*5.2. Meteorological Characteristics in Urban Environment*

According to [193], the local climate in cities often differs from surrounding areas. The "urban heat island" effect is a feature of the urban climate which is amongst others characterized by differences in temperature of up to 10 Kelvins in large cities. Additional changes can also be seen in air humidity, radiation, wind, air quality and noise.

Prevailing weather characteristics may also change on very small scales inside city boundaries creating the phenomena of micro-weather. For this purpose, the investigation of wind channeling, turbulence from buildings and urban canyoning, and the development of smart city sensing, micro-grid networks/weather models as well as high computational resources and machine learning approaches are required [187,192]. One of the biggest challenges is that "it is recognised that the weather information for UAS operations may be different from the one provided by today's meteorological service providers [. . . ]. UAS can fly near buildings and in areas where current aeronautical meteorological information is not always provided" [51].

According to [192], additional smart urban sensing can be achieved by optimally placed sensors. A contribution is expected in the development of urban climatology, the improvement of forecasts and the reduction of uncertainties, while targeting optimal UAM flight routes. Expected hurdles are communication bandwidth associated with high costs of expanding the network and possible congestion of current wireless networks due to the amount of data collectors required to achieve sufficient coverage. Processing and computational resources to sight and analyze collected data are needed. The "optimal placement" of weather sensors needs to be investigated thoroughly.

Equipping every VTOL aircraft with weather sensors and thereby increasing enormously the amount of real-time weather data could be a supplemental approach. This data could be then shared inside the UAM network e.g., through a U-space weather information service provider and can be used for weather analysis and forecasts. However, this also requires equipment investments and may probably lead to reduction of payload.
