UHI formula (Heat Overlay)

The Urban heat island effect is calculated using the following formula:
UHI_max = ( 2 - S_vf - F_veg ) * ( S * (T_max-T_min)³ / U )^-4
where:

• S_vf is the calculated average sky view factor;
• F_veg is the calculated average vegetation fraction;
• S is

Analysis

The formula has two parts, the factor and the temperature effect:
UHI_max = factor * temperature_effect

Factor

The factor is influenced by the sky view factor and the vegetation fraction, both ranging from 0 to 1. When both are low, i.e. barely any sky and no vegetation, the factor is near 2. When both are high, i.e. no surrounding buildings and a lot of vegetation, the factor is near 0.

Temperature effect

The S is calculated as followed: S = R_avg / (C_air * ρ_air)
where,

• R_avg is the daily average radiation in W/m²hr^-1