What is urban climate?

City climate is defined as "local climate which differs from that in neighbouring rural areas, as a result of urban development". This means air temperature, precipitation, concentration of air pollutants, and wind speed often differ from the surrounding areas. Differences in air temperature between a city and its surrounding areas can reach up to 10°C at night. City effects on precipitation intensity and wind fields are also documented.

Why do cities heat up but do not cool off as much?

Cities are characterised by densely built-up areas and surface sealing. This leads to higher air temperatures and reduced wind speeds. Buildings and streets store the energy of solar radiation during the day and release this energy in form of heat at night. Wind speed, and thus ventilation of the city, is often reduced by buildings. Vegetation such as urban trees or green spaces that could provide shade and cooling by evaporation can be rare or missing altogether.

What are the consequences?

Heat waves particularly affect the elderly and the sick, since cardiovascular diseases increase the risk of death during the heat. Increasing temperatures also increase the number of heavy precipitation events in cities, thereby causing floods whose impact is much more severe in cities than in the open countryside: the sealed surfaces of the city increase water runoff and prevent seeping of rainwater into the ground. The sewer system provides only limited buffer for rainwater and can overflow if precipitation is high. The increased likelihood of heavy precipitation and hail events can require additional insurance services due to the enormous damage they can cause in a city.

Why is it important to monitor urban climate?

The world's population living in urban areas is more than half today, and constantly rising. The increasing number of people in cities also require more living space and extended infrastructure. With increasing infrastructure density, further temperature increases are likely or need compensation measures even in cities where this is not yet necessary. Extensive monitoring of the urban climate is the basis for significantly improving the quality of life by evaluating suitable urban measures. Municipal companies can improve their efficiency (traffic, road and construction management, water budgets, etc.) if they have access to close-meshed (temperature, precipitation, etc.) measurements in the city. Organisers, planners, and Insurance companies can benefit from better risk assessment for heat, precipitation, wind and other factors.

How to better predict the urban climate and its effects?

The first step to better understand a city’s climate and its possible future change is to install a modern sensor network at suitable locations within the city to measure and store hourly data for air temperature, precipitation, and other variables. Based on these data, special city climate models fill the gaps between the measurements and generate a comprehensive climate information system for the city. This system can then produce risk maps and assessments, as well as forecasts, warnings, and planning scenarios. The system can be used to inform the citizens, support decision makers, and help develop plans to improve the city climate.