Ilmatieteen Laitos).
Temperature indeed generally higher in 2020
If you simply take a glance at the figures below, you can already see that it was often warmer in 2020 (dashed lines) than in 2021 (continuous lines).
However, it becomes even more clearly visible when one looks at the temperature distribution for both locations. In the figures below, histograms are shown for Kaisaniemi and Helsinki-Vantaa Airport. Especially looking at the maximum temperatures it is very clear that the late summer and fall of 2020 were generally warmer than in 2021.
It is nevertheless noteworthy that the relatively warm autumn of 2020 had some cold ‘outliers’. In both the airport and the city center, the lowest maximum temperature was actually lower in 2020. Also, the late summer/early fall 2020 had two more frosty nights than 2021 at the airport.
Despite the frosty nights, the overall numbers are clear: on more than 60 days, the maximum temperature was higher in 2020, and on more than 50 days, the minimum temperature was higher in 2020. This is the case for both weather stations.
Precipitation: fall 2021 much wetter
Besides colder, the late summer and first months of autumn in 2021 were also way wetter. Especially august 2021 saw a few very rainy days, with at the Helsinki-Vantaa airport almost 40 mm on one day towards the end of the month. Eventually, combining August, September and October, both weather stations ended up with about 60 mm more rain in 2021 compared to 2020.
Comparison to climatological mean
The average maximum temperature in 2020 in the aforementioned months was 16.3°C in Helsinki Kaisaniemi and 16.6°C at the Helsinki-Vantaa airport. The average in 2021 was about 2 degrees lower, 14.4°C in Helsinki Kaisaniemi and 14.5°C at the Helsinki-Vantaa airport. In 2020, the average was well above the climatological average (based on the climatological period 1991-2020) which is 15.0°C in Kaisaniemi and 15.2°C at the Helsinki-Vantaa airport. However, in 2021, the maximum temperatures were clearly below the climatological averages.
The average minimum temperature in Augustus, September and October was in both years above the climatological average, although 2021 was still clearly colder than 2020 (10.5°C against 9.3°C in Kaisaniemi, 8.9°C against 7.7°C in Helsinki-Vantaa). The climatological averages are 9.0°C and 7.5°C, respectively.
In 2021, the amount of precipitation was well above the climatological average for those months (about 250 mm in Kaisaniemi and about 300 mm in Helsinki-Vantaa, against climatological means of 210 mm and 219 mm, respectively). One year earlier, the amount of precipitation was clearly lower, with about 200 mm in Kaisaniemi and about 250 mm in Helsinki-Vantaa. In Kaisaniemi the total was in 2020 thus close to the climatological normal, whereas in Helsinki-Vantaa the total precipitation was still clearly above the climatological average.
All in all we can conclude that my gut was correct about the fact that the late summer and early autumn of 2021 was way wetter and colder than the previous year. I of course hoped that this was a presage of the upcoming winter.
