UT researcher builds the scale behind the new KNMI heat indicator: ‘hittekracht’

Tuesday 2 June 2026

Carolina Pereira Marghidan, PhD researcher at the University of Twente and the KNMI, developed a new indicator together with KNMI, TNO, VU and RIVM that will appear in the KNMI app from 2 June: 'hittekracht' (‘heat force’). A number from 0 to 10 that not only measures temperature, but also takes into account sun, wind and humidity. Pereira Marghidan developed the scientific underpinning and underlying methodology of the scale. "I hope that hittekracht will soon be in the dictionary."

Periods of extreme heat are becoming more frequent. Yet weather services have so far warned mainly using temperature. That only tells part of the story. On a windless, sunny day with high humidity, 25 degrees feels very different from a cloudy day with wind. The new hittekracht scale makes that difference visible.

Not the same as apparent temperature

The term apparent temperature already exists, but covers a more limited meaning. That formula only combines temperature and wind. Hittekracht goes further. The scale is based on the WBGT, the Wet Bulb Globe Temperature, a physical measure that has been used for decades in sports and work contexts to measure heat stress. WBGT combines temperature, humidity, wind speed and solar radiation, providing a more accurate picture of what heat does to the body.

The disadvantage of WBGT is that the value means little to those who are not experts. A WBGT of 23 degrees does not sound alarming, but can be equivalent to an air temperature of 30 degrees under physically demanding conditions. Pereira Marghidan developed the calculations to convert WBGT to a scale of 0 to 10. "Hittekracht 10 corresponds to a WBGT of 32 degrees or higher. In De Bilt, this has only occurred for a total of four hours in the past thirty years. That is exceptional", says Pereira Marghidan.

In the east, the counter is already higher

Hittekracht 10 is not equally rare everywhere in the Netherlands. "The highest hittekracht occurs more often and for longer in the east than many people realise", says Pereira Marghidan. In the east and southeast of the country, the number of hours with a hittekracht of 10 has risen to dozens of hours over the past thirty years. Along the western coast, that value has never been reached in the same period.

"These regional differences make it so important that the KNMI app now shows the hittekracht per location", says Pereira Marghidan. Existing heat information is often based on the nearest KNMI measuring station, which can be far away and is usually set up in an open field. This gives an incomplete picture of what people in a city or village actually experience.

Applications

It is not often that such a new scale is released. The UV index and wind force are now established concepts. Pereira Marghidan hopes that hittekracht will follow the same path. The scale is primarily intended for the general public, but hittekracht will likely also offer a useful alternative for professional users.

WBGT is already used as a benchmark at sporting events and in working conditions. Yet many employers still base their heat policy on the normal air temperature of the nearest KNMI station. WBGT and hittekracht offer a more effective alternative. Security regions, Rijkswaterstaat, NIPV and the police have already received an information session about the new scale.

About Carolina Pereira Marghidan

Carolina Pereira Marghidan is a PhD candidate at the Faculty of ITC of the University of Twente and the KNMI. Her research focuses on heat waves in the Netherlands and worldwide, and on the question of what heat information society needs to make better decisions. She is supervised by prof. dr. Maarten van Aalst and dr. Justine Blanford. In addition, she has a part-time appointment at the International Red Cross Climate Centre.