The water starts to evaporate before the structure is set, leaving baked goods dry. This also concentrates the sugars, which then further interferes with baked goods setting properly. Air bubbles and other leavening also expand more rapidly and then pop through the surface so your cakes and cookies end up flat and dense.
Compensating for these changes takes a little trial and error. Here are a few tips we've summarized from Shirley O. Corriher's book Bakewise:
• Add Some Extra Liquid - Corriher suggests trying extra-large eggs instead of large eggs. We also think that adding some moisture in the way of yogurt or folding in some whipped cream would help. Overall, try increasing the liquid by 2-4 tablespoons per cup of liquid in the recipe.
• Increase the Temperature - This will help the structure in the baked good to set more quickly, hopefully compensating for the rapid loss of liquid. Start by increasing the oven temperature by 25° and working from there.
• Decrease the Sugar - Less sugar will also help the structure to set more easily and quickly. Try decreasing the sugar by two tablespoons for every cup of sugar in the original recipe and see how that works.
• Decrease the Leavening - If your recipe uses baking powder, decrease it by 1/4 teaspoon. For baking soda, decrease it by just a pinch. (Yeast will be ok, though your bread dough may rise faster than expected.)
We also know of one really fantastic cookbook specifically written for high altitude baking: Pie in the Sky by Susan Purdy, available on Amazon.com for $21.
High altitude bakers, what other questions or advice do you have to share?