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.

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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.

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