It was a foggy start to the day in places which brought Marie to ask this question. Fog happens when the temperature of the air drops to the dewpoint temperature. At this point there will be condensation of invisible water vapor to droplets of liquid water suspended in the air. This is what the fog is made of.
Fall is our fog season as the nights are longer which helps temperatures to drop more at night. At the same time, water temperatures are still high from the end of summer. This allows for our dewpoint to be higher than the spring season which has similar amounts of sunshine.