I can certainly attest to the heat and humidity. It has been relentless here in Spain for a month now. As bböp had alluded to on earlier dates, perhaps the heat (and it being a Sunday night) resulted in a more sedate crowd, though my comparison in a Spanish context is the raucous Alicante crowd from August '23 (a similarly sweltering evening), which would take some living up to. Not to say that the show wasn't easily B+ and very well received by the sizeable crowd, just that it was the standard excellence we come to (unfairly?) expect from Wilco.
A special poster had been designed