Weather Whys: Fog

Q: What causes fog?

A: Fog – and mist – are simply clouds that form immediately above the ground, says Brent McRoberts of Texas A&M University. "Fog can occur at different times for different reasons," he explains. "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 longer nights allow 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, says McRoberts. "Fog and mist drops are very small," he adds. "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. In strict meteorological terms, fog occurs when surface visibility is reduced to less than 3,000 feet. Many times, airports are forced to close when visibility is reduced by far less. Fog normally disappears not long after sunrise. The warmth of the sun tends to evaporate fog and it gradually disappears. One type of fog, called valley fog, forms in mountain valleys during winter and can be as much as 1,500 feet thick, so thick that it often does not evaporate even during sunlight. This type of fog can last for several days."

Weather Whys is a service of the Department of Atmospheric Sciences at Texas A&M University.