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HOW COME? Baking? Factor in altitude

Why do cake boxes have different baking directions for higher altitudes? asks a reader.If you're baking a birthday cake, it's important to know the height of your kitchen. No, not floor to ceiling -- how high your oven is, compared with sea level. If it's 3,500 feet or above, you're doing high-altitude, cross-your-fingers baking.

But altitude doesn't just affect baking. It also changes the temperature at which water boils in a teakettle. That 212 degrees F. we learned in school? It doesn't apply up in the Rocky Mountains.

Cooking, it turns out, depends not just on heat, but on air pressure. At sea level, Earth's atmosphere weighs on each tiny square inch of us -- and everything else -- with about 14.7 pounds of force. As air thins out higher up, it exerts less and less pressure. Climb a high enough mountain, and you'll notice the difference: In thinner air, it's harder to breathe.

However, it's easier for water to boil. As air pressure on water drops, it takes less energy for H2O molecules to break the bonds between them. The higher the altitude, the less heat required to turn water from a liquid to a gas (steam).

At sea level, water boils at the familiar 212 degrees F. But hoist your teakettle up 500 feet in elevation, and its boiling point will drop by about 1 degree. At 14,000 feet, on Colorado's Pikes Peak or Washington's Mount Rainier, water boils at a not-so-hot 186 degrees. And at the 29,000-feet peak of Mount Everest, water bubbles away at just under 160 degrees.

So above 3,500 feet, foods cooked in water, like rice or beans, must simply be cooked longer before they're done.

But cake, bread, and other baked goods are trickier to get right. Gases released by yeast expand more in lower air pressure, making breads rise too quickly. Moisture evaporates faster. And because thin air is usually dry air, ingredients like flour may be extra dry, resulting in hockey-puck cookies and crumbling cakes. Which is why cake mix boxes and cookbooks make adjustments for altitude. Oven temperature often increases, along with baking time. Leavening (like baking powder) is reduced, and proportions change: more liquid in one recipe, less oil in another.

Want a cup of tea with your cake? Even though the kettle's boiling, the tea may be tasteless. Air travelers have long complained about the tea served on planes traveling at 35,000 feet. One airline, British Airways, has even partnered with a tea company to develop an "altitude" blend, claiming it produces flavorful tea, even six miles up.

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