UHI formula (Heat Overlay)

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The Urban heat island effect is calculated 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. The resulting heat island effect will be low then 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)

See also