UHI formula (Heat Overlay): Difference between revisions
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==Formula Decomposition== | ==Formula Decomposition== | ||
The formula has two parts, the factor and the temperature effect:<br> | The formula has two parts, the factor and the temperature effect:<br> | ||
UHI<sub>max</sub> = [[ | UHI<sub>max</sub> = [[#Factor|factor]] * [[#Temperature effect|temperature_effect]] <br> | ||
====Factor==== | ====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. | 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. | ||
Revision as of 13:32, 11 September 2019
The Urban heat island effect is calculated using the following formula:
UHImax = ( 2 - Svf - Fveg ) * ( S * (Tmax-Tmin)3 / U )-4
where:
- Svf is the calculated average sky view factor;
- Fveg is the calculated average vegetation fraction;
- S is the calculated daily average global radiation in K m/s
Formula Decomposition
The formula has two parts, the factor and the temperature effect:
UHImax = 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
Daily average global radiation
The S is calculated as followed:
ρair = p / Rspecific * (Tstation + 273.15)
S = Qql-avg / (Cair * ρair)
where,
- Qql-avg is the daily average global radiation in W/m2/hr
- Cair is the air heat capacity in J. We use a value of 1007.0.
- Tstation is the temperature measured at the station
- ρair is the calculated air density in kg/m3;
- Rspecific is the gas constant for dry air. We use a value of 287.058 J/(kg·K)