Urban climate

Taking a break to relax in a leafy city park on a hot summer day feels good. Not only is it greener and quieter than the surrounding urbanarea , it is also distinctly cooler. There are no buildings and road surfaces nearby that can store and emit the heat of the sun.

Still, every day in Germany, an area the size of about 50 soccer fields is paved over or falls victim to urban development. The building boom has an impact in particular on the climate in cities. The more densely developed a city is, the more pronounced the so-called heat island effect. In a German city center, the temperature can easily be three degrees higher than in the surrounding areas.

A study on the influence of soil and vegetation on the urban climate shows how important it is that cities have open, unsealed land. A team of researchers in the northern German city of Hamburg recently launched the Hamburg Urban Soil Climate Observatory (HUSCO) project.

Experts installed measuring facilities in the city to find out how much soil cools the surrounding climate and how various types of soil have different effects. One station was set up in a bog with a high groundwater level, and another in a dry area with a low groundwater level.

Sensors deep down

In both cases, the researchers built mini weather stations that measure the temperature, wind velocity and humidity. They also dug pits and installed soil sensors right under the surface as well as at a depth of 1.6 meters, project manager Annette Eschenbach said. "The sensors measure the soil's temperature and water content, among other things," the soil scientist from the University of Hamburg says.

The sensors have collected data over the past three years. The evidence shows that the sites dry out to varying degrees during periods with less precipitation, Eschenbach explains. "It all depends on the respective groundwater level."

Moist soil is replenished by groundwater, so it tends to dry out less quickly during dry periods than soil with low groundwater levels.

The researchers found that moist soil cools down the surrounding air, considerably more so than dry ground. Specifically, throughout the year, temperatures are half a degree lower in a park than in the adjacent heavily built-up neighbourhood. "That means city parks have a huge significance for the local climate," Eschenbach says.

Moist soil is effective

Allowing for more unsealed soil in cities could be an important element as humans adapt to climate change. "Building more parks is always a benefit for the urban climate," Annette Eschenbach says. But, the researcher adds, the Hamburg project has shown that it is best to create parks on sites with moist soil, where the cooling-down effect functions even better.

But, the desire for more urban parks conflicts with the widely-held ambition to create more residential space in cities to keep rents affordable. Experts fear the heat island effect may become even more pronounced in the future, not just because cities are increasingly compacted, but also due to global climate change.

"These past years have been quite damp, so what we urgently need now is a good, dry summer," Eschenbach concludes, adding that the HUSCO studies will continue in the future. A period of very hot weather, she says, will allow her team to get further conclusive evidence of the effect of parks on Hamburg's urban climate.