BAU-I-1 + 2 Heat stress in urban environments and summer heat island effect

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents

 

BAU-I-1: Heat stress in urban environments

In most cities examined, hot days and (not illustrated) tropical nights clearly occur more frequently in most years than indicated by the mean for Germany as a whole. Especially in years with above-average hot summer months, records show increased frequencies of situations in big cities, which are liable to expose the population to heat stress.

Four lines show the number of hot days in the cities of Berlin, Frankfurt am Main and Munich from 1986 to 2017. With the exception of Munich, there are individual data gaps in the time series. The area mean for Germany is also shown as dashed lines. All lines increase with fluctuations between the years. No trend analysis was carried out. In 2003 and 2015 there were the highest values with over 30 days in all cases.
BAU-I-1: Heat stress in urban environments

Four lines show the number of hot days in the cities of Berlin, Frankfurt am Main and Munich from 1986 to 2017. With the exception of Munich, there are individual data gaps in the time series. The area mean for Germany is also shown as dashed lines. All lines increase with fluctuations between the years. No trend analysis was carried out. In 2003 and 2015 there were the highest values with over 30 days in all cases.

Source: DWD (selected urban climate measuring stations; German climate atlas)
 

BAU-I-2: Summer-related heat island effect – case study

Between the inner city and the urban fringe of Berlin, a maximum day-time temperature difference of up to 9 Kelvin is possible. It is so far not possible to judge whether climate change is exacerbating the heat island effect. However, even if the air temperatures in the city and its vicinity rise ’only’ with equal strength, heat stress, especially among the city population will very frequently be high.

The frequency of heat island situations in the meteorological summer, i.e. from the beginning of June to the end of August, is presented in the form of stack columns using data for Berlin.
BAU-I-2: Summer-related heat island effect – case study

The frequency of heat island situations in the meteorological summer, i.e. from the beginning of June to the end of August, is presented in the form of stack columns using data for Berlin. Values for the years 2008 to 2010, 2012 to 2014, and 2016 and 2017 are shown. There are three categories of heat island intensity: 1-3 Kelvin, 4-6 Kelvin and over 6 Kelvin. With the exception of 2008, 2010 and 2017, there were always a few days in the highest category. The middle category clearly predominates in all years.
In addition, a line for the maximum heat island intensity and a line for the medium heat island intensity are shown for all years. A clear development is not discernible. No trend analysis was carried out.

Source: DWD (climate measuring stations Berlin Alexanderplatz and Schönefeld)
 

Heat stress in city environments

Cities are often subject to climatic conditions which are very different from the climate prevailing in their periphery. For example, the relative air humidity tends to be lower, and the mean temperature tends to be higher. In respect of temperature differences between urban and rural areas, climatologists refer to ’urban heat islands’. The intensity of the urban heat island effect is largely dependent on the size of the town or city, the density of buildings, their height, the proportion of green space and the construction materials used. Also cloud cover and wind patterns play important roles in terms of urban heat stress.

In hot summer months this can entail health problems for the population in cases where – owing to their characteristics – urban spaces heat up considerably during daytime without cooling down over night to the same extent as their rural periphery. This type of situation may occur more frequently in future. As indicated by climate projections for Central Europe, mean temperatures will rise and the overall weather characteristics will change too. For example, extreme thermal values are expected to occur more frequently. For instance, the frequency of so-called ‘hot days’ is expected to increase, where maximum air temperatures will reach or exceed 30 °C. Besides, the population will experience stress on so-called ’tropical nights’ when the thermometer will not go below 20 °C and there is little chance of enjoying a restful night.

As far as Germany’s surface area mean is concerned, the mean number of hot days has increased from approx. 3.5 days in the 1950s to presently approx. 10 days per annum (see p. 22). The time series for city climate measuring stations in Berlin, Frankfurt am Main and Munich do not go back as far as 1951. It is therefore not possible to make any comparisons with the development at a nationwide level. However, there is evidence even from shorter time series that cities are subject to special circumstances. Here, hot days – albeit with regional differences – clearly occur more frequently in most years than compared to the nationwide mean. In years with above-average hot summers, such as 2003, 2006 and 2015, the differences are particularly distinct. This applies even more strongly to tropical nights although these are not illustrated in this context. While Germany’s nationwide mean was between 1 and 1.5 tropical nights for the years mentioned, in Berlin as many as 12 and in Frankfurt am Main up to 14 such nights were recorded.

In contrast with hot days and tropical nights, the time series for urban heat island effects – on the basis of values measured every 10 minutes for the city of Berlin – does not refer to the frequency of times when the threshold value was exceeded. In fact, it refers to the maximum daily temperature differential between the centre and the city’s immediate periphery. In the summer months of June to August, these values are on average between 3 and 4 Kelvin. However, on peak days, temperature differences of almost 9 Kelvin are possible. Overall, high temperature differences occur especially in the evenings and at night. In other words, the inner city cools down distinctly more slowly in summer and less so than the periphery. This means that it is often too hot for the city population to get a sufficiently restful night.

So far the time series does not make it possible to judge whether climate change exacerbates the urban heat island effect. This may be because heat stress is increasing with equal strength in both cities and their periphery. This is roughly confirmed by indications given by projections for Frankfurt am Main38. Even without an increase in the intensity of urban heat islands, this would mean for the future that stressful situations will in all likelihood continue to be most frequent wherever the heat stress is already high at present.

38 - Früh B., Koßmann M., Roos M. 2011: Frankfurt am Main im Klimawandel – Eine Untersuchung zur städtischen Wärmebelastung. Berichte des Deutschen Wetterdienstes Nr. 237, Offenbach, 68 S

 

Interfaces

GE-I-1: Heat exposure and Public awareness

BAU-R-1: Recreation areas

IG-I-1: Heat-related loss in performance

 

Objectives

Mitigating climate-related increases in the heating effect in cities – as well as any associated heat stress – by means of suitable architecture as well as urban and spatial planning; safeguarding fresh air supply by means of unobstructed fresh-air corridors, especially in inner cities; inhibiting – with regard to urban development – any further sealing of open spaces by overbuilding with residential areas or traffic routes (DAS, ch. 3.2.1)