It's 30 °C which is still ridiculous to be called a "heat wave" from the point of view of most humans on this planet. And 15 °C overnight is considered "too warm", really?
> These thresholds vary by region, but an average threshold temperature is 30 °C by day and 15 °C overnight for at least two consecutive days.
As a German, I would say anything over 38 °C during the day and over 20 °C overnight is a "heat wave", and I think a lot of people down south, maybe most humans on this planet, may not consider this too warm. I remember visiting the Emirates for a week during summer... or working in the kitchen of a summer camp for disabled people in Oakhurst, New Jersey during the summer, without an AC in my room. The camp's kitchen's giant refrigerator room was a favorite meeting place for anyone who had access. The only way to sleep was with a fan running at full speed blowing right at me. Maybe that experience shifted my view of what a "heat wave" is. Oh, and in the Emirates they have "cooled swimming pools". I thought it was just a joke, but no, they really artificially cool the water of their swimming pools. I went into the Gulf water - for half a minute, then I went straight into the cooled swimming pool that I had scoffed at. Having grown up in East Germany and only knowing the Baltic I never knew that the sea could ever be too warm even to enter for more than a minute.
Yesterday or the day before, reading UK news (BBC or The Guardian) as I often do, I saw the announcement of the "heat wave" and the warning of temperatures over 30 °C(!) and thought exactly the same as OP, even if he got the exact number wrong.
The UK is usually very humid too when it's hot so the heat index (the 'feels like' temperature) is much higher. For example 30°C could 'feel like' 38°C if the humidity is at 80%. Also terrible if you are sweating and it can't evaporate.
And 15 °C overnight is considered "too warm", really?
In some places, buildings are designed on the assumption that temperatures at night will be significantly cooler, and give the building an opportunity to shed heat collected during the day. When that assumption is violated, the building remains hot and continues getting hotter each day, which is dangerous to people inside.
I live in one such building. It takes one week of high temperatures and it remains hot inside no matter the temperatures, it takes a week of cool weather. That has nothing to do with the topic though, and 15 °C at night is not a heat wave no matter how bad the design.
By the way, I found by personal experiment that all it takes is one layer of something like cloth in front of the stone to prevent if from heating up. I completely cover my large balcony on the outside now, including the thick stones. In all previous years they always stored to much heat during the day that the balcony remained hot well until morning, heat radiating from all sides, making it almost unbearable to go there (or to open the wide door) even when the outside air temperatures had dropped below 20 °C. The difference this summer is huge. So it does not take anything fancy or expensive at all, just prevent direct sun exposure of those heat sink surfaces (like solid concrete).
