They were stored vertically in my basement – so a constant temperature year round. They didn’t leak stored like that. I only noticed it when I brought them upstairs to change the water out, one fell over and lost about a tablespoon out of the main opening in less than a minute. So i put the other two on their sides as well to see what they would do, and both of them also leaked copiously from the main opening. They didn’t do that when they were new. I’m still using them, but I take more care now to ensure they always stay upright.