A full month more above 20°C

Hi, this is Lisa. I’m responsible for the blog at Datawrapper. Today I’m exploring and visualizing again a topic that affects all of us: the weather.

It was warm in Berlin last week. Not just warm – hot. 37°C (98.6°F) hot. Every time it’s that hot, people are either barricading themselves in their cooler apartments or rushing to the lakes. And every time I wonder: Is this normal? Or rather: Is this the new normal?

Looking at the past five years of weather data from Berlin, we see that it's at least not unusual. We've had heat periods in the years before, even earlier in the year. (In fact, this year's April and May were unusually mild, with only seven days warmer than 20°C.) The data also shows that it's likely we get at least one other hot week in July or August.

But to answer the question if this the "new normal," we need to zoom out a bit. Luckily, the weather station in Berlin Tempelhof has measured temperatures daily since 1948. Here's a comparison of the first and the last (full) 10 years in the records:

The difference between these two decades seems... not... really... big? We had cold days in the winters of the '50s, and we still have them. And there were lots of warm and even hot days back then. If you squint, the top half of the heatmap does look a bit less orange-red: There are a few more bright, 5-20°C gaps in between. But it's hard to tell.

But just because we can't see something, doesn't mean it's not there.

Let's try again.

There are a few ways to separate the climate trend from the weather fluctuations. In most cases, we compare average temperatures over a long time, e.g. decades – like I did in a Weekly Chart three years ago, with a vertical arrow plot.

Today I want to try another approach. In the following chart, I'm plotting the number of days in a year that a certain temperature is reached:

We can still see a lot of up and down in the individual values. But there's now a clear difference between our two decades 1948-1957 and 2011-2020: On average 32 more days >20°C and 28 fewer days <5°C.

I chose to visualize these two temperature thresholds (min 20°C and max 5°C) because they used to make up roughly the same number of days per year, until in the '80s the lines started to diverge. But the same trend is visible for other temperatures:

I guess nobody complains about a few dozen more sunny 21°C days. But that increase in really hot days –that's dangerous for humans, animals, and plants.

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You can find a lot more climate visualizations in the Climate Vis section of our blog. And if you're interested in inspiring data visualizations in general, don't miss our new blog series "Data Vis Dispatch": Every Tuesday, we'll publish the best of small and big visualizations. Find the first edition here. We'll see you next week!