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Which Dutch city centers suffer the most from heat islands on hot summer days? This series tries to find an answer to this question with the help of the ECOSTRESS data provided by NASA, where ground temperature data in areas measuring 70 by 70 meters can be sought. Recently, these super-local measurements are also available for the Netherlands. A new episode is published every Tuesday and Friday, with three new city maps every time. Read the other episodes of this series here.

November is drawing near now. That means that this summer heat island series is almost over. In this second-to-last episode, the Dutch city of Leeuwarden is playing the starring role. Why specifically this city? Because a very innovative art project passed through the city center last summer, bearing the somewhat nondescript name of ‘Bosk’.

The Frisian art collective Acardia treated the historic city center to a green metamorphosis between May and August with the help of more than a thousand trees in planters on wheels. This meant that every few weeks a different location had its turn. What? Some things are best shown rather than explained. You can see the impact of all that greenery on the streetscape in this aftermovie.

Not only did the impact of so many trees transform the streetscape, theoretically it should also be detectable on the ECOSTRESS satellite measurements that have been used to create many a city map over the past period. Nevertheless, even in this special episode we will begin with an overall picture. Has the Frisian capital managed to master the heat in its streets even in the absence of a mobile jungle on wheels? Not exactly.


Despite the fact that it was about two to three degrees less hot here than many other places in the country, the ground temperature figures here are pretty much on par with the rest. Ironically, hitting around 53℃, a parking lot of an indoor ice rink is the very hottest location in the Frisian capital.

While the center is admittedly free of such outliers, at the same time a sizeable area with values above 45℃ has been measured here. This could rise to around 46.5℃: this did not exactly make Leeuwarden the best kid in the class in this series, but also not that serious of a case. Waagplein and the old narrow streets north of Nieuwestad warmed up the most.

The Frisian capital did, however, have a noticeable exception in the Huizum neighborhood. It reached a whopping 47℃ in the streets of this pre-war neighborhood on August 12. You would expect conditions like these to be more common in a business park or furniture mall. Several other heat islands also stand out in a number of neighborhoods around the city center.

Conditions in almost all suburbs were not too bad to cope with on this hot summer day. Still, you will have to head toward the water for the biggest source of cooling. Something of which there is absolutely no shortage of in the newly built Blitsaerd neighborhood and Zuiderburen Vinex district.

Interestingly, former “Vogelaar neighborhoods” Heechterp-Schieringen and the Vrijheidswijk are among the cooler zones. It seems that the large-scale refurbishment of several years ago has paid off, at least in this area. This is quite different in comparable neighborhoods in many other cities.

Mobile airco?

Back to the inner city. Because just what exactly is the effect of those thousands of Bosk trees on ground temperatures, and can this data prove that point? ECOSTRESS data from June 17, Aug. 1, Aug. 12 and Aug. 16 should provide an answer. Why is July missing? Because during that month, the satellite only passed over the Netherlands in the middle of the night or in cloudy weather. As a result, the data from this month is unfortunately unusable.

But not to worry. These four snapshots in time provide an excellent basis for analysis, with you, the reader, also at the controls. You can zoom in or out on the environment using the Easyzoom magnifying tool. This works fairly easily, you can do this with your fingers on mobile devices, while desktop readers can use their scroll wheel to operate the magnifying glass.

The map of Aug. 16 – compiled two days after the roaming forest left town – serves as the basis for this. Bosk was in a different location in the city each time on June 17, August 1 and August 12.

Where the trees were at those times is indicated on these maps by a green circle. The black circles indicate a former or future location. The little trees indicate permanent vegetation with no artistic merit or tour schedule.

Photo – Jelmer Visser

17 June

Usually, the Nieuweburen is a little warmer than the surrounding streets on the canal and the oldest part of the center. That was not the case on June 17, at least among the rolling trees. The street is by no means a refreshing oasis, but temperatures did not rise significantly at that time here.

This is about one degree higher on the August 16 control map. The adjacent area to the east across the canal is also slightly trailing in temperatures on that map, whereas it is the other way around here.

Will those thousand extra trees have caused this difference? They just might have! But as far as irrefutable proof is concerned, it seems that there is no question of that as yet. Did the situation return to ‘normal’ at the second time – Monday, August 1? In any event, the plant platoon has since left and can be found at Gouveneursplein, in the far west of the city center.

1 August

At this second location, the differences seem to be much more pronounced than on June 17. This becomes even more notable given that the maximum temperature according to the Royal Netherlands Meteorological Institute (KNMI) reached ‘only’ 19 ℃ on August 1, while the ECOSTRESS algorithm was not hampered by any cloud coverage during the measurement. Despite the cool summer day, it still warmed up considerably on the ground surface in parts of the city. However, Gouveneursplein remained remarkably cool when Bosk came to visit….

The differences compared to the immediate environment and the rest run as high as 2.5 ℃. This bears the strong hallmarks of a significant disparity. Still, it cannot be ruled out that this conspicuously cool zone could be attributable to a breeze from the west, a statistical blip, the substantially lower air temperatures or some other cause.

In order to genuinely demonstrate the effect of Bosk in a fully scientific way, much more accurate measuring instruments and points in time are needed. The last stop of the mobile city park should therefore be visible on the final map of Friday, Aug. 12, where the citizens of Leeuwarden were able to bit farewell to their travelling trees over the course of two more days.

12 August

Although the eastern part of the square hardly differs from the streets that surround it, it is as much as three degrees cooler on the ground in front of the historic Oldehove on Friday, August 12. Albeit that there is room for nuance here as well. A blistering hot spot like this square is not found on any map. Still, the differences are considerably greater than on June 17, August 1 and 16.

Is the Bosk effect at play here? That probability seems increasingly likely judging by the four maps. Use the interactive tool to zoom out in particular to see the whole picture. Then you can see how Nieuweburen is all the way back to ‘normal’, while the cobblestones of Gouveneursplein hit a higher reading than the surfaces of its southern neighbors.

Has the ECOSTRESS satellite thus conclusively established that Bosk is responsible for 1.5℃ to as much as 3℃ of cooling? Not quite. Further research is needed to determine this which requires for more precise measuring equipment, more measurement times and the expertise of experts who are able to rule out other factors.

Yet we would not at all be surprised if these findings reveal parallels to this global method. On Tuesday, August 16, the tour was over, which left downtown Leeuwarden much less green in one fell swoop. Are these kinds of elaborate green art projects the future? Who knows. And if the heat stress failed to dissipate, at least it looked impressive and beautiful!

But will Bosk fix all of Leeuwarden’s heat stress? No, unfortunately. This would take far more than a thousand trees. And that also without wheels or a touring schedule. So, a temporary remedy on a hyperlocal level? Plausibly.

The following is once more the ‘control chart’ from Aug. 16.