Sometimes it's right on the box of rice mix—the high altitude version of cooking instructions. Usually this means that your rice will have to cook a little bit longer if you are in Denver or at the top of Mount Everest. Of course that's just a joke. No one cooks rice at the top of Everest. But why are the instructions even different? Why does it matter where you cook? The answer has to do with boiling water.

Go ask some people on the street about the boiling temperature of water. Some might say 212°F or even better 100°C—but that's not always true. As you increase your altitude above sea level, the boiling point of water decreases by about 1°F for every 500 feet increase. That means your water in Denver is going to be 203°F and this will have an impact on your cooking.

But why?

Water Vapor Pressure

There are many awesome things about water—one interesting "factoid" is that on the surface of the Earth you can find water in all three phases: solid (we call this ice), liquid water, and as a gas. We call the gas phase of water "water vapor."

You might think that you need to boil liquid water to create water vapor—but you don't. You just need some liquid water at room temperature (or any temperature). Picture a glass of water. If you could zoom in with super vision (not actually possible), you would see that this water is made of a bunch of molecules—water molecules. Although these molecules are themselves made of three atoms (two hydrogens and one oxygen), let's just think of them as tiny balls.

These tiny water balls are moving around in the water but stay fairly close to their ball neighbors. This motion isn't due to currents in the water, instead this is thermal motion. Imagine these tiny balls jiggling around in a giant collection of balls. The hotter the water, the greater the motion of these water balls. But wait! The speeds of the water particles aren't all the same. Although there is an average ball speed, some are going faster and some are going slower. It's just like the height of a group of adult humans. There is an average height, but everyone is not the same. Some people are VERY tall, but that's just a small fraction of the total group.

If you have a glass of water sitting out on a table, the water balls don't just stay as a liquid. Some of these balls have enough thermal energy to break away and become free. Free from the liquid stage means the water ball is now a gas—water vapor. Boiling is not needed to get this water vapor. But wait! It works the other way too. Some of the water balls in the gas stage can interact with the liquid water and join the liquid water balls.

Water in a closed container (like a water bottle) will eventually reach an equilibrium state between water vapor and liquid water. At this equilibrium state the rate that water balls are freed from the liquid state are the same as the rate of water balls entering the liquid state. The pressure of this water gas in equilibrium is called the vapor pressure (assuming it's all water gas and no air in the container).

You can see evidence of this water vapor in a closed container by looking at the water that condenses on the walls. Here is a picture.