You live in an area with naturally high humidity. Warm air holds more humidity than cool air. If you open your saddle bag when you leave work in the afternoon, it gets a big gulp of warm, humid air. Over night, the temperature drops and when it goes below the dew point, the humidity starts to condense out of the trapped air, just like moisture condenses on the side of a glass holding a cold drink. Anything inside the saddle bag gets wet. The same thing happens to your gas tank, but because the volume of air above the gas is less than the size of the saddle bag, less moisture condenses in there. If a machine sits for several weeks with a half full gas tank, there may be some water in the tank that wasn't there when the machine was parked. This is why it is preferable to store a motor vehicle with a full tank of fuel (less air volume above the fuel). Every day, as the temperature rises, the air expands and some air will squeeze out of the tank, then in the evening, as the temp goes down, the air will contract and the tank will "inhale" some new, humid air. As the temp continues to fall, the new moisture will condense and you have water in the fuel. The smaller the volume of air in the tank, the less moisture is generated with each heating and cooling cycle.
