Water typically exists in its gaseous state in the atmosphere. Fog contains droplets of liquid water suspended in the air. Thus, to produce a fog, vapor in the air would undergo condensation with a state change from gas to liquid. Intermolecular interactions between water molecules strengthen as the vapor condenses to produce a liquid. The condensation of vapor thus involves a production of energy. With all the extra intermolecular interactions, molecules in the fog would find it more difficult to move around and, therefore, see a decrease in their motion.
