UHI formula (Heat Overlay)

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The Urban heat island effect is calculated[1] using the following formula:
where:

Formula Decomposition

The formula has two parts, the factor and the 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. In that case, the resulting heat island effect will be low as well.

Temperature effect

The Urban heat island temperature effect is calculated as:

Daily average global radiation

The S is calculated as followed:

where,

  • is the daily average global radiation in W/m2/hr
  • is the air heat capacity in J. We use a value of 1007 J.
  • is the hourly temperature measured at the station
  • is the calculated air density in kg/m3;
  • is the gas constant for dry air. We use a value of 287.058 J/(kg·K)
  • converts the used temperature from celcius (°C) to kelvin (K)

See also



References

  1. A diagnostic equation for the daily maximum urban heat island effect for cities in northwestern Europe • N.E. Theeuwes et al. • http://www.meteo.wur.nl/medewerkers/steeneveld/Theeuwes_JOC_2016.pdf • last visited 13-02-2020