How cities shape the weather

8 March 2018, by Stephanie Janssen

Photo: UHH/CEN/Janssen

Differs from the countryside: Sarah Wiesner analyzes urban climate.

At the Cluster of Excellence for climate research, Sarah Wiesner has taken a closer look at the city of Hamburg’s climate. Together with colleagues she has now published a comprehensive study on how the city impacts individual weather factors. In an interview, the researcher talks about apparent temperatures and urban heat.

Urban climate is made up of factors like temperature, wind and humidity. Which is the most interesting?
For me it’s recently become the wind; it’s so changeable. It constantly surprises me when effects don’t occur where I expect them to: a gust of wind doesn’t sweep right around the corner, but instead releases its full force a few meters farther away.

Which is the most important weather factor?
For most people it’s the apparent temperature, which largely determines the comfort factor. We call this thermal comfort, and it mainly consists of air temperature, wind and the surface temperature of the ground and buildings together.

You study urban climate. How does this differ from the climate in the countryside?
When the prevailing “normal” weather in the countryside hits a city, most of the individual weather factors are affected. This is due to the buildings, patchy vegetation, and the characteristics of the various surfaces. Streets and buildings absorb heat during the day, and release that heat when the cooler evening comes. Unlike green areas, sealed surfaces can’t release cooling moisture through evaporation. At the same time, the corners of the buildings make the wind harsher, producing gusts.

In a recently published study, you investigate which urban effects are produced by cities themselves, and which are caused by global climate change, for instance. Why is it important to differentiate?
Once we know which climate change-related phenomena are intensified in cities, future urban planners will be able to implement targeted measures accordingly. But this aspect isn’t always easy to identify: when one of a city’s weather parameters changes, it also changes the other parameters – these are highly complex interactions. That’s why we’re working to clearly identify those factors that cities themselves influence.

Which weather factors fall into that category?
Wind, air temperature, humidity and surface temperature – in other words, factors that make up the thermal comfort – are all significantly changed by cities. In the summertime, the afternoons are much warmer in the city than they are in the surrounding countryside. For example, in Hamburg we record an average of 31 days per year with a high of over 25 degrees Celsius, but only 22 such days in the countryside. When it comes to hotter days, with a high of more than 30 degrees, the average number in the city is six per year – which is twice the number in the surrounding area. For people with health issues, the growing heat could mean real problems – and as climate change progresses, we’re likely to see more and more of these hot days.

Which weather factors aren’t affected?
When it comes to precipitation and hours of sunlight, we haven’t found any differences – at least not in your average major city in Central Europe, which Hamburg is a good example of. In China things can be a very different story; for example, in its metropolises the sunlight is limited due to heavy smog.

What can we do about urban heat if climate change continues?
Open bodies of water and greening efforts can produce a cooling effect; we should try to avoid adding more sealed surfaces. These aspects need to be directly addressed in the context of urban planning – which will improve citizens’ thermal comfort. At a higher level, the City of Hamburg can also help by reducing emissions and doing its part to achieve the globally agreed-upon climate protection goals.

Original article:
Sarah Wiesner, Benjamin Bechtel, Jana Fischereit, Verena Gruetzun, Peter Hoffmann, Bernd Leitl, Diana Rechid, K. Heinke Schlünzen, Simon Thomsen (2018): Is It Possible to Distinguish Global and Regional Climate Change from Urban Land Cover Induced Signals? A Mid-Latitude City Example; Urban Science 2 (1), 12