Q: What causes fog?
A: Fog—and mist—are simply clouds that form immediately above the ground, said Brent McRoberts of Texas A&M University.
"Fog can occur at different times for different reasons," he explained. "The most common form is radiation fog, usually called ground fog. If the air near the ground is cooled sufficiently, it becomes saturated and fog can develop. This often occurs at night and this time of year. The nights are longer in fall and winter and that allows the ground more time to grow cold enough to condense water vapor into the air, resulting in fog."
Q: How big are the water drops in fog?
A: Very tiny, said McRoberts.
"Fog and mist drops are very small," he added. "They vary from about 1-500th of an inch to even 1-25,000th of an inch, so just a little moisture can cause a lot of fog and mist."