Many moons ago I went to Mulhouse for a 24 hour Kart Race. I was warned about the heat but more about the Humidity. I trained in a Sauna for a month beforehand to get used to that and it helped. Temp by the way was up to 35C in the late morning and humidity was way up there. One hour at a time driving and you could not do more--Except I did a 4 hour stint at night to give the co-driver a rest. On the bike I have never found heat a problem till I stop. Wind circulating the wicking jersey keeps me cool but once stopped- instant overheat and I am done.
And high humidity means plenty of Isotonic drinks.
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How long was I in the army? Five foot seven.
Spike Milligan