How was the weather in December 2020? A review of weather data for Taipei City.

As a nerdy scientist one of my interests is weather observation. Almost every season or month I am wondering if this month was colder or rainier or sunnier than in previous years. Usually personal impression and memory can fool you, so I decided to look at the data from weather stations and compare the current month with previous years. For December 2020 my feeling was that it was colder and rainier than usual in Taipei City. But when looking at the climate data provided by the Central Weather Bureau Taiwan (ROC) and comparing the data from December 2020 with each December for the previous 20 years, as well as with the long-term average, December 2020 was not that unusual, except for one thing.

In case you don’t know what the long-term average is. The long-term average is the average climate data for a time period of 30 years. In this case it is data collected from 1981 to 2010. The long-term average can be considered to represent normal conditions. By comparing recent data with the long-term average scientists can say if climate data, e.g. temperature, precipitation, sunshine, in a region was normal or exceptional.

Before I start examining the details I need to give an important information: All data presented here is from the weather station in Taipei City (Station Taipei, No. 466920), and all my conclusions are only valid for Taipei City. If you live in another part of Taiwan, then this will not apply to you. It is possible to do this for whole Taiwan, but this would require a much more intense data analysis. And I don’t have the time to do this. The data I am using here can be downloaded from the CWB Observation Data Inquire System (CODiS) and the long-term averages for Taiwan can be found on the website of the Central Weather Bureau Taiwan.


Was December 2020 colder or warmer than other Decembers? Even though the last days of December experienced a cold surge with temperatures dropping down to 7.6°C, the monthly average temperature in December 2020 was 18.1°C. This is 0.2°C warmer than the long-term average of 17.9°C. Additionally, the monthly average maximum temperature in December 2020 was 20.5°C, which is 0.2°C cooler than the long-term average. However, the monthly average minimum temperature was 16.5°C and 0.9°C warmer than the long-term average. The monthly average minimum temperature is calculated by taking the lowest temperature measured for each day in a month and calculating the average value. Similar for the monthly average maximum temperature, but here the highest temperature per day is used.

The temperature data shows that the monthly average temperatures in December 2020 in Taipei City were not exceptional from the long-term average. When looking at the temperature graph you can see that the monthly average temperatures for the last 20 years fluctuate between 16.4°C and 20.1°C and the monthly average temperature in December 2020 was in average of other previous Decembers. The only exception in temperature data is that the monthly average minimum temperature was 0.9°C warmer than the long-term average. Interestingly since 2015 the monthly average minimum temperatures have been constantly higher than the long-term average. Which means that since five years the Decembers in Taipei City are not getting as cold as they have been in the past. I am not a climate expert, but I am wondering if this could be a result of global warming.

Precipitation / Rainfall

Was December 2020 rainier than other Decembers in the last 20 years? Not really. The total monthly precipitation in December 2020 in Taipei City was 145 mm. This is nearly the double of the long-term average of 73.3 mm. But compared to other months it is somewhat in the middle. There have been drier and wetter Decembers in the past 20 years. Interestingly, to me it felt like it was constantly raining through the whole December, but when looking at the numbers, it was just average. Also the total duration of precipitation was 124.6 hours, which is average for the last 20 years.

Sunshine and Clouds

So far December 2020 was kind of average. But now comes something exceptional. The duration of sunshine was with 16.9 hours the lowest in 20 years! This was more than 5 times lower than the long-term average of 90.7 hours in Taipei City. The previous negative record holder was December 2011 with only 23.5 hours of sunshine. Also the amount of clouds in December 2020 was the highest in the last 20 years. With an average cloud index number of 9.3 (10 is the highest). Of course when the sky is largely covered by clouds it will affect the amount of sunshine. This very low amount of sunshine is what I remember from December 2020. There was an entire week from 3rd to 10th December 2020 without any sun and it started to make me feel depressed.

One thing about the sunshine data is a bit odd. When looking at the chart then the values from the years 2000 to 2010 are higher than the values from the years 2011 to 2020. I am wondering if this is because the Central Weather Bureau changed the method of measuring sunshine duration or if it has a climate reason.

That’s it for now. Let’s see how January 2021 and the rest of the winter will turn out. I think it will be a wet and cold winter. But I also hope that it will be a short winter and spring will come soon.

Please let me know in the comments if this was interesting and understandable or if it was too scientific. The big challenge for scientists is to communicate results in a way that non-scientists can understand them.

Title photo by Timo Volz from Pexels

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