What is the lowest possible cloud ceiling height you can have without it being fog?

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Dear Tom,

What is the lowest possible cloud ceiling height you can have without it being fog?

Thanks,
David Wechsler
Glenview

Dear David,

The Glossary of Meteorology, published by the American Meteorological Society, defines fog as water droplets or ice crystals suspended in the atmosphere near the earth’s surface that affect visibility. By international definition, the visibility must be reduced to at least 1 kilometer (0.62 miles or less). Fog differs from a cloud, in that the base of the fog is at the surface. Cloud heights can be measured by instrument, balloon, aircraft or observer estimation rounded to the nearest 100 feet. If the cloud is on or near the ground and the sky is obscured, the base of the fog is usually reported as zero. If there is some horizontal visibility and the base of the cloud deck is discernable, the lowest reported ceiling value would be 100 feet.

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