Fruits are energy sinks for the plants. The more they bear, the less they can handle heat.
Imagine running at 80°F with an empty backpack. It’s relatively easy. Now, start adding bricks to the bag. It won’t be long before it becomes exhausting. Full clusters are like bricks for the plants. This is why, in Step 1, we pruned them. Making sure plants are not overloaded during heat waves.
As a rule of thumb, we want to keep the day's temperature around 75°F. This is for an 8 clusters’ plant. If you have 10 clusters, because of late ripening, aim for 70°F. The other way around is the same. If you only have 6 clusters, your plant can take around 79°F to force its energy into fruit production.