Why Does It Take a Longer Time For Water To Boil At Higher Altitudes?

When it’s boiling, water in New York is a little hotter than water in Mexico City. And hotter water will get an egg to a given state of doneness in a shorter time.

A little thought will show that the biggest difference between New York and Mexico City, apart from the relative difficulty of finding a good corned beef sandwich, is the altitude. The average stove in Mexico City is 7,347 feet higher than the average stove in New York City. And the higher the altitude, the lower the temperature at which water boils.

How much lower? If pure water boils at 212 degrees Fahrenheit (100 degrees Celsius) in New York City (and it may not), it will boil at only 199 degrees Fahrenheit (93 degrees Celsius) in Mexico City. Not a huge difference, but your exemplary three-minute New York City egg will certainly take longer to create in Mexico City.

The reason is simple, once you realize what boiling consists of: water molecules becoming energetic enough to break away from their brethren in the pot, then gathering together into rising bubbles, and finally flying off into the air as steam.

In order to escape, water molecules have to have enough energy, that is, they have to be at a high enough temperature, to overcome two separate forces: (a) They have to break apart the stickiness that holds them together in the liquid, and (b) they have to overcome the pressure that the atmosphere is applying to the surface of the water. That pressure is caused by air molecules that are continually bombarding the surface of the water like a barrage of ricocheting hailstones.

The sum-total force of those collisions is transmitted through the water to every molecule within it. Molecules at the surface can just fly off into the huge spaces between the air molecules, but those in the interior of the water must overcome this sum-total pressure in order to get out.

The stickiness of liquid water molecules to each other is the same, of course, whether they’re part of a Manhattan or a margarita. But atmospheric pressure is another story. In Mexico City, the air is only 76 percent as dense as it is at sea level. That means that only about three-quarters as many air molecules are bombarding the surface of the water every second. The water molecules are therefore able to muscle their way upward and boil off without having to have quite so much energy: that is, without having to get quite so hot.

An extreme: The highest point on this planet is Mount Everest, which is 29,028 feet (8,848 meters) above sea level. At this altitude, the atmospheric pressure is only 31 percent of what it is at sea level, and the boiling temperature of water is only 158 degrees Fahrenheit (70 degrees Celsius).

That’s not hot enough to cook much of anything, no matter how hungry you may have gotten while climbing.

Does that mean that we could make water boil hotter if we artificially increased the pressure on it?

That’s precisely what a pressure cooker does.

Let’s clamp a tight fitting lid with only a small hole for escaping steam onto a cooking pot. Then we’ll place a weight on top of that hole to keep a certain, calculated amount of the steam pressure in, instead of letting it escape freely into the atmosphere.

Or else we can use some sort of pressure regulator to fix the pressure at a predetermined value. The pressure of the “atmosphere” inside the pot will then be maintained at that higher value.

At a typical pressure-cooker pressure of ten pounds per square inch (0.70 kilogram per square centimeter) above normal atmospheric pressure, the boiling temperature, and hence the temperature of the steam inside, is 240 degrees Fahrenheit (115 degrees Celsius).

That’s hot enough to make short work of any otherwise long simmering dish, such as a stew. Moreover, the space inside a pressure cooker is filled with steam, which is a much better conductor of heat air.

Thus, any heat anywhere in the pot will be conducted into the food more efficiently than if the pot were filled with air. This also makes for faster cooking.