Heatwave Indicator: "The sensed temperature level provides a measure of comfort"
Scorching French Heatwave: Unmasking the Real Feeling of the Heat
June 29, 2023: 73 French departments are on Orange Heat Alert, all thanks to the refrigerant forecaster, Météo France, expecting an early summer heatwave episode with progressive expansion towards the north. Yet, these weather predictions seldom prepare us for the true sensation of the heat. What about the so-called "Perceived Temperatures"?
According to Météo France meteorologist, Nemo Pawlowski, "Perceived Temperatures" aren't accurate measurements—they're calculated indices reflecting thermal comfort in extreme temperatures. In cold or hot situations, Pawlowski clarifies, they provide an estimation of the comfort humans experience.
Unveiling the Mystery of Perceived Temperatures
To measure air temperature under precise conditions, Météo France stations set up their sensors at 1.5 meters above the ground, within protected meteorological shelters, shielding them from precipitation, sunlight, and earth's heat. Nevertheless, such measurement fails to fully represent the actual heat sensation felt by our bodies, influenced by variables like wind speed, humidity, environmental exposure, and individual physiological conditions.
Humidex: The Dread Indicator
Two discomfort indexes, Humidex and Windchill, have been conceived for public health concerning extreme temperatures. Humidex is employed when temperatures rise, formulated from air temperature and relative humidity. Origins of this index are obscure, with indications of airtight research by Canadian scientists more than forty-five years ago.
Based on the Humidex equation, the higher the air temperature and humidity, the higher the index.
"In tropical zones, Humidex is generally quite high. However, the sensation is intense due to humidity, as opposed to the relatively manageable discomfort felt in hotter, drier places, like London, with temperatures of 35°C.", explains Nemo Pawlowski. The highest recorded Humidex was 46.7 in Tours, France, on July 30, 2024, with 37.3°C in the shelter and 42% relative humidity.
Météo France's Delicate Handling of Humidex
"When we communicate forecasted temperatures, we never mention the Humidex", says Nemo Pawlowski. "It results from an empirical formula that lacks actual measurable physical value." The forecaster highlights the arbitrary nature of the thresholds, citing their subjective interpretation of individual physiological circumstances.
Moist Heat: The Heatwave's Silent Killer
Humidity dramatically intensifies discomfort and danger during heatwaves by impeding the body's natural cooling process. During hot periods, when delicate rings develop on shirts, it may seem paradoxical, but sweating aids thermal balance. However, the combination of air moisture and heat stops sweat evaporation, leading to impaired body cooling and potential compromise of vital functions.
The elderly and young children are particularly susceptible to high temperatures, prompting the necessary preventive measure—hydration, even in humid conditions.
The only effective defense against a heatwave is staying hydrated, laments Nemo Pawlowski. In humid weather, this becomes even more crucial because we perspire intensely.
[1] Météo France, "Humidex"[2] National Oceanic and Atmospheric Administration (NOAA), "Heat Index"[3] Government of Canada, "Health Hazard Information—Heat and Sun"[4] Environment Canada, "Temperature-Humidity Index"[5] World Meteorological Organization (WMO), "Humidex"
- The origins of Humidex, a discomfort index used in contexts of extreme temperatures, can be traced back to research conducted by Canadian scientists over forty-five years ago, focusing on the effects of heat and humidity on human comfort.
- In the field of environmental science, climate-change research reveals that humidity plays a significant role in exacerbating the effects of heatwaves, particularly by hampering the body's natural cooling process, with its potential consequences being most profound for vulnerable groups like the elderly and young children.
