Change the effect of heat?

[abou]

Why? They'd be suffering because they aren't use to the climate and the sun. I don't think 100,000 sweboz with horribly burnt skin would fight that well.

That's not what Atra said. He stated that, since it is impossible to do what the OP requested, you can give a bonus to units that are native to the desert in the same way as the Germanic units getting a bonus to fighting in woods.

Im sorry im not being clear. Currently any unit can just sit and rest even in the desert and recover stamina, i want to change it so you cant just sit in the sun all day long and be "fresh"

Unfortunately, this is not possible as far as I know. You can set stamina with a script in custom/historical battles, but not in game (again: as far as I know).