It may be a simple question and I've read the PHAK and searched online, but I can't seem to understand why adding moisture to a dry cold air mass would create fog. I understand when an air parcel is cooled to the dewpoint neared the surface, it will become saturated and fog appears. However, why would adding moisture create fog?
You can either cool the air to reduce it's capacity to hold water, or you can add water to the air. Either way will eventually make the air saturated
You can either make the bucket smaller, or you can add water in the bucket. Either way you will eventually make the bucket spill ...