Which indicators are most effective for describing the impact of heat across Europe?

Heatwaves can impact human health with the extreme heat wave in August 2003 resulting in 15,000 excess deaths alone in France, with temperatures in excess over the seasonal average by 11-12°C for nine consecutive days. The increased energy demand for cooling needs due to air conditioning and refrigeration may create an overloading of the power supply resulting in blackouts which may further exacerbate health problems. The effect of climate change owing to increased greenhouse gases being released into the atmosphere will lead to higher temperatures in the future.

What is the best way to communicate this heat to the public? There have been several heat indicators developed in the scientific literature, such as "number of summer days", perceived temperature (PERCT), and physiological equivalent temperature (PET). The fact that so many have been developed suggests that there may not be one perfect indicator.

Regional Differences in Heat Indices

Why is this? Is this because different regions require different indices (e.g. northern Europe vs southern Europe)? Do different regions measure different parameters? Does the public of certain regions have a better understanding of heat indices so that more complex indicators can be used (e.g. southern Europe which is known for hot weather)?

Heat indicators such as the number of summer days, hot days, or tropical nights represent the number of days where the maximum or minimum temperature exceeds certain threshold temperatures. E.g. A hot day, where the maximum temperature exceeds 30°C, in central Europe may not be considered a hot day by the people of southern Europe. These purely meteorologically-based indices, although easy to calculate, may not give a direct relation to human health.

Insights from the KNOWING Research Project

Thermal comfort indices provide a direct measure of the heat on humans. They consider the effect of meteorological quantities such as temperature, mean temperature, wind speed, and humidity on a human model described by its metabolic rate and insulation through clothing. Examples of such indices are the already mentioned perceived temperature (PERCT), physiologically equivalent temperature (PET), or universal thermal climate index (UTCI). Are such thermal comfort indices a more valuable heat indicator than meteorologically-based indices?

The KNOWING research project uses models that are also used to assess heat pollution in urban areas. They calculate both types of heat indicators. This provides a broader range of information that can flow into the diverse sector models (e.g. energy, transport, housing, etc). However, could limiting the use to only a few indicators make following the knock-on effect of heat to each sector easier to follow?