Fog is a cloud of tiny water droplets or ice crystals suspended in the air that touches the ground. Its density can vary, but it can be thick enough to considerably reduce visibility, sometimes to the point where you can hardly see your hand in front of your face. This common weather phenomenon can wreak havoc on travel plans, but fog can also completely transform the visual aspect of a destination, especially a city where buildings, monuments, and landmarks often take on a surreal and unworldly quality when shrouded in this damp, fine water vapor.