Difference between revisions of "Fog"

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<div class="definition"><div class="short_definition">Water droplets suspended in the [[atmosphere]] in the vicinity the earth's surface that affect  [[visibility]].</div><br/> <div class="paragraph">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&deg;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 K&ouml;hler theory. [[Mist]] may be considered an intermediate  between fog and haze; its [[particles]] are smaller (a few &#x003bc;m 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.</div><br/> </div>
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<div class="definition"><div class="short_definition">Water droplets suspended in the [[atmosphere]] in the vicinity the earth's surface that affect  [[visibility]].</div><br/> <div class="paragraph">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|radiation  fog]] or [[upslope fog]]), or by adding moisture and thereby elevating the dewpoint (producing [[steam fog|steam  fog]] or [[frontal fog]]). Fog seldom forms when the dewpoint spread is greater than 4&#x000b0;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|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 K&#x000f6;hler theory. [[mist|Mist]] may be considered an intermediate  between fog and haze; its [[particles]] are smaller (a few &#x003bc;m 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.</div><br/> </div>
 
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Latest revision as of 15:59, 25 April 2012



fog

Water droplets suspended in the atmosphere in the vicinity the earth's surface that affect visibility.

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.