You might find https://helmets.org/cooling.htm of interest.
That aside, part of the game here is realising the degree to which helmets are about comfort, a mix of psychological and physical. If it were only about protection everyone would be using heavier, more protective lids, but it's not: it's choosing a point where the comfort and reassurance are at a happy balance point for the wearer*.
If the comfort is primarily psychological, and a rider simply doesn't feel right without a lid, then obviously that has them in a lid, but given that helmet exemptions to the Spanish helmet laws make an exemption for "extreme heat" it's not unreasonable to assume there can come a time when the perceived cons outweigh the perceived pros, and in such a case one can see that it's not necessarily the case that a by-default helmet wearer will always want to be wearing one. In very hot weather (particularly combined with big hills, also note that hill-climb TTs in the UK don't require helmets unlike most competitive cycling) it's worth putting it on the table that it might be worth making an exception: you have more options to cool your head if you don't require a helmet than if you do. Like the Spanish law, being too hot can be a reason to change habits in exceptional circumstances.
Pete.
* (it would be interesting if there were a range of formal protection standards generally available, with more protection needing greater weight and/or less ventilation, rather than the de-facto "EN1078 or nothing" and we could see what people actually chose given a choice, but I digress and we don't really have that option, particularly as the better Snell standard is increasingly rare)