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Fog is a visible aggregate of minute water droplets suspended in the atmosphere at or near the surface of the earth. When air is almost saturated with water vapor, this means that the relative humidity is close to 100%, and that fog can form in the presence of a sufficient number of condensation nuclei, which can be smoke or dust particles. Some fogbanks are denser than others because the water droplets have grown bigger through accretion. In fog conditions, droplets can absorb more water and grow considerably in size. There are different ways to classify fog. An often-used classification is the one used by the International Civil Aviation Organization (ICAO). According to this system, fog can be classified into 4 categories:
- Category I: visual range 1220 meters
- Category II: visual range 610 meters
- Category IIIa: visual range 305 meters
- Category IIIc: visual range 92 meters