Results of the tram-based measurements from the Hannover urban climate project (2017 to 2020)

The current climate change is a great challenge, especially for cities. The already elevated temperature levels rise further, hence, the urban heat island effect is intensified. To adapt adequately, it is essential to understand the climatic conditions of a city and how they might vary within the city. In the context of preventive disaster control, the state of Lower Saxony and the German Meteorological Service (DWD) work closely together. In a joint project, which took place between July 2017 and December 2020, the climate of the city of Hannover was investigated. To obtain a complete picture of the climate in the city, various measurement methods were used.  Here, we want to present the results of the tram measurements. Three regular trams from the Hannover public transport (Üstra) were equipped with measuring sensors. During their regular trips through the city, these trams collected meteorological data, specifically the air temperature on their route. These measurements enabled a comprehensive spatial resolution of meteorological data. For the evaluation, special route sections were selected that were of particular interest from a structural standpoint. These ranged from purely commercial areas to residential areas and even sparsely built green spaces. The differences (spatial temperature anomalies) between the tram data and a reference station located in the surrounding area, were evaluated. It was found that the structural characteristics along the route alone do not always have to be the cause of high or low overheating. The conditions in the immediate neighborhood also play a significant role. For example, a route section located in a green strip experiences strong overheating when the wind blows from the east, where the mostly strong overheated Nordstadt is located. On the other hand, heavily built and sealed areas can benefit from a sparsely built and greener environment when winds come from the appropriate direction.

In summary, the measurement of meteorological variables using trams is a powerful tool for analyzing the microclimate of a city. The high spatial resolution offers the possibility to investigate the climatic conditions of all city areas. This is a great advantage, as it is not always possible to install temporary measurement stations everywhere.