I imagine the buildings there just aren't built to support that heat plus the body height of hundreds or thousands of attendees?
The human body has a natural resting temperature of about 37°C, and metabolism of course generates more heat constantly, so we constantly have to shed that heat. When the temperature is low, we can rely purely on conducting the heat into the atmosphere to shed the heat (which is probably why internal body temperature is higher than the atmosphere!). At higher temperatures, conduction is less efficient, or sometimes even adds heat load into the system (at above 37°C, obviously), so we start relying on evaporative cooling (i.e., sweat) to cool us down.
The wet-bulb temperature is the minimum temperature that can be reached by evaporative cooling. So when the wet-bulb temperature is in the mid-30s °C… people start to become literally unable to regulate their core body temperature, and the heat is lethal. Wet-bulb is largely a combination of the temperature and humidity, but unfortunately, it's not typically reported in most weather reports, so people go off of the air temperature (and the humidity) that is reported.
Which is a long-winded way of saying "the humidity matters a lot for how much a given temperature is bearable." I don't know what environment you come from purely by rural New South Wales, but my first guess is the semi-arid and thus low-humidity bush regions of the state, which means the apparent wet-bulb temperature of 37-40°C would be a lot lower than the equivalent 37-40°C for most of the humid continental climates of Europe.
> The total heat transfer of a human is the same as the metabolic heat production, and is approximately 100 watts, or 400 kJ/hr, or 60-70 kcal/hr at rest. It occurs by:
> Radiation (50%)
> Convection (30%)
> Evaporation (20%)
> Conduction (usually 0%)
It may be that you sweat the same at 30C (i.e., water coming out of your pores), it's just that with higher humidity the sweat cannot evaporate and collects on your skin in beads, which you notice.
Last summer my house got to 39, and I didn't have AC (it was broken). I think I'm still recovering.
There’s something about 85F/30C and 80%+ humidity that prevents the temp from going much higher for a longer period of time.
When the heat wave struck, initially that thermal inertia works in your favour as the cool building keeps the interior cool. But after a day or two it heats up. Once that happens, the temperature at night doesn't drop. As you say, ventilation is not good, so the humidity rises too. The thing becomes an overnight sauna.
After one night of this, I went looking for a fan. In my Australian home, every room has a ceiling fan. In this German dwelling (rented by my daughter, an Aussie, for the past few years) there wasn't a single fan in the house, and worse none of the nearby shops sold fans. It looked to me like there was no "social knowledge" of fans or how to handle heat in general. I eventually got a fan off Amazon, who thankfully delivered the next day.
We were also there in another year in winter. These same buildings were amazing the cold. They could maintain a cosy inside temperature with very little extra energy. But geeze, you don't want to be in one in a heatwave that goes on for days.
https://www.cell.com/trends/genetics/fulltext/S0168-9525(20)...
The humidity here it's hell. You feel 35C like ~42C in dry climates.
If you live in a town in New South Wales where the average humidity is less than 50% in the wettest (or, should I say, least dry) season, you might not understand what it feels like in London where the average doesn’t dip below 65% any time of year.
Today London will feel at least 4 degrees Celsius hotter than Hong Kong. The latter is already an extremely unpleasant place to be in these conditions (and had in fact its own very hot weather warning issued), and unlike London it has a very strong culture of air conditioning.
4 degrees might seem like not a lot, but heat extremes are a tricky beast. Once your body cannot evaporate heat fast enough, you’re literally toast.
I grew up in a humid city and summers were unbearable. Now I live in a dry climate and 30°C is pretty comfortable.
EDIT: actually yesterday was the hottest single day ever in recorded french history, and it happened in the merry month of June, not August. Truly your typical heat wave.
EDIT2: I was wrong, the record is not actually 23 years old, but… 1 year old, as the record had already been broken in 2025. Can’t wait to get my next dose of very typical 44C temps in may next year.