Why can a sea-level cake recipe fall in a mountain kitchen?

Answer: Gas expands too fast

Water boils hotter — This reverses the real altitude effect. Lower air pressure makes water boil at a lower temperature, not a higher one, and liquids evaporate faster. That can dry batters and change concentration, but the collapse risk comes from gases expanding before the cake structure is ready. The mountain changes the pressure rules.

Gas expands too fast ✓ — Right: lower air pressure lets leavening gases expand faster and farther. A sea-level formula may rise before the crumb has enough strength, so bubble walls stretch, merge, or burst. High-altitude advice often reduces leavener and adjusts liquid, sugar, or flour for exactly this reason. The same recipe can fail because the atmosphere changed, not the baker.

Oven heat turns weaker — Oven thermostats do not become weaker just because elevation rises; a 350°F oven is still aiming for 350°F. What changes is the pressure outside the batter and the boiling point of water inside it. Heat can still arrive, but the batter behaves differently under lower pressure. That is why recipe adjustments target leavener, liquid, sugar, and structure.