According to international definition, fog reduces visibility below 1 km (0.62 miles). Fog differs from
cloud only in that the base of fog is at the earth's surface while clouds are above the surface. When composed of
ice crystals, it is termed
ice fog. Visibility reduction in fog depends on concentration of
cloud condensation nuclei and the resulting distribution of
droplet sizes. Patchy fog may also occur, particularly where air of different
temperature and
moisture content is interacting, which sometimes make these definitions difficult to apply in practice. Fogs of all types originate when the temperature and
dewpoint of the air become identical (or nearly so). This may occur through cooling of the air to a little beyond its dewpoint (producing
advection fog,
radiation fog or
upslope fog), or by adding moisture and thereby elevating the dewpoint (producing
steam fog or
frontal fog). Fog seldom forms when the dewpoint spread is greater than 4°F. According to U.S. weather observing practice, fog that hides less than 0.6 of the sky is called
ground fog. If fog is so shallow that it is not an
obstruction to vision at a height of 6 ft above the surface, it is called simply
shallow fog. In
aviation weather observations fog is encoded F, and ground fog GF. Fog is easily distinguished from
haze by its higher
relative humidity (near 100%, having physiologically appreciable dampness) and gray color. Haze does not contain activated droplets larger than the critical size according to Köhler theory.
Mist may be considered an intermediate between fog and haze; its
particles are smaller (a few μm maximum) in size, it has lower relative humidity than fog, and does not obstruct visibility to the same extent. There is no distinct line, however, between any of these categories. Near industrial areas, fog is often mixed with
smoke, and this combination has been known as
smog. However, fog droplets are usually absent in
photochemical smog, which only contains unactivated haze droplets.
