How does atmospheric pressure change boiling temperature?

The boiling temperature of water changes directly with atmospheric pressure. The boiling point is defined as the temperature at which the water's internal vapor pressure becomes equal to the external atmospheric pressure.

• When atmospheric pressure increases: It pushes down harder on the liquid's surface. This means more energy (a higher temperature) is needed for the water's vapor pressure to overcome this external pressure. This is the principle behind a pressure cooker, which traps steam to increase the pressure, raising the boiling point to over 120°C (250°F) and cooking food much faster.
• When atmospheric pressure decreases: (like at high altitudes or in a vacuum), there is less force on the surface. This allows water to boil with less energy, resulting in a lower boiling temperature.