Cactus grown outdoors in summer actually gets cooked by the heat

I collected seeds a few days ago. But it just so happened that I had no time these two days and didn't go up. Yesterday evening I went up, and I was ready to quit the hobby. A large area was completely cooked. (Thanks to hb33 for the material)

My cacti have always been in the sun, so sun resistance is not a problem, but when the high temperature reached the critical value there was still no way. Directly cooked. Just a short time. There was no hope of saving them.

The day before yesterday I was still showing off, saying that next year I'd see about collecting seeds and swapping for a good bicycle, and then this. Now even many of the mother plants are gone. The key thing is the mother plants. They are very hard to buy now. They are also the painstaking collection of so many years. I am truly depressed.

The problem: high temperature. 60+ degrees.

Many friends will say that on the ground in a greenhouse, open-air outdoor growing is no problem. Yes, my Long Zhao [Cereus] growing outdoors in the sun is fine, and ground-level greenhouses, because they are close to the ground, dissipate heat much better. Mine is on the rooftop, and the rooftop itself has heat radiation, so the temperature is indeed very high. Even with fans on it will basically be over 50 degrees. (Not one unit, but 7 units.) My greenhouse is not sealed either; the surroundings are open, but it is just this hot.

Actually, those grown outdoors in the sun do not get that hot, and the rooftop floor slab is only 52-53 degrees. It never reaches 60. But... when the greenhouse fans were not on, it went to 60+.

Why did the problem happen?

For some reason Xiaomi's automatic switch failed. Yesterday it did not turn on. (2 greenhouses, two switches; the main greenhouse's did not turn on), the seedling greenhouse's did turn on, and the temperature was over 50 degrees. But the seedlings were fine. There was also temperature control and alarms, but I did not look at my phone... and just like that, it happened perfectly...

Of course I also did not use shade. If I had used shade, the temperature could have been several degrees lower. It probably would not have been this serious. But because I have gone without shade like this for 6-7 years, and got through it safely every time, it did not start this year... so I was negligent...

Look at the pictures. The Chilean [cacti], Long Zhao [Cereus], and Tuan Shan [Opuntia] were all cooked through (the outdoor ones were totally fine).

Some I even specially photographed while collecting seeds, one day apart. Heaven and earth...

This one placed openly on the terrace, in direct sun, is fine.

This one placed on the rooftop ground in direct sun is also fine, (the temperature was around 53 degrees, and the Tuan Shan [Opuntia] were all fine.)

This is yesterday's greenhouse temperature (the temperature of the seedling greenhouse with a fan). With a fan, it still reached 55 degrees at the highest. As for the one without a fan, you can imagine

Final cause: after it was turned off on the 29th, there was no operation to turn it on on the 30th. At 18:38 on the 30th I went up to the rooftop and discovered the problem, and only then turned on the fans.

Another piece of data: the rooftop electricity usage. On the 30th, it dropped by a large amount (because only 2 fans and other equipment were running).