Isole di calore urbano

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Responsible:Niccolò Calandri

UTFVI — Urban Thermal Field Variance Index

Overview

The UTFVI (Urban Thermal Field Variance Index) quantifies the intensity of the urban heat island (UHI) effect by measuring the relative deviation of each pixel's land surface temperature from the spatial mean temperature of the analysis area. It is derived from the Night LST product.

The UTFVI formula is:

UTFVI = (LST - LST_mean) / LST_mean

Where:

LST = Local land surface temperature at a given pixel (K)
LST_mean = Spatial mean LST across the entire analysis area (K)

Positive UTFVI values indicate temperatures above the area average (heat island), while negative values indicate cooler areas (typically vegetated or water surfaces). The index is classified into 6 ecological evaluation levels:

Class	UTFVI Range	Interpretation
Excellent	< -0.100	Strong cooling effect (water bodies, dense vegetation)
Good	-0.100 to -0.050	Moderate cooling (parks, forests)
Normal	-0.050 to -0.010	Near-average temperature
Bad	-0.010 to 0.005	Slight warming (suburban areas)
Worse	0.005 to 0.015	Significant warming (dense urban)
Worst	> 0.015	Extreme heat island (industrial, highly impervious)

Calculation Methodology

UTFVI derivation pipeline:

Night LST generation — High-resolution nighttime LST is produced via Random Forest downscaling of MODIS thermal data to 25 m (see Night LST layer documentation)
Spatial mean calculation — LST_mean is computed as the area-weighted spatial average across all valid pixels within the analysis area
UTFVI computation — For each pixel: UTFVI = (LST - LST_mean) / LST_mean
Classification — UTFVI values are binned into 6 classes using the thresholds above
Spatial analysis — Urban vs. rural UTFVI distributions are compared for SUHI characterization

Key characteristics:

UTFVI is a relative index: values depend on the analysis area extent
Nighttime UTFVI is preferred over daytime because it captures heat retention rather than instantaneous solar heating
Dense urban areas with high imperviousness typically show the highest UTFVI values
Vegetated areas (parks, forests, gardens) act as cooling islands with negative UTFVI

Data Sources

Code	Name	Provider	Type	Coverage	Resolution	Availability
`MODIS/061/MOD11A1`	MODIS Terra LST Daily	NASA LP DAAC	Thermal raster	Global	~1 km → 25 m	2000 — present
`MODIS/061/MYD11A1`	MODIS Aqua LST Daily	NASA LP DAAC	Thermal raster	Global	~1 km → 25 m	2002 — present
`COPERNICUS/S2_SR_HARMONIZED`	Sentinel-2 L2A (NDVI)	ESA/Copernicus	Multispectral	Global	10 m	2017 — present
`ESA/WorldCover/v200`	ESA WorldCover 2021	ESA	Land cover	Global	10 m	2021
`USGS/SRTMGL1_003`	SRTM DEM	NASA/USGS	Elevation	60°N–56°S	30 m	2000

Quality Range and Levels

Indicator	Unit	Range	Inverted
`utfvi`	dimensionless	[-0.2, -0.1, -0.05, -0.01, 0.005, 0.015, 0.1]	Yes

Inverted = Yes: a higher UTFVI value indicates a worse condition (stronger heat island).

Technical Notes

UTFVI is highly sensitive to the spatial extent of the analysis area; comparing UTFVI values between sites of different sizes should be done with caution
The index works best for urban and peri-urban areas where temperature gradients are significant
In homogeneous landscapes (uniform agriculture, dense forest), UTFVI values cluster near zero with low information content
Seasonal variations are significant: summer UTFVI typically shows stronger patterns than winter
Cloud-free conditions are required for the underlying LST data

References

Liu, L., Zhang, Y. (2011). "Urban heat island analysis using the Landsat TM data and ASTER data: A case study in Hong Kong". Remote Sensing, 3(7), pp. 1535–1552.
Zhang, J., Wang, Y., Li, Y. (2006). "A C++ program for retrieving land surface temperature from the data of Landsat TM/ETM+ band 6". Computers & Geosciences, 32(10), pp. 1796–1805.
Oke, T.R. (1982). "The energetic basis of the urban heat island". Quarterly Journal of the Royal Meteorological Society, 108(455), pp. 1–24.

Data Sources & References

Sources

WRD_MAQLT_991d

WRD_MTRLT_991d

Methodology

L'UTFVI (Urban Thermal Field Variance Index) è un indice derivato dalla Land Surface Temperature notturna per valutare l'intensità dell'effetto isola di calore urbana (Urban Heat Island - UHI). L'indice viene calcolato normalizzando la LST rispetto alla media dell'area: UTFVI = (LST - LST_mean) / LST_mean, dove LST è la temperatura del pixel e LST_mean è la media dell'area. Valori positivi indicano temperature superiori alla media (aree più calde), valori negativi indicano temperature inferiori alla media (aree più fresche). L'indice viene classificato in 6 livelli di comfort termico: 1-Excellent (UTFVI < -0.1): condizioni termiche ottimali, tipiche di parchi e aree verdi; 2-Good (-0.1 a 0): buone condizioni, aree con vegetazione moderata; 3-Normal (0 a 0.005): condizioni nella media dell'area; 4-Bad (0.005 a 0.01): disagio termico moderato, tipico di aree suburbane dense; 5-Worse (0.01 a 0.015): disagio termico elevato, aree commerciali/industriali; 6-Worst (> 0.015): condizioni critiche, centri urbani densi con scarsa vegetazione. L'analisi può includere anche SUHI (Surface Urban Heat Island) basata su zone concentriche: Ua (aree urbane >50% costruito), FUa (buffer 500m), PUa (buffer 2km).

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