Towards a universal thermal climate index (UTCI)

The heat wave over Europe in summer 2003 exemplified the vulnerability of our society against excess heat. About 25000 to 30000 deaths were directly associated with the heat wave. Besides the extreme impact of such high temperatures, the deviation from our usual thermal environment influences our well-being and our productivity. Predictions of the thermal conditions can help to warn people when there is a danger of unusual conditions. But the effect weather exerts on people is caused by more than just the air temperature. Humidity and wind speed as well as the intensity of the sun, all influence how we respond to the outdoor environment.

The assessment and forecast of the thermal environment is therefore an important task for public information, allowing people to optimize their comfort, performance and health. Previously due to lack of knowledge and technology, later also by ignorance, more than 100 simple thermal indices - most of them two-parameter indices - have been developed in the last 150 years to describe the complex conditions of heat exchange between the human body and its thermal environment. Among them some well-known and still popular examples are the heat index and the wind chill index. However, due to their simple formulation, these indices never fulfilled the essential requirement that there must be a unique thermophysiological effect for each index value, regardless of the combination of the input meteorological values.

Dealing with the exchange between the human body and its environment requires the application of a complete heat budget model that takes all mechanisms of heat exchange into account. In addition to the meteorological variables air temperature, water vapor pressure, wind velocity and mean radiant temperature, including the short- and long-wave radiation fluxes of the atmosphere, parameters such as metabolic rate and clothing insulation are also considered. Such models possess the essential attributes to be utilized operationally in most biometeorological applications in all climates, regions, seasons, and scales.

Although each of the existing heat budget models are basically appropriate for the use in any kind of assessment of the thermal environment none of the models is accepted as a fundamental standard, neither by modelers nor by users. On the other hand it is difficult to accept that after more than 30 years experience with heat budget modeling and easy access both to IT and meteorological data people still use oversimplified and thus unreliable indices or even just air temperature in the assessment issues.