How vapor pressure affects boiling point at altitude?

Boiling occurs when a liquid’s vapor pressure equals the surrounding atmospheric pressure. At higher altitudes, atmospheric pressure is lower because the air column above is thinner. Since vapor pressure rises with temperature, a liquid doesn’t need to reach as high a temperature to match the lower outside pressure. As a result, the boiling point decreases at altitude.

For example, at sea level (1 atm or 101.3 kPa), water boils at 100°C. In a mountain region where the air pressure might be around 80 kPa, water boils near 93°C. This means foods take longer to cook because the boiling water isn’t as hot.

The relationship is straightforward: lower external pressure → lower boiling temperature → lower energy requirement for molecules to escape into vapor. Conversely, in a pressure cooker, the pressure is artificially increased, raising the boiling point and speeding up cooking.

Therefore, vapor pressure directly controls boiling behavior: when vapor pressure equals local atmospheric pressure, boiling starts—whatever that temperature happens to be.