I'll approach it from a different perspective:

Let's say you are going out for a day hike and something goes wrong, what would be reasonable to take along for a "most of the time" worst case scenario, say a night or two out on your own with a broken limb. In that case, the list of stuff I would want with me to cover most of what could be reasonably planned for would tip the scales right about 20 lbs. Yes, you could go overboard generating a what-if list, but if you consider the type of activity you will be involved in, the type of terrain, and the relative isolation, then things such as 2-3 liters of water, some foul weather gear, Doug's kit, a good knife, some e-rats, you get the idea.

20 lbs is not an unreasonable load. It is about what I pack with me when out hunting elk in the woods, excluding my dedicated hunting tools.

As far as the heat goes, I've gone from 140 degree heat in Iraq, dry as parchment, to 100+ moisture laden air in Brisbane, to 90+ saturated air in Florida, to 100+ thin and dry air in Denver. My conclusion: when it is hot, I go sit in the AC and be happy I am not making another stinking afternoon bunker run to the "Sweat Box" anymore.