What is the UTCI (Universal Thermal Climate Index)?

What UTCI measures

UTCI was developed by a commission of the International Society of Biometeorology together with the European COST Action 730, which ran from 2005 and completed the index in 2009. The goal was a single index based on the most advanced model of human thermoregulation, valid in all climates, seasons and scales.

UTCI is the air temperature of a reference condition that produces the same model response as the actual conditions. The response comes from the Fiala multi-node thermoregulation model, coupled with a clothing model whose insulation depends on temperature and is reduced by wind and walking. The difference between UTCI and the actual air temperature, the offset, is the combined effect of radiation, wind and humidity. The reference condition is:

• Activity: a person walking at 4 km/h (1.1 m/s), with a metabolic rate of 2.3 MET, or 135 W/m².
• Wind: 0.5 m/s at 10 m height, about 0.3 m/s at 1.1 m.
• Radiation: mean radiant temperature equal to air temperature.
• Humidity: 50% relative humidity, with vapour pressure held at 20 hPa when the air is above 29 °C.

The four inputs, and where they come from

For sun falling on people, SolarCal, a simplified whole-body model published in 2015 by Arens and colleagues at UC Berkeley's Center for the Built Environment for occupants in sunlight indoors, estimates how much short-wave radiation raises the mean radiant temperature. The same approach is used in ASHRAE Standard 55 for occupants in direct sun.

• Air temperature (°C), from a weather station, a weather file or on-site measurement.
• Mean radiant temperature (°C): the temperature of a uniform black enclosure that would exchange the same radiant heat with a person as the real surroundings. It captures sun, sky and surrounding surfaces and is the hardest input, usually modelled from solar radiation and the site's geometry; the operational procedure accepts values from 30 °C below to 70 °C above the air temperature.
• Wind speed (m/s) at 10 m above ground, the standard height of weather-station anemometers.
• Humidity, as relative humidity or water vapour pressure.

The ten thermal stress categories

The categories were derived from simulated physiological responses in the reference condition, such as sweat rate, skin and core temperature and shivering. Within the no-stress band, the developers note that 18 to 26 °C may correspond closely to the physiological definition of a thermal comfort zone.

• Above +46 °C: extreme heat stress.
• +38 to +46 °C: very strong heat stress.
• +32 to +38 °C: strong heat stress.
• +26 to +32 °C: moderate heat stress.
• +9 to +26 °C: no thermal stress.
• 0 to +9 °C: slight cold stress.
• −13 to 0 °C: moderate cold stress.
• −27 to −13 °C: strong cold stress.
• −40 to −27 °C: very strong cold stress.
• Below −40 °C: extreme cold stress.

Limitations to keep in mind

• Fixed reference activity: UTCI always describes the walking reference person and its clothing model, not someone seated at a café table or a worker in protective clothing.
• An approximation: the operational procedure (Bröde et al., 2012) replaces the full physiological model with a polynomial. It is valid for air temperatures of −50 to +50 °C, mean radiant temperatures from 30 °C below to 70 °C above air temperature, wind speeds of 0.5 to 17 m/s at 10 m and vapour pressures up to 50 hPa, and its root mean square error against the full model is about 1.1 °C.
• Only as good as the mean radiant temperature: UTCI from a weather file with a generic sun exposure describes an open site, not a shaded courtyard or a narrow street.
• Station wind is not street wind: wind measured at 10 m on open ground can differ a lot from the wind at pedestrian height between buildings.
• Outdoors only: it was developed to assess the outdoor atmospheric environment; indoor comfort is assessed with methods such as PMV or adaptive comfort.

Using UTCI in outdoor design

• Map stress by season and hour to see when terraces, playgrounds, entrances and squares are usable, and which hours need intervention.
• Test interventions through their inputs: shade lowers the mean radiant temperature, windbreaks lower the wind speed. Compare exposed and sheltered cases, because the right answer can differ between summer and winter.
• Focus on the hours people actually use the space, not on annual totals.
• Check future climate with a future weather file, since heat stress over the building's life may differ from today's.

How C4B shows UTCI

C4B calculates UTCI for any location with the Bröde et al. (2012) procedure, adding a SolarCal-based solar correction to the mean radiant temperature, and shows it as heatmaps by season and hour in seven stress bands. It is available on the Free plan with weather-station data. Projects on paid plans add outdoor comfort strategies for four sun and wind exposure cases, directional outdoor comfort and future climate overlays.