Urban Thermal Field Variance Index (UTFVI) is a dimensionless index that measures how much hotter or cooler each pixel’s land surface temperature is compared with the mean LST of the whole urban area, and is widely used to map and evaluate urban heat islands.​

The commonly used UTFVI formula is: $\text{UTFVI}=\frac{Ts-Tm}{Tm}$UTFVI=TmTs−Tm, where $Ts$Ts is pixel LST and $Tm$Tm is mean LST of the study area. Positive UTFVI values indicate pixels hotter than the city average (potential heat hotspots), while negative values indicate cooler-than-average surfaces, often linked with vegetation or water.​

Thermal Comfort Classes

A frequently adopted ecological evaluation scheme (with slight variations across studies) is:

• UTFVI < 0: no thermal stress, excellent comfort.​
• 0–0.005: weak UHI impact, good comfort.​
• 0.005–0.01: middle impact, normal comfort.​
• 0.01–0.015: strong impact, bad comfort.​
• 0.015–0.02: stronger impact, worse comfort.​
• 0.02: strongest impact, worst ecological condition and severe heat stress.

These classes are typically mapped to show how much of the city lies in “bad–worst” thermal conditions versus “excellent–good” ones.

Relationship to UHI and LULC

UTFVI is used as a quantitative expression of the surface urban heat island, linking elevated LST to built-up density and vegetation loss. Studies repeatedly show that declining NDVI and increasing NDBI are associated with rising LST and higher UTFVI values, highlighting the role of urban expansion and loss of green cover in intensifying thermal stress.

Link(s) and Source(s):

https://www.sciencedirect.com/science/article/pii/S2212095524000440