Mental conditioning and altered behavior.

If you’re not used to it the heat can feel oppressive. Some kind of lizard-brain response telling you that the heat is a threat that must be mitigated immediately. Once you get used to it you realise that it’s just mild discomfort and that it’s not harmful (except for sun burn, heat stroke, et cetera).

You also alter your behavior in simple ways that may not occur to you if you’re not used to a hot climate. Like… make mid-day the least active part of your day. You’re not going to just duck down to the shops for a bite at 1pm.