BTW, there is an English expression exactly corresponding to Hundstage: https://en.wikipedia.org/wiki/Dog_days

Apparently it originates in Latin so it's no surprise that it's shared between many European languages.

It comes from when Sirius (the "Dog Star") rises at the same time as the Sun - I think it is more about the astrological culture than the language, although that will be a vector.

It's funny, because in french, there is : "un temps de chien". But it refers to heavily rainy days, not hot days.

'Dog days', for those getting the link cut short.

FYI Hundstage literally translates to dog days. Hund = hound, tag = day.

Air conditioners don't generate much extra heat, they just move heat from inside to outside.

I am afraid you are mistaken, the ambient temperature increase is meaningful[0] in this case up to a predicted 2 degree increase for Paris if they double the numbers of their ACs.

ACs are more than just fans – they are basically fridges for your room, and like every thermodynamic machine they produce more heat than they can cool. Only not in your room but outside.

AC means cooling your room while heating your city.

[0]: https://www.cnrm-game-meteo.fr/IMG/pdf/demunck_2013_ijc.pdf