My understanding is that this term actually doesn't have anything to do with the temperature, but is, in fact, an old maritime term.

Lore has it that ancient mariners out on the high seas referred to the period of the year that ran from the end of July into the middle/late part of August as the "Dog Days" because NHL and NBA and of course all the college sports had ended for the year, and football (and the following year's college sports) was of course still yet to come, and there were really no other sports to listen to on satellite radio or watch on cable or satellite television and to post about on internet message boards except for baseball.

Is that how everybody else understands it, or do I just have it wrong?