I have spent some years in the deep south. During the summer, I don't know how anybody can do anything in direct sunlight from about 11AM till around 6:30PM in the evening.
Although during the Civil War, they didn't have air conditioning, refrigerators or electricity. So maybe they were acclimated. Myself, I couldn't imagine voluntarily living in the deep south without a few serious cooling devices.
In gaming terms, I probably would reduce marching distances during the summer in the coastal states-Louisiana, Mississippi, Florida, coastal areas of Texas and probably Georgia as well. Any forced march would produce extra hits.
Also weren't there quite a bit of serious and common tropical diseases in the deep south during the Civil War...malaria, yellow fever, etc. I imagine tropical diseases would sweep through and devastate troops from time to time. Myself, I have seen clouds of mosquitos carry off old men, small children and the more frail women from time to time.
