UHI formula (Heat Overlay): Difference between revisions
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UHI<sub>max</sub> = ( 2 - S<sub>vf</sub> - F<sub>veg</sub> ) * ( S * (T<sub>max</sub>-T<sub>min</sub>)<sup>3</sup> / U )<sup>-4</sup> <br> | UHI<sub>max</sub> = ( 2 - S<sub>vf</sub> - F<sub>veg</sub> ) * ( S * (T<sub>max</sub>-T<sub>min</sub>)<sup>3</sup> / U )<sup>-4</sup> <br> | ||
where: | where: | ||
* S<sub>vf</sub> is the calculated average sky view factor; | * S<sub>vf</sub> is the calculated [[Sky_view_avg_result_type_(Heat_Overlay)|average sky view factor]]; | ||
* F<sub>veg</sub> is the calculated [[Vegetation_avg_result_type_(Heat_Overlay)|average vegetation fraction]]; | * F<sub>veg</sub> is the calculated [[Vegetation_avg_result_type_(Heat_Overlay)|average vegetation fraction]]; | ||
* S is the calculated [[#Daily average global radiation|daily average global radiation]] in K m/s | * S is the calculated [[#Daily average global radiation|daily average global radiation]] in K m/s | ||
Revision as of 13:45, 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
- Tmax is the maximum temperature measured at a weather station between 8 AM and 7 AM the next day.
- Tmin is the minimum temperature measured at a weather station between 8 AM and 7 AM the next day.
- U is the daily average wind speed measured at 10 meters above ground at a weather station.
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
The Urban heat island temperature effect is calculated as:
- the daily average global radiation S,
- the maximum-minimum temperature difference ΔT
- the daily average wind speed, measured at 10m above ground.
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 J.
- Tstation is the hourly 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)