The terrain may not be flat on Colorado’s Front Range, where Ginger lived for several years, but the cookies are. They puff up quickly in the oven, then deflate just as dramatically. It was not possible for us to test every recipe at various altitudes, but we did discover some general “rules” about cookies and altitude. For example, leavened drop cookies and cake-like bar cookies are most vulnerable to the low atmospheric pressure at high altitudes. Although they may spread a little more than they do at sea level, rolled and refrigerator cookies are less affected by altitude. Chewy cookies such as chocolate chips may need a bit more liquid and flour at altitudes above 5,500 or so. Adding an additional egg can help, especially with moister cookies.
On this site we provide general guidelines on producing good cookies of a given type at altitude. And on several of the cookies that seemed especially affected by altitude when we tested them, we give tips for adjusting the recipes.
Humidity also affects cookies, drying them out or making them chewier than usual. Meringue-based cookies will not work properly on a humid day. They absorb wetness from the air and turn gummy rather than crisp.
If you live in a desert-dry climate, on the other hand, the atmosphere will suck moisture from cookies. That’s good news for crisp meringues, bad news for chocolate chip cookies. In fact, all but the most buttery cookies will dry out in a day or two, even in airtight tins. If you won’t be eating them quickly, it’s best to wrap them well and pop them into the freezer.