The hottest lawns are not always in the city

Posted on May 15, 2021 by lembrechtsjonas

According to the first results of our citizen science project ‘CurieuzeNeuzen in de Tuin’, lawns in urban gardens can also be quite cool. This came as a bit of a surprise.

[English summary based on today’s discussion of our results in De Standaard by Ine Renson]

Today, we are launching something amazing: the daily updated temperature maps of our 4400 temperature sensors in our citizen science network. These maps have been highly anticipated, as we all were wondering about the patterns that would show up on them. Most importantly, one thing we wanted to see: are garden soils in cities warmer than those in the countryside due to the heat island effect?

It turns out that they are not. The map of soil temperatures shows a fairly diffuse picture, where cities do not immediately stand out. This is perhaps different from what we expected to see, but it is precisely why it provides interesting insights.

Our TOMST-built lawn daggers have three temperature sensors: one at a depth of 10 centimeters, one just at the ground surface and one at 12 centimeters above the ground. Those three curves show a different rhythm through the day. The air temperature at or just above the ground fluctuates strongly, while the soil temperature goes up and down in a gentle manner. So an earthworm feels a very different temperature than the one we experience ourselves in the garden.

Soil as a buffer: the average soil- and air temperature in Flemish lawns on April 13th. The two curves show the air temperature (green) and the soil temperature (brown). Soil Temperature remains rather stable, while air temperature fluctuates heavily. Soil warms much slower than the air, and reaches its maximum temperature late in the afternoon. Infographic as appeared in De Standaard newspaper.

We see that the soil buffers the temperature fluctuations, and that this pattern occurs consistently in every garden in Flanders. Here, our gardens nicely follow the rules of the soil physics textbooks. But when we compare soil and air temperatures, we encounter a remarkable phenomenon. When we plot the nighttime minima of air temperature on a map, the cities clearly stand out (see map below). In the early morning, city gardens are clearly warmer than the surrounding countryside. On the map of maximum soil temperatures, the pattern is a lot more subtle. There you see a lot more local variation across the whole urban-rural gradient. Very interesting: you’ll find most of the warm gardens there in the city outskirts, so halfway the urban-rural gradient. From the soils’ point of view, urban gardens are not necessarily warmer than average.

Heat islands in the coldest nights. This map shows the **minimum air temperature** (12 cm height) on April 13th, and visualizes the ground frost in early morning. Cities immediately pop out as warmer, with differences across the region of up to 15°C (-10 to +5°C)! Infographic as appeared in De Standaard newspaper.

We believe this goes to the heart of how the heat island effect works. Structures like buildings and roads absorb a lot more heat, so the air temperature close to the ground heats up quickly. Wind and air currents cause that warm air to spread, even to places in the shade. So the city as a whole heats up. Much of that heat is re-emitted in the evening and retained between the buildings. That’s why the heat island effect in the city is so obvious at night.

In the soil, things are different. It absorbs heat by radiation from the sun, but that heat is not transmitted laterally as much as in the air. A soil that is covered with plants, and not asphalt or concrete, will heat up less. Also, a soil that is shaded by buildings or by trees and shrubs stays cooler The fact that cities don’t really stand out on the soil temperature map might thus be because city gardens are often smaller and therefore catch extra shade, highlighting the critical aspect of shade for cool garden microclimates, even in the heart of the city.