This Recommendation specifies a quantitative evaluation framework for the thermal environment of data centres characterized by high heat-flux density, stringent environmental requirements, and refrigeration systems with year-round operation. The framework defines multi-level indicators: room-, row-, cabinet-, device-, and chip-level, covering airflow organization and cooling effectiveness. The framework supports identifying hot spots, optimizing airflow patterns, reporting safety levels, and quantifying energy use and savings potential, and it enables comparison across data centres of different sizes, layouts, and airflow schemes.