High-altitude baking causes many complications, and these issues can affect the outcome of baked goods. Wet ingredients are subject to a faster rate of evaporation, which can lead to unwanted texture variations. Additionally, the low air pressure in high-altitude locations can extend bake times beyond the expected. When adding baking soda or baking powder for leavening, bakers may also find that dough rises at a much faster pace than it would in other areas.
Based on these facts, the claims about Costco’s high-altitude stores running into cookie problems make sense. And while there’s no way to verify that this is the case, a Reddit commenter shared another bit of compelling evidence. According to the person, “My friend was at the Costco by me in WA and he said the cookies didn’t look like they were all cooked the same. He said they all looked like they were cooked a different amount of time.” Fortunately, the Redditor’s friend relayed that the cookie was “tasty,” despite any potential issues posed by high altitudes.