Fog

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(New page: 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)...)
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Water droplets suspended in the atmosphere in the vicinity the earth's surface that affect visibility. 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. 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.+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. 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.
 +
 +[[Image:Aerial Site and Fog Cypress 2008.jpg]]
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Revision as of 17:20, 26 June 2009

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. 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.

Image:Aerial Site and Fog Cypress 2008.jpg


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