The Universal Thermal Climate Index (UTCI) is a metric that attempts to quantify how people perceive given weather conditions by equating them to an equivalent temperature based on standard conditions. It calculates a comfort value for the actual conditions, and then determines an equivalent temperature that would create the same comfort value for that “standard” condition. The standard condition is a location at 50% relative humidity, low air speed, and mean radiant temperature equal to dry bulb.
UTCI is a straight forward metric that is easy to understand and easy to apply. It is widely used around the world to gauge thermal comfort, including in the City of London’s thermal comfort criteria.
Although it is easy to use, it does have several important limitations as well.
It uses a fixed metabolic rate or activity level (walking at 4.4 km/hr or 135 W/m2). This means that it will misrepresent comfort in areas where more active or more passive activities take place.
It automatically controls clothing level. This inability to explicitly manipulate clothing type creates two concerns. Firstly, UTCI speculates that individuals are dressing “weather appropriate” which is both subjective and has western attire implications. Secondly, the metric fails to consider a person’s inability to change their clothing based on changing weather conditions throughout the day.
These considerations mean that UTCI can be a challenge when applied in very warm climates or for non-western cultures. For this reason, Orbital Stack prefers to use the SPMV* metric for thermal comfort.
| UTCI (°C) range | Stress Category |
|---|---|
| above +46 | extreme heat stress |
| +38 to +46 | very strong heat stress |
| +32 to +38 | strong heat stress |
| +26 to +32 | moderate heat stress |
| +9 to +26 | no thermal stress |
| +9 to 0 | slight cold stress |
| 0 to -13 | moderate cold stress |
| -13 to -27 | strong cold stress |
| -27 to -40 | very strong cold stress |
| below -40 | extreme cold stress |