All last summer I was riding after work, about 5PM, with temps as high as 103, but the humidity here is very low (most locals will not say so, but when 60% is considered hight, that seems low to me who grew up on the east coast).

You do acclimate, but you have to give yourself time. Also, I find that i really have to keep hydrated all the time, not just on the bike. I usually drink between 3/4 to 1 gallon of water a day, not counting my ride water. I don't use an electrolite substitute, but I don't seem to need it for most rides.

Barbara