For you flatlanders, you know who you are, mountain weather is vastly different from what you are used to. In Mt. Rainier National Park they will tell you it can snow anytime of the year.
When we tent camped, it was unbelievably cold at Bryce Canyon NP. Asking at the lodge during breakfast the next morning we found out it had gotten down to 25 degrees F during the middle of July! BTW Bryce is about 7000ft.
You always need a warm jacket at high altitudes.
A charter member of the Buffalo Plaid Brigade!
Whether you think you can or think you can't, you're right.