Värmebölja means 'heatwave' in Swedish.
Värme means 'heat' or 'warmth', and bölja means 'wave', so it's an easy translation. You'll sometimes hear värmevåg used too, using another Swedish word for 'wave', but värmebölja is the most common term.
It's true what they say, Swedes love to talk about the weather, so it's always good to expand your vocabulary with as many meteorological terms as possible.
Readers from warmer climes may be wondering exactly when they would have cause to use the word värmebölja in a country not known for its high temperatures.
In Sweden, there's a very precise answer. A warm spell is officially a värmebölja when there are five consecutive days with a daily high of over 25C. Nothing compared to the definition in the northeastern US (32.2C) or parts of Australia (35C). But don't forget that Swedish buildings are designed to keep in the heat, and private homes rarely have air conditioning, so with summers getting warmer, yes, you may well have reason to complain about the heat here.
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A similar term exists in Danish, varmebølge, which is also based on temperatures above 25C, but our Danish neighbours also have a more extreme version, hedebølge (which literally means 'heatwave', while varmebølge is technically a 'warmth wave') based on temperatures of above 28C.
Sweden has no equivalent to the hedebølge.
Although there is a separate word for heat (en hetta), it's good to remember that the Swedish languages uses varm in most contexts where English uses 'heat'. A few other examples are varm choklad (hot chocolate), värmeflaska (hot water bottle) and centralvärme (central heating).
Värmebölja på väg med 30 grader på vissa håll
Heatwave on the way with 30C in some locations
Usch, vad varmt! Jag hoppas att värmeböljan är över snart
Yuck, it's so hot! I hope the heatwave is over soon