It is generally not advisable to leave salt uncovered for extended periods of time, as this can cause the salt to become damp and clump together. If you must leave salt uncovered, be sure to store it in a cool, dry place away from direct sunlight. It's also important to note that some types of salt are more prone to clumping than others. For example, kosher salt and sea salt typically don't clump as easily as table salt. So if you're concerned about your salt clumping together, you may want to consider using one of these types of salt instead.