UHI formula (Heat Overlay): Difference between revisions

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* T<sub>min</sub> is the [[Daily_temperature_min_(Heat_Overlay)|minimum temperature]] measured at a weather station between 8 AM and 7 AM the next day.
* T<sub>min</sub> is the [[Daily_temperature_min_(Heat_Overlay)|minimum temperature]] measured at a weather station between 8 AM and 7 AM the next day.
* U is the [[Daily_avg_wind_speed_(Heat_Overlay)|daily average wind speed]] measured at 10 meters above ground at a weather station.
* U is the [[Daily_avg_wind_speed_(Heat_Overlay)|daily average wind speed]] measured at 10 meters above ground at a weather station.


==Formula Decomposition==
==Formula Decomposition==

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:

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:

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)