Many cooks believe that storing meringues on a rainy or humid day is futile because the confections contain a lot of sugar, and sugar is hygroscopic—meaning that it attracts moisture from the air, which causes the cookies to turn sticky and marshmallowy. Disproving this theory took just a simple test. We waited for a rainy day and baked up several batches of meringues. As soon as the cookies cooled (after about 10 minutes), we placed them in airtight plastic containers with tight-fitting lids. We put one container in the fridge (a relatively humid environment), one on the counter, and, as a control, one in the freezer (which tends to be arid). Two weeks and several rainy days later, we opened the containers. The cookies stored in the more humid refrigerator and on the counter were every bit as dry and crisp as those stored in the freezer.
THE UPSHOT: As long as you pack meringues in an airtight container immediately after cooling them, any humidity in the air doesn’t matter.