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The Urban Heat Island (UHI) effect occurs when built-up areas record significantly higher average temperatures than the surrounding rural zones. The phenomenon is primarily caused by the reduction of vegetated surfaces, the high heat absorption of construction materials (concrete, asphalt), and anthropogenic heat production. The KPI shows, for the months from April to August, the distribution of UHI intensity levels, calculated through two indicators: the UTFVI (Urban Thermal Field Variance Index) and the night-time land surface temperature (Night LST).
UHI has direct impacts on human health (heat waves, summer mortality), energy consumption for cooling, urban biodiversity, and air quality. The severity of the phenomenon is directly correlated with the density and type of urban land cover (Oke, 1982). The KPI is inverted: higher UHI intensity values indicate more critical conditions.
The UTFVI quantifies the UHI effect intensity through the variance of the urban thermal field. It is calculated as:
UTFVI = (LST_pixel - LST_mean) / LST_mean
where LST_pixel is the land surface temperature of the pixel and LST_mean is the mean temperature of the analysed area. Positive values indicate pixels warmer than average (UHI hotspots); negative values indicate cooler pixels (mitigation zones).
The night-time LST directly measures surface temperature during night-time hours (typically 01:30 local time for MODIS Aqua). Night-time temperatures are particularly significant for UHI since urban surfaces retain heat accumulated during the day.
The KPI uses MODIS LST data at 1 km resolution, downscaled to 25 m via a machine learning model exploiting:
Calculation pipeline:
Analysis period: April — August (most representative months for the UHI phenomenon)
Units:
| Code | Name | Provider | Resolution | Availability |
|---|---|---|---|---|
WRD_MAQLT_99 | MODIS Aqua LST | NASA / USGS | 1000 m | continuous |
WRD_MTRLT_99 | MODIS Terra LST | NASA / USGS | 1000 m | continuous |
WRD_ESAXX_99 | ESA WorldCover | ESA / Impact Observatory | 10 m | 2020, 2021 |
WRD_S2XXX_99 | Sentinel-2 | ESA/Copernicus | 10 m | 2017 — present |
WRD_SRTMX_99 | NASA SRTM | NASA | 30 m | static |
| Indicator | Unit | Range | Inverted |
|---|---|---|---|
utfvi | — | [0, 0.2, 0.4, 0.6, 0.8, 1] | Yes |
Inverted = Yes: a lower value indicates lower UHI intensity and better conditions.
| Level | UTFVI | Interpretation |
|---|---|---|
| A (Excellent) | 0 – 0.2 | UHI effect absent or negligible; area at or below average temperature |
| B (Good) | 0.2 – 0.4 | Slight UHI effect; mild warming above average |
| C (Moderate) | 0.4 – 0.6 | Moderate UHI effect; impact on thermal comfort conditions |
| D (Poor) | 0.6 – 0.8 | Significant UHI effect; health risk during heat waves |
| E (Critical) | > 0.8 | Severe UHI effect; strong warming; critical health risk areas |
| Indicator | Unit | Range | Inverted |
|---|---|---|---|
night_lst | °C | [-10, 20, 25, 30, 35, 45] | Yes |
Inverted = Yes: lower night-time temperatures indicate better conditions and lower nocturnal UHI intensity.
| Level | Night LST | Interpretation |
|---|---|---|
| A (Excellent) | < 20°C | Cool night-time temperatures; excellent thermal mitigation |
| B (Good) | 20 – 25°C | Comfortable temperatures; slight nocturnal warming |
| C (Moderate) | 25 – 30°C | Mild temperatures; moderate nocturnal UHI impact |
| D (Poor) | 30 – 35°C | High temperatures; warm nights; thermal stress |
| E (Critical) | > 35°C | Very high temperatures; tropical nights; elevated health risk |
utfvi
night_lst
(°C)MODIS LST data (Terra: WRD_MTRLT_99, Aqua: WRD_MAQLT_99) at 1 km resolution, downscaled to 25 m via ML model. Downscaling features: Land Cover (ESA WorldCover, WRD_ESAXX_99), NDVI (Sentinel-2, WRD_S2XXX_99), Altitude (NASA SRTM, WRD_SRTMX_99). Analysis period: April–August. UTFVI = (LST_pixel - LST_mean) / LST_mean. Night LST: mean night-time surface temperature for April–August period. Cloud filtering via MODIS QC bands. Two inverted indicators: utfvi [0, 0.2, 0.4, 0.6, 0.8, 1] and night_lst [-10, 20, 25, 30, 35, 45].