Why do many foods have High Altitude Cooking Instructions?

Most any packaged food that involves boiling (like boxed macaroni-and-cheese dinners) will have "high altitude" cooking instructions. I have here a box of Hamburger Helper that says:

High Altitude (3500-6500 ft): Decrease hot water to 3-3/4 cups. Increase simmer time to 17 minutes.

The reason foods have these instructions is because the boiling point of water changes with altitude. As you go higher, the boiling temperature decreases.

At sea level, the boiling point of water is 212 degrees F (100 degrees C). As a general rule, the temperature decreases by 1 degree F for every 540 feet of altitude (0.56 degrees C for every 165 meters). On top of Pike's Peak, at 14,000 feet, the boiling point of water is 187 degrees F (86 degrees C). So pasta or potatoes cooked at sea level are seeing 25 degrees more heat than pasta or potatoes cooked on Pike's Peak. The lower heat means a longer cooking time is needed.

Pressure cookers work in the opposite direction. A pressure cooker raises the pressure so that the water boils at a higher temperature. A typical pressure cooker applies 15 pounds of pressure, so the boiling point of water rises to 250 degrees F (121 degrees C) at sea level. The higher temperature means that foods take less time to cook.

Here are some interesting links:


National Body Challenge 2008: Restaurants