UHI formula (Heat Overlay)
Revision as of 09:56, 12 September 2019 by Frank@tygron.nl (talk | contribs)
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.
- U is the daily average wind speed measured at 10 meters above ground at a weather station.
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 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)
See also
