More than one hundred thermal indices are in use, and each new study seems to add another. For people who want to apply an index—in weather services, public health, occupational safety, urban planning, tourism or sports—the question is not which index is scientifically the most elaborate, but which one answers their question. This perspective gives a practical overview of nine indices that dominate application today: the empirical Heat Index (HI) and Discomfort Index (DI), the occupational screening index Wet Bulb Globe Temperature (WBGT), which is well established but not founded on thermophysiology, the thermal comfort index Predicted Mean Vote (PMV), which rests on a simplified heat balance, and the heat-balance equivalent temperatures Physiologically Equivalent Temperature (PET), modified PET (mPET), Universal Thermal Climate Index (UTCI), Perceived Temperature (PT) and Standard Effective Temperature (SET*). For each index, the purpose it was built for, the input variables, the assumptions about the human being, typical fields of application, strengths and limitations are summarised. Three tables condense the comparison, translate it into a simple decision path and compare PET, mPET and UTCI. The main message is simple: an index is a tool made for a purpose. Simple indices remain useful where radiation and wind are of minor importance or where data are scarce; equivalent temperatures based on the human energy balance are needed whenever sun, shade, wind and surfaces matter, that is, in most outdoor situations. Whatever index is chosen, its assumptions, its input data—above all the mean radiant temperature in sunshine—and its assessment scale must be reported transparently.
- Open Access
- Perspective
Which Thermal Index for Which Purpose? A Practical Guide to Simple Heat Indices and Equivalent Temperatures for Users
Author Information
Received: 08 October 2026 | Revised: 09 November 2026 | Accepted: 11 November 2026 | Published: 13 November 2026
Abstract
Keywords
thermal indices | heat stress | PET | UTCI | mean radiant temperature | human biometeorology
References
- 1.
de Freitas, C.R.; Grigorieva, E.A. A Comprehensive Catalogue and Classification of Human Thermal Climate Indices. Int. J. Biometeorol. 2015, 59, 109–120. https://doi.org/10.1007/s00484-014-0819-3.
- 2.
de Freitas, C.R.; Grigorieva, E.A. A Comparison and Appraisal of a Comprehensive Range of Human Thermal Climate Indices. Int. J. Biometeorol. 2017, 61, 487–512. https://doi.org/10.1007/s00484-016-1228-6.
- 3.
Staiger, H.; Laschewski, G.; Matzarakis, A. Selection of Appropriate Thermal Indices for Applications in Human Biometeorological Studies. Atmosphere 2019, 10, 18. https://doi.org/10.3390/atmos10010018.
- 4.
Jing, W.; Qin, Z.; Mu, T.; et al. Evaluating Thermal Comfort Indices for Outdoor Spaces on a University Campus. Sci. Rep. 2024, 14, 21253. https://doi.org/10.1038/s41598-024-71805-5.
- 5.
Subedi, R.; Rijal, H.B.; Khadka, S.; et al. Development of Adaptive Thermal Comfort Model for Urban Park Based on Field Survey and Literature Review. Front. Built Environ. 2025, 10, 1526919. https://doi.org/10.3389/fbuil.2024.1526919.
- 6.
Li, Y.; Fei, W.; Yang, M.; et al. How to Realize Large-Scale Outdoor Thermal Comfort Studies? A Systematic Review Based on OTC Characterization, Methods and Research Trends. Front. Sustain. Cities 2025, 7, 1552994. https://doi.org/10.3389/frsc.2025.1552994.
- 7.
Binarti, F.; Koerniawan, M.D.; Triyadi, S.; et al. A Review of Outdoor Thermal Comfort Indices and Neutral Ranges for Hot-Humid Regions. Urban Clim. 2020, 31, 100531. https://doi.org/10.1016/j.uclim.2019.100531.
- 8.
Gunadhi, A.C.R.; Rijal, H.B.; Subedi, R.; et al. Review of Microclimate Effects on PET and UTCI for Developing Outdoor Adaptive Models. Results Eng. 2026, 32, 112624. https://doi.org/10.1016/j.rineng.2026.112624.
- 9.
Thorsson, S.; Lindberg, F.; Eliasson, I.; et al. Different Methods for Estimating the Mean Radiant Temperature in an Outdoor Urban Setting. Int. J. Climatol. 2007, 27, 1983–1993. https://doi.org/10.1002/joc.1537.
- 10.
Kántor, N.; Unger, J. The Most Problematic Variable in the Course of Human-Biometeorological Comfort Assessment—the Mean Radiant Temperature. Open Geosci. 2011, 3, 90–100. https://doi.org/10.2478/s13533-011-0010-x.
- 11.
VDI 3787 Part 2; Environmental Meteorology—Methods for Human-Biometeorological Evaluation of the Thermal Component of the Climate. Beuth: Berlin, Germany, 2022.
- 12.
Steadman, R.G. The Assessment of Sultriness. Part I: A Temperature–Humidity Index Based on Human Physiology and Clothing Science. J. Appl. Meteorol. 1979, 18, 861–873.
- 13.
Rothfusz, L.P. The Heat Index “Equation” (or, More Than You Ever Wanted to Know about Heat Index); Technical Attachment SR 90-23; NWS Southern Region Headquarters: Fort Worth, TX, USA, 1990.
- 14.
Anderson, G.B.; Bell, M.L.; Peng, R.D. Methods to Calculate the Heat Index as an Exposure Metric in Environmental Health Research. Environ. Health Perspect. 2013, 121, 1111–1119. https://doi.org/10.1289/ehp.1206273.
- 15.
Lu, Y.C.; Romps, D.M. Extending the Heat Index. J. Appl. Meteorol. Climatol. 2022, 61, 1367–1383. https://doi.org/10.1175/jamc-d-22-0021.1.
- 16.
Thom, E.C. The Discomfort Index. Weatherwise 1959, 12, 57–61. https://doi.org/10.1080/00431672.1959.9926960.
- 17.
Giles, B.D.; Balafoutis, C.; Maheras, P. Too Hot for Comfort: The Heatwaves in Greece in 1987 and 1988. Int. J. Biometeorol. 1990, 34, 98–104. https://doi.org/10.1007/bf01093455.
- 18.
Yaglou, C.P.; Minard, D. Control of Heat Casualties at Military Training Centers. AMA Arch. Ind. Health 1957, 16, 302–316.
- 19.
ISO 7243; Ergonomics of the Thermal Environment—Assessment of Heat Stress Using the WBGT (Wet Bulb Globe Temperature) Index. International Organization for Standardization: Geneva, Switzerland, 2017.
- 20.
Budd, G.M. Wet-Bulb Globe Temperature (WBGT)—Its History and Its Limitations. J. Sci. Med. Sport 2008, 11, 20–32. https://doi.org/10.1016/j.jsams.2007.07.003.
- 21.
Liljegren, J.C.; Carhart, R.A.; Lawday, P.; et al. Modeling the Wet Bulb Globe Temperature Using Standard Meteorological Measurements. J. Occup. Environ. Hyg. 2008, 5, 645–655. https://doi.org/10.1080/15459620802310770.
- 22.
Lemke, B.; Kjellstrom, T. Calculating Workplace WBGT from Meteorological Data: A Tool for Climate Change Assessment. Ind. Health 2012, 50, 267–278. https://doi.org/10.2486/indhealth.ms1352.
- 23.
Epstein, Y.; Moran, D.S. Thermal Comfort and the Heat Stress Indices. Ind. Health 2006, 44, 388–398. https://doi.org/10.2486/indhealth.44.388.
- 24.
ISO 7933; Ergonomics of the Thermal Environment—Analytical Determination and Interpretation of Heat Stress Using Calculation of the Predicted Heat Strain. International Organization for Standardization: Geneva, Switzerland, 2023. https://doi.org/10.3403/03204852u.
- 25.
Staiger, H.; Matzarakis, A. Accuracy of Mean Radiant Temperature Derived from Active and Passive Radiometry. Atmosphere 2020, 11, 805. https://doi.org/10.3390/atmos11080805.
- 26.
Lamberti, G.; Della Vista, D.; Leccese, F.; et al. On the Geometrical Limitations of the Globe Thermometer for Assessing Pedestrian Radiative Exposure in Outdoor Settings. Energy Build. 2026, 350, 116609. https://doi.org/10.1016/j.enbuild.2025.116609.
- 27.
Błażejczyk, K.; Epstein, Y.; Jendritzky, G.; et al. Comparison of UTCI to Selected Thermal Indices. Int. J. Biometeorol. 2012, 56, 515–535. https://doi.org/10.1007/s00484-011-0453-2.
- 28.
Akbari, R.; Asghari, M.; Amrolahi, M.; et al. Validity and Field Applicability of the Environmental Stress Index (ESI) for Heat Stress Assessment in Outdoor Workers of Arid/Semi-Arid Climates. Int. J. Biometeorol. 2026, 70, 219. https://doi.org/10.1007/s00484-026-03282-6.
- 29.
Mousavi, S.M.; Dehghan, H.; Yazdanirad, S. Climate-‑Related Performance of the Wet-‑Bulb Globe Temperature (WBGT) Index: A Fuzzy Analytical Hierarchy Process Approach. Theor. Appl. Climatol. 2026, 157, 657. https://doi.org/10.1007/s00704-026-06597-x.
- 30.
Yazdanirad, S.; Abbasi, M.; Dehghan, A.; et al. Validation of Heat Strain Risk Assessment (HSRA) Indices Compared to Wet Bulb Globe Temperature (WBGT) and Predicted Heat Strain (PHS) Indices. BMC Public Health 2025, 25, 4332. https://doi.org/10.1186/s12889-025-25655-z.
- 31.
Fanger, P.O. Thermal Comfort: Analysis and Applications in Environmental Engineering; Danish Technical Press: Copenhagen, Denmark, 1970.
- 32.
ISO 7730; Ergonomics of the Thermal Environment—Analytical Determination and Interpretation of Thermal Comfort Using Calculation of the PMV and PPD Indices and Local Thermal Comfort Criteria. International Organization for Standardization: Geneva, Switzerland, 2005.
- 33.
Mayer, H.; Höppe, P. Thermal Comfort of Man in Different Urban Environments. Theor. Appl. Climatol. 1987, 38, 43–49. https://doi.org/10.1007/bf00866252.
- 34.
Höppe, P. The Physiological Equivalent Temperature—A Universal Index for the Biometeorological Assessment of the Thermal Environment. Int. J. Biometeorol. 1999, 43, 71–75. https://doi.org/10.1007/s004840050118.
- 35.
Matzarakis, A.; Mayer, H.; Iziomon, M.G. Applications of a Universal Thermal Index: Physiological Equivalent Temperature. Int. J. Biometeorol. 1999, 43, 76–84. https://doi.org/10.1007/s004840050119.
- 36.
Lin, T.P.; Matzarakis, A. Tourism Climate and Thermal Comfort in Sun Moon Lake, Taiwan. Int. J. Biometeorol. 2008, 52, 281–290. https://doi.org/10.1007/s00484-007-0122-7.
- 37.
Matzarakis, A.; Rutz, F.; Mayer, H. Modelling Radiation Fluxes in Simple and Complex Environments—Application of the RayMan Model. Int. J. Biometeorol. 2007, 51, 323–334. https://doi.org/10.1007/s00484-006-0061-8.
- 38.
Fröhlich, D.; Matzarakis, A. Spatial Estimation of Thermal Indices in Urban Areas—Basics of the SkyHelios Model. Atmosphere 2018, 9, 209. https://doi.org/10.3390/atmos9060209.
- 39.
Chen, Y.C. Thermal Indices for Human Biometeorology Based on Python. Sci. Rep. 2023, 13, 20825. https://doi.org/10.1038/s41598-023-47388-y.
- 40.
Maronga, B.; Banzhaf, S.; Burmeister, C.; et al. Overview of the PALM Model System 6.0. Geosci. Model Dev. 2020, 13, 1335–1372. https://doi.org/10.5194/gmd-13-1335-2020.
- 41.
Fröhlich, D.; Matzarakis, A. Calculating Human Thermal Comfort and Thermal Stress in the PALM Model System 6.0. Geosci. Model Dev. 2020, 13, 3055–3065. https://doi.org/10.5194/gmd-13-3055-2020.
- 42.
Geletič, J.; Lehnert, M.; Krč, P.; et al. High-Resolution Modelling of Thermal Exposure during a Hot Spell: A Case Study Using PALM-4U in Prague, Czech Republic. Atmosphere 2021, 12, 175. https://doi.org/10.3390/atmos12020175.
- 43.
Chen, Y.C.; Matzarakis, A. Modified Physiologically Equivalent Temperature—Basics and Applications for Western European Climate. Theor. Appl. Climatol. 2018, 132, 1275–1289. https://doi.org/10.1007/s00704-017-2158-x.
- 44.
Jendritzky, G.; de Dear, R.; Havenith, G. UTCI—Why Another Thermal Index? Int. J. Biometeorol. 2012, 56, 421–428. https://doi.org/10.1007/s00484-011-0513-7.
- 45.
Fiala, D.; Havenith, G.; Bröde, P.; et al. UTCI-Fiala Multi-Node Model of Human Heat Transfer and Temperature Regulation. Int. J. Biometeorol. 2012, 56, 429–441. https://doi.org/10.1007/s00484-011-0424-7.
- 46.
Bröde, P.; Fiala, D.; Błażejczyk, K.; et al. Deriving the Operational Procedure for the Universal Thermal Climate Index (UTCI). Int. J. Biometeorol. 2012, 56, 481–494. https://doi.org/10.1007/s00484-011-0454-1.
- 47.
Jendritzky, G.; Menz, G.; Schirmer, H.; et al. Methodik Zur Raumbezogenen Bewertung der Thermischen Komponente Im Bioklima des Menschen (Fortgeschriebenes Klima-Michel-Modell); Deutscher Wetterdienst: Hannover, Germany, 1990.
- 48.
Staiger, H.; Laschewski, G.; Grätz, A. The Perceived Temperature—A Versatile Index for the Assessment of the Human Thermal Environment. Part A: Scientific Basics. Int. J. Biometeorol. 2012, 56, 165–176. https://doi.org/10.1007/s00484-011-0409-6.
- 49.
Gagge, A.P.; Fobelets, A.P.; Berglund, L.G. A Standard Predictive Index of Human Response to the Thermal Environment. ASHRAE Trans. 1986, 92, 709–731.
- 50.
Gagge, A.P.; Stolwijk, J.A.J.; Nishi, Y. An Effective Temperature Scale Based on a Simple Model of Human Physiological Regulatory Response. ASHRAE Trans. 1971, 77, 247–262.
- 51.
ANSI/ASHRAE Standard 55-2023; Thermal Environmental Conditions for Human Occupancy. ASHRAE: Atlanta, GA, USA, 2023.
- 52.
Pickup, J.; de Dear, R. An Outdoor Thermal Comfort Index (OUT_SET)—Part I—The Model and Its Assumptions. In Biometeorology and Urban Climatology at the Turn of the Millennium; WMO: Geneva, Switzerland, 2000; pp. 279–283.
- 53.
Lindberg, F.; Holmer, B.; Thorsson, S. SOLWEIG 1.0—Modelling Spatial Variations of 3D Radiant Fluxes and Mean Radiant Temperature in Complex Urban Settings. Int. J. Biometeorol. 2008, 52, 697–713. https://doi.org/10.1007/s00484-008-0162-7.
- 54.
Bruse, M.; Fleer, H. Simulating Surface–Plant–Air Interactions inside Urban Environments with a Three Dimensional Numerical Model. Environ. Model. Softw. 1998, 13, 373–384. https://doi.org/10.1016/s1364-8152(98)00042-5.
- 55.
ISO 7726; Ergonomics of the Thermal Environment—Instruments for Measuring Physical Quantities. International Organization for Standardization: Geneva, Switzerland, 1998.
- 56.
Jia, S.; Wang, Y.; Wong, N.H.; et al. Estimation of Mean Radiant Temperature Across Diverse Outdoor Spaces: A Comparative Study of Different Modeling Approaches. Energy Build. 2024, 310, 114068. https://doi.org/10.1016/j.enbuild.2024.114068.
- 57.
Tartarini, F.; Schiavon, S. Pythermalcomfort: A Python Package for Thermal Comfort Research. SoftwareX 2020, 12, 100578. https://doi.org/10.1016/j.softx.2020.100578.
This work is licensed under a Creative Commons Attribution 4.0 International License.
