Why is it colder at higher altitudes?

Answer: Air expands and cools as it rises

Closer to the cold vacuum of space — Wrong. Mountain tops are still in the atmosphere—not even close to space! Temperature drops because rising air expands in lower pressure and cools (adiabatic cooling), not because of proximity to space.

Air expands and cools as it rises ✓ — Correct! As air rises, atmospheric pressure decreases. Lower pressure allows air to expand, and when gases expand, they cool down - this is adiabatic cooling. Temperature drops about 6.5°C per kilometer of altitude. So even though you're closer to the sun on a mountain, the air is much colder!

Mountains block warm air — Wrong. Mountains don't block warm air in a way that explains altitude temperature. The cooling happens because rising air expands in lower pressure and loses heat through adiabatic cooling.