There is no universal definition of a city in the European Union. Therefore, in COOLCITY, a city is defined as an urbanized area managed by a single administrative authority. To define city areas, we use three input datasets:

• administrative division LAU (Local Administrative Units),
• degree of urbanization DEGURBA, which considers population size, density, and built-up area density,
• a point layer of cities enriched with demographic attributes from Eurostat and local statistical offices.

Based on these data, urbanized areas are identified according to the following criteria:

• population of at least 5,000 residents (based on local statistical offices),
• urban area of at least 2 km² (except for cities located within metropolitan areas, where this criterion is not mandatory).

Ultimately, 11,041 urbanized areas in the European Union meet these criteria.

The Performance Index is an additional indicator used to assess the extent to which cities effectively utilise their resources and develop high-quality blue-green infrastructure (BGI). It shows the difference between the COOLCITY Index value for a given city and the average value observed in other cities with similar socio-geographical conditions. It reflects the magnitude of benefits achieved through the implementation of blue-green infrastructure in urban adaptation relative to local conditions and opportunities.

The Performance Index models the influence of socio-geographical conditions on a city’s adaptation potential and allows for assessing how much the state of BGI in a given city differs from that of cities developing under similar geographic conditions and levels of urbanisation. This makes it possible to evaluate not only which cities have high adaptation potential, but also which ones make better use of their resources than would be expected given their conditions. A positive value of the index indicates a better-than-expected result, while a negative value may indicate untapped potential.