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Online Encyclopedia
Originally appearing in Volume V10, Page 590 of the 1911 Encyclopedia Britannica.
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FOG, the name given to any distribution of solid or liquid particles in the surface layers of the atmosphere which renders surrounding objects notably indistinct or altogether invisible according to their distance. In its more intense forms it hinders and delays travellers of all kinds, by sea or land, by railway, road or river, or by the mountain path. It is sometimes so thick as to paralyse traffic altogether. According to the New English Dictionary the word " appears to be " a back formation from the adjective " foggy," a derivative of " fog used with its old meaning of aftermath or coarse grass, or, in the north of Britain, of " moss." Such a formation would be reasonable, because wreaths of fog in the atmospheric sense are specially characteristic of meadows and marshes where fog, in the more ancient sense, grows. Two other words, mist and haze, are also in common use with reference to the deterioration of transparency of the surface layers of the atmosphere caused by solid or liquid particles, and in ordinary literature the three words are used almost according to the fancy of the writer. It seems possible to draw a distinction between mist and haze that would be fairly well supported by usage. Mist may be defined as a cloud of water particles at the surface of land or sea, and would only occur when the air is nearly or actually saturated, that is, when there is little or no difference between the readings of the dry and wet bulbs; the word haze, on the other hand, may be reserved for the obscuration of the surface layers of the atmosphere when the air is dry. It would not be difficult to quote instances in which even this distinction is disregarded in practice. Indeed, the telegraphic code of the British Meteorological Office uses the same figure for mist and haze, and formerly the Beaufort weather notation had no separate letter for haze (now indicated by z), though it Description of Effects. Name. No. On Land. On Sea. On River. Slight Fog or Mist I Objects indistinct, but Horizon invisible, but Objects indistinct, but traffic by rail or road lights and landmarks navigation unimpeded unimpeded visible at working distances 2 Traffic by rail requires Lights, passing vessels Navigation impeded, ad- additional caution and landmarks gener- ditional caution re- Moderate Fog 3 Traffic by rail or road ally indistinct under quired impeded a mile. Fog signals are sounded 4 Traffic by rail or road Ships' lights Navigation suspended impeded and vessels i invisible 4 at mile or Thick Fog ' 5 Traffic by rail or road less totally disorganized 588 distinguished between /',fog, and m, mist. It is possible, however, that these practices may arise, not from confusion of idea, but from economy of symbols, when the meaning can be made out from a knowledge of the associated observations. As regards the distinction between mist and fog, careful consideration of a number of examples leads to the conclusion that the word " fog " is used to indicate not so much the origin or meteorological nature of the obscurity as its effect upon traffic and travellers whether on land or sea. It is, generally speaking, " in a fog " that a traveller loses himself, and indeed the phrase has become proverbial in that sense. A " fog-bell " or " fog-horn " is sounded when the atmosphere is so thick that the aid of sound is required for navigation. A vessel is " fog-logged " or " fog-bound " when it is stopped or detained on account of thick atmosphere. A " fog-signal " is employed on railways when the ordinary signals are obliterated within working distances. A " fog-bow " is the accompaniment of conditions when a mountain traveller is apt to lose his way. These words are used quite irrespective of the nature of the cloud which interferes with effective vision and necessitates the special provision; the word " mist " is seldom used in similar connexion. We may thus define a fog as a surface cloud sufficiently thick to cause hindrance to traffic. It will be a thick mist if the cloud consists of water particles, a thick haze if it consists of smoke or dust particles which would be persistent even in a dry atmosphere. It is probable that sailors would be inclined to restrict the use of the word to the surface clouds met with in comparatively calm weather, and that the obscurity of the atmosphere TABLE I. Air travelling from Northern Africa to when it is blowing hard and perhaps raining hard round by the Azores. a cloud of minute water globules, of no great vertical thickness, which disperses the sunlight by repeated reflection but is fully translucent. In dust-storms and sand-storms dark or coloured fog clouds are produced such as those which are met with in the Harmattan winds off the west coast of Africa. In large towns the fog cloud is darkened and intensified by smoke, and in some cases may be regarded as due entirely to the smoke. The physical processes which produce fogs of water particles are complicated and difficult to unravel. We have to account for the formation and maintenance of a cloud at the earth's surface; and the process of cloud-formation which is probably most usual in nature, namely, the cooling of air by rarefraction due to the reduction of pressure on ascent, cannot be invoked, except in the case of the fogs forming the cloud-caps of hills, which are perhaps not fairly included. We have to fall back upon the only other process hitherto recognized as causing cloudy condensation in the atmosphere, that is to say, the mixing of masses of mist air of different temperatures. The mixing is brought about by the slow motion of air masses, and this slow motion is probably essential to the phenomenon. Over the sea fog is most frequently due to the cooling of a surface layer of warm air by the underlying cold water. The amount of motion of the air must be sufficient to prevent the Successive Temperatures of sea 68° 68° 67° 59° 54°F. air 68° 70° 67° 6o° 56°F. States of the atmosphere . clear clear clear shower mist Northern Russia, Wind Force. 0& I 2 3 4 5 6 7 8–12 All Winds. Number of occasions of fog 8 7 9 14 6 3 End of Article: FOG

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