Jock Scot’s report of the record temperature in Scotland translates from 91.2°F to 32.888889°C.
Of course Highland (and Lowland) regiments have served in much hotter climes.
Pre-20th century, not all Highland regiments wore the kilt overseas, but those that did certainly adapted their uniform to the conditions.
Shirtsleeve dress seems to have predominated, although full formal uniforms were often to be seen, too.
Regards,
Mike