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<sup>1</sup>)
* Q<sub>ql-avg</sub> is the 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 13:25, 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


Analysis

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 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)