What about the mountains.... is it colder in the summer than at desert. Climate wise?

Answer 1

As Martin alluded, yes it does depend somewhat. Other factors being ignored, however, it will be cooler at higher elevations than lower. I forget the lapse rate offhand, but I believe it is approximately 4 degrees F per 1000 feet. Thus, if it was 80 degrees at an elevation of 2000 feet and you drove up a nearby mountain to 8000 feet, you would expect the temperature to fall to somewhere around 56 degrees F. The same lapse rate would apply regardless of the season.

Other weather characteristics and the actual shape of the mountains and valleys will almost always skew your "experiment" results one way or the other, though.

Answer 2

yes, no, maybe
depends on
a) which mountains
b) which desert