I need to adjust my valves today but it's 104 degrees outside and probably going to be 100 in my garage. Will it have any effect on getting them adjusted correctly? I know there supposed to be done cold but what exactly is "cold"?

I wouldn't think that's warm enough to change anything... you may be surprised that the engine will still feel cool to the touch...

weather hot != engine hot

Yeh.....you're gonna sweat a whole lot more!