Why are temperatures of 25C considered a heatwave in Sweden?

For readers from a lot of other countries, a heatwave of 25C might not feel like a big deal, but in Sweden, it's warm.

There is no standard international definition of a heatwave, with each country deciding how to define it.

There's no firm Swedish definition either, but when temperatures hit at least 25C at some point over the course of the day for at least five days, that's usually considered to be a heatwave.

In Sweden, the national meteorological institute SMHI defines “high temperatures” as the mercury hitting at least 26C at some point during the day, three days in a row, and “very high temperatures” as at least 30C three days in a row.

If temperatures hit 30C for five days in a row, or reach 33C for three days in a row, this qualifies as “extremely high temperatures”.

“Extreme heat is a new problem in Sweden,” Martina Söderström, from engineering consultancy company Sweco, told The Local. “It’s only really become an issue in the last ten years, with 2018 a bit of a wake-up call, with very high temperatures and a long heatwave.”

Summer 2018 saw a record number of wildfires ravage the country, Sweden’s hottest July in over 250 years, and the warmest summer since records began across most of southern and central Sweden.

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Not only are Swedes in general more used to cold temperatures, but the country’s infrastructure is built with cold temperatures in mind.

“It’s how we’ve built our buildings and our society. Buildings aren’t designed to protect people from heat and sunlight, rather they are designed to be insulated,” Söderström said.

Swedish buildings are also unlikely to feature ceiling fans or air conditioning, which, coupled with the fact that many of them are well-insulated, means that it can be difficult to cool them down once they're already hot.

'Natural solutions are an important piece of the puzzle'

Sweco in 2024 investigated how a number of different European cities, including Stockholm, could build heatwave resilience as climate change looks likely to increase their frequency. In Stockholm, the number of heatwave days per year is expected to increase by 150 percent by 2100, with similar figures for the other Scandinavian capitals Copenhagen (160 percent) and Oslo (140 percent).

“The increase in temperature is greater here [in the Nordic countries] and we have probably come the least far in planning for it. We’ve been more focused on protecting ourselves from the cold,” Söderström said. “If you look at Rotterdam and Brussels, for example, they’re further ahead both in measuring the heat and maybe also have a clearer plan of the measures they’re going to take to mitigate it.”

Sweden also has a lot of sunlight during the summer – Stockholm gets over 18.5 hours of sunshine around midsummer, Malmö in the south gets 17.5 hours, while in the far north of the country the sun doesn’t set for months. This means that temperatures often stay high during the night, providing fewer opportunities for people to recuperate from the heat and putting more stress on vulnerable individuals.

Cities are not designed with heat in mind, either, meaning that they often lack shade or trees, which contribute towards keeping temperatures low.

• READ ALSO: How to keep cool in Sweden during summer heatwaves

“Natural solutions are an important piece of the puzzle,” Söderström said.

“It’s important to get green and blue infrastructure into city planning. Trees are great, as they give shade and also they release water vapour into the air which has a cooling effect, and bodies of water can have the same cooling effect – when it’s cooler than the surrounding air, it helps to cool it down. So water, green solutions, proximity to parks and shade, all of those things are important to have in a city environment.”

People in cities are also more affected by heatwaves, Söderström added, as hard surfaces like asphalt, concrete and metal roofs absorb heat, further warming the air as this heat is emitted.

“It’s important to think about this when planning areas. Think about getting plants and water into urban areas, but also the materials being used.”

Sweden can learn a lot from hotter countries

Sweden has “a lot to learn” from countries which are used to warmer temperatures already, she added, saying that there’s no need to “reinvent the wheel”.

“Things like how to plan buildings and cities, lighter materials, how to place buildings to allow the wind to move through them, building covered verandas, providing shade, for example.”

Although children, the elderly and people with underlying health conditions are more susceptible to high temperatures, they are by no means the only groups to feel the consequences of prolonged heatwaves.

“These types of temperatures put stress on all aspects of society,” Söderström said. “It’s not just individuals who are particularly sensitive to heat, but it has consequences for the entire population. It affects agriculture, forestry and our ability to secure a steady supply of water.”

“We need to start thinking about how to protect society [against heatwaves] and plan ahead. Measure, get an idea of where risk areas are or how they come about, and make a long-term plan for them,” she added.

“It’s time to act now. This is going to continue for at least another 80 years and if we act now, we can adapt so our society and our cities are more resilient in the future.”

Do you come from somewhere with a warmer climate than Sweden? How do you find the heat here? Does it affect you more or less than it did back home? Let us know in the comments below.