Water boils at a lower temperature at high altitudes, because

The correct answer is the air pressure is less.

Key Points

• The boiling point of water is lower at higher altitudes due to the decreased atmospheric pressure.
• A liquid/water boils when its vapour pressure exceeds the total atmospheric pressure over it.
• Unlike Pressure Cooking where the boiling point is higher due to high pressure,  at higher-altitude cooking takes place at lower atmospheric pressure.
  • This requires more cooking time.
  • Water boils at 212º F at sea level. 
  • At higher altitudes, the boiling point of water decreases, which leads to longer cooking times.
• Relation of Pressure and altitude:
  • Pressure decreases with increasing altitude.