New heat intensity index debuts; Many cities still unprepared for extreme heat
The Royal Netherlands Meteorological Institute (KNMI) has introduced a new heat intensity index today. In the meantime, new data show that most Dutch municipalities still lack local heat plans, even as runners in Groningen, Utrecht, and Amersfoort recently collapsed during races.
KNMI launched a 0-to-10 “hittekracht” scale in its app on Tuesday, combining temperature, humidity, wind and solar radiation to better reflect how heat actually feels.
The system is based on the Wet-Bulb Globe Temperature standard and is intended to complement existing heat warnings and the National Heat Plan from the National Institute for Public Health and the Environment (RIVM).
On Heat Action Day, KNMI said the tool helps explain why conditions can feel severe even when temperatures are not extreme. On Monday, heat intensity in De Bilt reached 8 around 2:00 p.m., while marathons in Groningen, Utrecht and Amersfoort saw multiple participants become unwell. Heat intensity was 6 in Groningen and 5 in the other cities.
Two years ago, the Dam tot Damloop between Amsterdam and Zaandam was stopped when emergency services could no longer handle overheated runners, despite a temperature of 23 degrees Celsius and a heat intensity of 5.
KNMI researcher Carolina Pereira Marghidan said temperature alone does not capture heat stress. “Not every 25 degrees Celsius feels the same,” she said. “Heat stress does not depend only on temperature. Sunlight, humidity and wind make a major difference.” She added that high humidity makes it harder for the body to cool itself through sweating, while wind and cloud cover can reduce heat stress.
The index runs from 0 to 10 and is comparable to scales for wind or solar intensity. KNMI said typical heat waves in the past 30 years produced values of 6 or 7, though levels of 8 or higher can also occur outside heat waves. A level of 10 is extremely rare: it has not been recorded in parts of Noord-Holland and Noord-Brabant, and in De Bilt it has appeared for only about four hours since 1991. Peak values are usually short-lived.
KNMI developed the system with TNO and RIVM following requests from emergency services, including police, fire departments and event organizers, who said a clearer index could help them assess risks earlier. The agency said conditions can become dangerous even without extreme temperatures when wind is low and humidity is high.
Outdoor workers are already adapting informally. In Almere, a roofer working in rising heat near 30 degrees Celsius rated the conditions a “9 out of 10.” “I think I would rather be at the beach sometimes,” he said. KNMI later assessed the heat intensity in Almere at 7. When told the official rating, he responded, “It could be worse,” and added, “We just push through and don’t let it get to us.”
The index is expected to increase from spring into summer and decline in autumn. KNMI said people will gradually learn what different levels mean for their own health, including when to drink more water, avoid exertion or seek shade. Vulnerability varies: older adults, young children, pregnant people and those with heart or lung conditions face higher risk, as do people doing heavy physical labor or endurance sports.
Municipal preparedness, however, remains uneven. An inventory by Climate Alliance Netherlands and the Red Cross found that 128 of 342 municipalities now have local heat plans, up from 90 last year. The national target is at least 180 plans.
Heat plans outline how cities should assist vulnerable residents during extreme heat, including elderly people, children, pregnant individuals, those with health conditions, and homeless populations. The organizations say more action is needed as summers grow hotter.
“Fixed drinking water points or cooling spaces help all residents,” said Harm Goossens, director of the Red Cross.
Edwin van der Strate, an adviser on heat adaptation at Climate Alliance Netherlands, said municipalities must go further than planning alone. “Work on cooler public spaces, heat-resistant housing and protecting the health of vulnerable residents,” he said.
