UHI formula (Heat Overlay): Difference between revisions
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S = Q<sub>ql-avg</sub> / (C<sub>air</sub> * ρ<sub>air</sub>)<br> | S = Q<sub>ql-avg</sub> / (C<sub>air</sub> * ρ<sub>air</sub>)<br> | ||
where, | where, | ||
* Q<sub>ql-avg</sub> is the daily average global radiation in W/m<sup>2</sup>/hr | * Q<sub>ql-avg</sub> is the [[Daily_avg_radiation_(Heat_Overlay)|daily average global radiation]] in W/m<sup>2</sup>/hr | ||
* C<sub>air</sub> is the air heat capacity in J. We use a value of 1007.0. | * C<sub>air</sub> is the air heat capacity in J. We use a value of 1007.0. | ||
* T<sub>station</sub> is the temperature measured at the station | * T<sub>station</sub> is the temperature measured at the station | ||
* ρ<sub>air</sub> is the calculated air density in kg/m<sup>3</sup>; | * ρ<sub>air</sub> is the calculated air density in kg/m<sup>3</sup>; | ||
* R<sub>specific</sub> is the [https://en.wikipedia.org/wiki/Specific_gas_constant|specific gas constant] for dry air. We use a value of 287.058 J/(kg·K) | * R<sub>specific</sub> is the [https://en.wikipedia.org/wiki/Specific_gas_constant|specific gas constant] for dry air. We use a value of 287.058 J/(kg·K) | ||
Revision as of 13:41, 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.
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 1007.0.
- 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)