UHI formula (Heat Overlay): Difference between revisions
Jump to navigation
Jump to search
No edit summary |
No edit summary |
||
| Line 1: | Line 1: | ||
The Urban heat island effect is calculated using the following formula:<br> | The Urban heat island effect is calculated using the following formula:<br> | ||
UHI<sub>max</sub> = ( 2 - S<sub>vf</sub> - F<sub>veg</sub> ) | UHI<sub>max</sub> = ( 2 - S<sub>vf</sub> - F<sub>veg</sub> ) · ( S · (T<sub>max</sub>-T<sub>min</sub>)<sup>3</sup> / U )<sup>-4</sup> <br> | ||
where: | where: | ||
* S<sub>vf</sub> is the calculated [[Sky_view_avg_result_type_(Heat_Overlay)|average sky view factor]]; | * S<sub>vf</sub> is the calculated [[Sky_view_avg_result_type_(Heat_Overlay)|average sky view factor]]; | ||
| Line 11: | Line 11: | ||
==Formula Decomposition== | ==Formula Decomposition== | ||
The formula has two parts, the factor and the temperature effect:<br> | The formula has two parts, the factor and the temperature effect:<br> | ||
UHI<sub>max</sub> = [[#Factor|factor]] | UHI<sub>max</sub> = [[#Factor|factor]] · [[#Temperature effect|temperature_effect]] <br> | ||
====Factor==== | ====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 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. | ||
| Line 22: | Line 22: | ||
=====Daily average global radiation===== | =====Daily average global radiation===== | ||
The S is calculated as followed:<br> | The S is calculated as followed:<br> | ||
ρ<sub>air</sub> = p / R<sub>specific</sub> | ρ<sub>air</sub> = p / R<sub>specific</sub> · (T<sub>station</sub> + 273.15)<br> | ||
S = Q<sub>ql-avg</sub> / (C<sub>air</sub> | 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_avg_radiation_(Heat_Overlay)|daily average global radiation]] in W/m<sup>2</sup>/hr | * Q<sub>ql-avg</sub> is the [[Daily_avg_radiation_(Heat_Overlay)|daily average global radiation]] in W/m<sup>2</sup>/hr | ||
Revision as of 14:33, 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 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)