I have a box of regular mac and cheese I’m making tonight and I see “high altitude microwave directions” and while I am a college graduate I have no idea why this exists. What does the altitude have to do with macaroni and cheese? I’m sure this also has chemical compounds at play but physics made more sense to tag it with I don’t know if I’m doing this right but I’m hoping someone can break this down for me. Thanks.
Water boils at a lower temperature at higher altitudes because there is less air pressure above it. Because of this water can only be a liquid at lower temps. That means it takes longer for things that need to be in boiling water because the water can’t get hotter without becoming a gas.
High altitudes have a relatively low air pressure. As a result of that, water will boil at a slightly lower temperature. The effect is significant enough that you’ll need to boil pasta for a little bit longer than you would at sea level.
It’s the opposite of a pressure cooker.
Pressure cookers maintain high pressure inside the cooking vessel – higher than normal atmospheric pressure. At high pressure, the boiling point of water is increased. That is, the water needs to be hotter before it turns into steam. This allows you to cook food inside water that is much hotter than you can normally make it. Normally, you can heat it to 100 C and that’s it. The water will stay at that temperature while slowly evaporating. In a pressure cooker, you can get your water up to 120 C. This allows you to cook food faster.
At high altitude, it’s exactly the opposite. The atmospheric pressure there is lower, and so the boiling point of water is lower as well. (Basically, you can imagine the water molecules pushing, trying to get out of the liquid, but the air is pushing back. If you make the water hotter, that translates to the water molecules moving faster and thus pushing harder. How hot you need to make it depends on how much the air is pushing back.) Because the boiling point is lower, it takes longer to cook things, as you now have to cook in water that gets no hotter than, say, 90 C. How much longer depends on the altitude. At the top of Mt Everest (elevation: 8849 m), the pressure is so low that water boils at around 70 C. In Cusco, Peru (3350 m), around 89 C. In Denver, Colorado (1600 m) it’s around 94 C.
The air pressure is lower at high altitudes, which reduces the boiling point of water. Since many foods – pasta in particular – cook at the boiling temperature of the water they’re in (or that is in them), things at high altitude sit at lower temperatures, and therefore need longer cooking times.