97
4.2.1 Physiologically Equivalent Temperature
The Physiologically Equivalent Temperature (PET) is the equivalent temperature at
a given place (outdoors or indoors) to the air temperature in a typical indoor setting
with core and skin temperatures equal to those under the conditions being assessed.
Thereby, the heat balance of the human body with a work metabolism 80 W (light
activity, added to basic metabolism) and a heat resistance of clothing 0.9 clo) is
maintained (Höppe 1999).
The following assumptions are made for the indoor reference climate:
• mean radiant temperature equals air temperature (Tmrt = Ta).
• air velocity (wind speed) is fixed at v = 0.1 m/s.
• water vapor pressure is set to 12 hPa (approximately, equivalent to a relative
humidity of 50 % at Ta = 20 °C).
The procedure for the calculation of PET contains the following steps:
1. calculation of the thermal conditions of the body with MEMI for a given combination of meteorological parameters.
2. insertion of the calculated values for mean skin temperature and core temperature into the model MEMI and computation the energy balance equation system
for the air temperature Ta (with v =0.1 m/s, VP = 12 hPa and Tmrt = Ta).
3. the resulting air temperature is equivalent to PET.
PET offers the advantage of a widely known unit (degrees Celsius), which makes
results more comprehensible to regional or urban planners, who are not necessarily
very familiar with the modern human-biometeorological methods (Matzarakis et al.
1999). The assessment classes of PET (Table 4.1) are valid only for the assumed
values of internal heat production (80 W) and thermal resistance of the clothing
(0.9) (Matzarakis and Mayer 1997).
The meteorological input parameters have to be measured or transferred to the
average height of a standing person’s gravity center, 1.1 m above ground (Matzarakis
et al. 2009). These meteorological parameters can be measured or calculated by
numerical models. PET can be calculated by models like RayMan (Matzarakis et al.
2007, 2010).
4.2.2 Universal Thermal Climate Index
The Universal Thermal Climate Index UTCI (Jendritzky et al. 2012) is defined as
the air temperature (Ta) of the reference condition causing the same model response
as the actual condition. Thus, UTCI represents the air temperature, which would
produce, under reference conditions, the same thermal strain as in the actual thermal
4 Relevance of Thermal Indices for the Assessment of the Urban Heat Island
4.2.1 Physiologically Equivalent Temperature
The Physiologically Equivalent Temperature (PET) is the equivalent temperature at
a given place (outdoors or indoors) to the air temperature in a typical indoor setting
with core and skin temperatures equal to those under the conditions being assessed.
Thereby, the heat balance of the human body with a work metabolism 80 W (light
activity, added to basic metabolism) and a heat resistance of clothing 0.9 clo) is
maintained (Höppe 1999).
The following assumptions are made for the indoor reference climate:
• mean radiant temperature equals air temperature (Tmrt = Ta).
• air velocity (wind speed) is fixed at v = 0.1 m/s.
• water vapor pressure is set to 12 hPa (approximately, equivalent to a relative
humidity of 50 % at Ta = 20 °C).
The procedure for the calculation of PET contains the following steps:
1. calculation of the thermal conditions of the body with MEMI for a given combination of meteorological parameters.
2. insertion of the calculated values for mean skin temperature and core temperature into the model MEMI and computation the energy balance equation system
for the air temperature Ta (with v =0.1 m/s, VP = 12 hPa and Tmrt = Ta).
3. the resulting air temperature is equivalent to PET.
PET offers the advantage of a widely known unit (degrees Celsius), which makes
results more comprehensible to regional or urban planners, who are not necessarily
very familiar with the modern human-biometeorological methods (Matzarakis et al.
1999). The assessment classes of PET (Table 4.1) are valid only for the assumed
values of internal heat production (80 W) and thermal resistance of the clothing
(0.9) (Matzarakis and Mayer 1997).
The meteorological input parameters have to be measured or transferred to the
average height of a standing person’s gravity center, 1.1 m above ground (Matzarakis
et al. 2009). These meteorological parameters can be measured or calculated by
numerical models. PET can be calculated by models like RayMan (Matzarakis et al.
2007, 2010).
4.2.2 Universal Thermal Climate Index
The Universal Thermal Climate Index UTCI (Jendritzky et al. 2012) is defined as
the air temperature (Ta) of the reference condition causing the same model response
as the actual condition. Thus, UTCI represents the air temperature, which would
produce, under reference conditions, the same thermal strain as in the actual thermal
4 Relevance of Thermal Indices for the Assessment of the Urban Heat Island