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Originally appearing in Volume V10, Page 590 of the 1911 Encyclopedia Britannica.
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FOG , the name given to any See also:

distribution of solid or liquid particles in the See also:surface layers of the See also:atmosphere which renders surrounding See also:objects notably indistinct or altogether invisible according to their distance. In its more intense forms it hinders and delays travellers of all kinds, by See also:sea or See also:land, by railway, road or See also:river, or by the See also:mountain path. It is sometimes so thick as to paralyse See also:traffic altogether. According to the New See also:English See also:Dictionary the word " appears to be " a back formation from the See also:adjective " foggy," a derivative of " fog used with its old meaning of aftermath or coarse grass, or, in the See also:north of See also:Britain, of " See also: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 See also:ancient sense, grows. Two other words, mist and haze, are also in See also:common use with reference to the deterioration of transparency of the surface layers of the atmosphere caused by solid or liquid particles, and in See also:ordinary literature the three words are used almost according to the See also: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 See also:cloud of See also:water particles at the surface of land or sea, and would only occur when the See also: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 See also: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 See also:code of the See also:British Meteorological See also:Office uses the same figure for mist and haze, and formerly the See also:Beaufort See also:weather notation had no See also:separate See also: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 See also:

Horizon invisible, but Objects indistinct, but traffic by See also:rail or road See also:lights and landmarks See also: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 See also: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 See also:idea, but from See also: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 See also: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-See also:bell " or " fog-See also:horn " is sounded when the atmosphere is so thick that the aid of See also:sound is required for navigation. A See also:vessel is " fog-logged " or " fog-See also:bound " when it is stopped or detained on See also:account of thick atmosphere. A " fog-See also:signal " is employed on See also:railways when the ordinary signals are obliterated within working distances. A " fog-See also:bow " is the See also:accompaniment of conditions when a mountain traveller is See also:apt to lose his way. These words are used quite irrespective of the nature of the cloud which interferes with effective See also:vision and necessitates the See also:special See also: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 See also:

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 See also:calm weather, and that the obscurity of the atmosphere TABLE I. Air travelling from See also:Northern See also:Africa to when it is blowing hard and perhaps raining hard See also:round by the See also:Azores. a cloud of See also:minute water globules, of no See also:great See also:vertical thickness, which disperses the sunlight by repeated reflection but is fully translucent. In dust-storms and See also:sand-storms dark or coloured fog clouds are produced such as those which are met with in the See also:Harmattan winds off the See also:west See also: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 See also:physical processes which produce fogs of water particles are complicated and difficult to unravel. We have to account for the formation and See also:maintenance of a cloud at the See also:earth's surface; and the See also: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 See also: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 See also: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 See also: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 See also:

Russia, See also: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 <I <I 47 per moo observations . . 5 6 I I 22 20 12 6 2 84 • Number of occasions of mist per moo observations . are not visible within working distances the sound- See also:ing of a fog-horn becomes a See also:duty. The number of occasions upon which fog and mist may be noted as occurring with winds of different strengths may be exemplified by the following results of See also:thirty years for St See also:Mary's, Scilly Isles, where the observations have always been made by men of nautical experience. Successive Temperatures of sea. ,, air . f, See also:State of atmosphere as well should be indicated by the terms " thick weather " or " very thick weather " and not by " fog "; but the See also:term " fog " would be quite correctly used on such occasions from the point of view of cautious navigation. If cloud, drizzling See also:rain, or hea y rain cause such obscurity that passing ships condensation taking See also:place at the sea surface without showing itself as a cloud.

In a See also:

research on the See also:Life See also:History of Surface Air Currents the changes incidental to the See also:movement of the air over the north See also:Atlantic Ocean were traced with great care, and the above examples (Tables I, II) taken from See also:page 72 of the See also:work referred to are typical of the formation of sea fog by the cooling of a relatively warm current passing over cold water. In conformity with this See also:suggestion we find that fog is most liable to occur over the open ocean in those regions where, as off the Newfound- land See also:banks, cold-water currents underlie warm air, and that it is most frequent at the See also:season of the See also:year when the air temperature is increasing faster than the water temperature. But it is difficult to bring this See also:hypothesis always to See also:bear upon actual practice, because the fog is representative of a See also:tempera- See also:ture difference which has ceased to exist. One cannot therefore observe under ordinary circumstances both the temperature difference and the fog. Doubtless one requires not only the initial temperature difference but also the slow See also:drift of air which favours cooling of the See also:lower layers without too much mixing and consequently a layer of fog See also:close to the surface. Such a fog, the characteristic sea fog, may be called a cold surface fog. From 67° 66° See also:fair 63° 64° shower 54° F. 53° F. mist with shower The use of the word " fog " in the connexion " high fog," to describe the almost See also:total darkness in the daytime occasionally noted in See also:London and other large cities due to the persistent opaque cloud in the upper air without serious obscuration of the surface layers, is convenient but incorrect. Regarding " fog " as a word used to indicate the state of the atmosphere as regards transparency considered with reference to its effect upon traffic, a See also:scale of fog intensity has been introduced for use on land or at sea, whereby the intensity of obscurity is indicated by the See also:numbers i to 5 in the table following. At sea or in the See also:country a fog, as a See also:rule, is See also:white and consists of the conditions of its formation it is likely to be less dense at the See also:mast-See also:head than it is on See also:deck. One would expect that a cold-air current passing over a warm sea surface would give rise to an ascending current of warmed air and hence cause cumulus cloud and possibly See also:thunder showers rather than surface fog, but one cannot resist the conclusion that sea fog is sometimes formed by slow transference of cold air over relatively warm water, giving rise to what may be called a " steaming-pot " fog.

In such a case the actual surface layer in contact with the warm water would be clear, and the fog would be thicker aloft where the mixing of cold air and water vapour is more See also:

complete. Such fogs are, however, probably rare in comparison with the cold-water fogs. If the existence of a cold current over warm water were a sufficient cause of fog, as a current of warm air over cold water appears to be, the See also:geographical distribution of notable fog would be much more widespread than it actually is, and the seasonal distribution of fog would also be other than it is. The formation of fog over land seems to be an even more complicated process than over the sea. Certainly in some cases mistiness amounting to fog arises from the replacement of cold surface air which has chilled the earth and the objects thereon by a warm current. But this process can hardly give rise to detached masses or banks of fog. The ordinary land or valley fog of the autumn evening or See also:winter See also:morning is due to the See also:combination of three causes, first the cooling of the surface layer of air at or after sunset by the See also:radiation of the earth, or more particularly of See also:blades of grass, secondly the slow downward flow (in the See also:absence of wind) of the air thus cooled towards lower levels following roughly the course of the natural water drainage of the land, and thirdly the See also:supply of moisture by evaporation from warm moist See also:soil or from the relatively warm water surface of river or See also:lake. In this way steaming-pot fog gradually forms and is carried downward by the natural though slow descent of the cooled air. It thus forms in wreaths and banks in the lowest parts, until perhaps the whole valley becomes filled with a cloud of mist or fog. A case of this See also:kind in the Lake See also:District is minutely described by J. B. See also:Cohen (Q.

J. See also:

Roy. Met. See also:Soc. vol. 30, p. 211, 1904) . It will be noticed that upon this hypothesis the circumstances favourable for fog formation are (1) a site near the bottom level of the drainage See also:area, (2) cold surface air and no wind, (3) an evening or See also:night of vigorous radiation, (4) warm soil, and (5) abundant moisture in the surface-soil. These conditions define with reasonable accuracy the circumstances in which fog is actually observed. The persistence of these fog wreaths is always remarkable when one considers that the particles of a fog cloud, however small they may be, must be continually sinking through the air which holds them, and that unless some upward motion of the air keeps at least a See also:balance against this downward fall, the particles of the cloud must reach the earth or water and to that extent the cloud must disappear. In sheltered valleys it is easy to suppose that the See also:constant downward drainage of fresh and colder fog-laden material at the surface supplies to the layers displaced from the bottom the necessary upward motion, and the result of the See also:gradual falling of drops is only that the surface cloud gets thicker; but there are occasions when the extent and persistence of land fog seems too great to be accounted for by persistent radiation cooling. For example, in the See also:week before See also:Christmas of 1904 the whole of See also:England See also:south of the See also:Humber was covered with fog for several days. It is of course possible that so much fog-laden air was poured down from the sides of mountains and hills that did project above the surface of the fog, as to keep the lower reaches supplied for the whole See also:time, but without more particulars such a statement seems almost incredible.

Moreover, the drifting of fog banks over the sea seems capricious and unrelated to any known circumstances of fog-formation, so that one is tempted to invoke the aid of electrification of the particles or some other abnormal See also:

condition to account for the persistence of fog. The observations at See also:Kew See also:observatory show that the See also:electrical potential is abnormally high during fog,but whether that is the cause or the result of the presence of the water particles, we are not yet in a position to say. It must be remembered that a fog cloud ought to be regarded as being, generally speaking, in process of formation by mixing. Observations upon clouds formed experimentally in globes tend to show that if a See also:mass of fog-bearing air could be enclosed and kept still for only a See also:short while the fog would See also:settle and leave the air clear. The apparently capricious behaviour of fog banks may be due to the fact that mixing is still going on in the persistent ones, but is completed in the disappearing ones. One remarkable characteristic of a persistent fog is the coldness of the foggy air at the surface in spite of the See also:heat of the See also:sun's rays falling upon the upper surface of the fog. A remarkable example may be quoted from the case of London, which was under fog all See also:day on 28th See also:January 1909. The maximum temperature only reached 310 F., whereas at Warlingham in See also:Surrey from which the fog lifted it was as high as 46° F. A priori we might suppose that the formation of fog would See also:arrest cooling by radiation, and that fog would thus See also:act as a See also:protection of See also:plants against See also:frost. The condensation of water evaporated from wet ground, which affords the material for making fog, does apparently act as a protection, and heavy watering is sometimes used to protect plants from frost, but the same cannot be said of fog itself—cooling appears to go on in spite of the formation of fog. A third process of fog-formation, namely, the descent of a cloud from above in the See also:form of See also:light drizzling rain, hardly calls for remark. In so far as it is subject to rules, they are the rules of clouds and rain and are therefore See also:independent of surface conditions.

These various causes of fog-formation maybe considered with See also:

advantage in relation to the geographical distribution of fog. See also:Statistics on this subject are not very satisfactory on account of the uncertainty of the distinction between fog and mist, but a See also:good See also:deal may be learned from the distribution of fog over the north Altantic Ocean and its various coasts as shown in the Monthly Meteorological Charts of the north Atlantic issued by the Meteorological Office, and the See also:Pilot charts of the North Atlantic of the See also:United States Hydrographic Office. Coast fog, which is probably of the same nature as land fog, is most frequent in the winter months, whereas sea fog and ocean fog is most extensive and frequent in the See also:spring and summer. By See also:June the fog area has extended from the Great Banks over the ocean to the British Isles, in See also:July it is most intense, and by See also:August it has notably diminished, while in See also:November, which is proverbially a foggy See also:month on land, there is hardly any fog shown over the ocean. The various meteorological aspects of fog and its incidence in London were the subject of reports to the Meteorological See also:Council by See also:Captain A. See also:Carpenter and Mr R. G. K. Lempfert, based upon special observations made in the winters of 1901–1902 and 1902–1903 in See also:order to examine the possibility of more precise forecasts of fog. The study of the properties and behaviour of fog is especially important for large towns in consequence of the economic and hygienic results which follow the incidence of dense fogs. The fogs of London in particular have See also:long been a subject of inquiry. It is difficult to get trustworthy statistics on the subject in See also:con-sequence of the vagueness of the practice as regards the See also:classification of fog.

For large towns there is great advantage in using a fog scale such as that given above, in which one deals only with the See also:

practical range of vision irrespective of the meteorological cause. Accepting the classification which distinguishes between fog and haze or mist, but not between the two latter terms, as See also:equivalent to specifying fog when the thickness amounts to the figure 2 or more on the fog scale, we are enabled to compare the frequency of fog in London by the comparison of the results at the London observing stations. The comparison was made by Mr See also:Brodie in a See also:paper read before the Royal Meteorological Society (Quarterly See also:Journal, vol. 31, p. 15), and it appears therefrom that in See also:recent years there has been a notable diminution of fog frequency, as indicated in the following table of the total number of days of fog in the years from 1871:- But from any statistics of the frequency occurrence of fog it must not be understood that the atmosphere of London is approaching that of the surrounding districts as regards transparency. Judged by the autographic records it is still almost opaque to See also:sunshine strong enough to See also:burn the card of the See also:recorder during the winter months. The bibliography of fog is very extensive. The titles referring to fog, mist and haze in the Bibliography of See also:Meteorology (See also:part ii.) of the U. S. Signal Office, published in 1889, number 306. Among more recent authors on the subject, besides those referred to in the See also:text, may be mentioned :-Koppen, " Bodennebel," Met. Zeit.

(1885); Trabert, Met. Zeit. (19o1), p. 522; See also:

Elias in Ergebnisse See also:des aeronautischen Observatoriums bei See also:Berlin, ii. (Berlin, 1904) ; See also:Scott, Q.J.R. Met. Soc. xix. p. 229; A. G. McAdie, ' Fog Studies,"Amer. Inv. ix. (See also:Washington, D.C., 1902), p.

2o9; See also:

Buchan, " Fogs on the Coasts of See also:Scotland," Journ. See also:Scot. Met. Soc. xii. p. 3. (W. N. S.) FOGAllARO, See also:ANTONIO (1842- ), See also:Italian novelist and poet, was See also:born at See also:Vicenza in 1842. He was a See also:pupil of the Abate See also:Zanella, one of the best of the See also:modern Italian poets, whose See also:tender, thoughtful and deeply religious spirit continued to animate his See also:literary productions. He began his literary career with See also:Miranda, a poetical See also:romance (1874), followed in 1876 by Valsolda, which, republished in 1886 with considerable additions, constitutes perhaps his See also:principal claim as a poet, which is not inconsiderable. To the classic grandeur of See also:Carducci and D'See also:Annunzio's impetuous torrent of See also:melody Fogazzaro opposes a Wordsworthian simplicity and pathos, contributing to modern Italian literature wholesome elements of which it would other-See also:wise be nearly destitute. His novels, Malombra (1882); Daniele Cortis (1887), Misterio del Poeta (1888), obtained considerable literary success upon their first publication, but did not gain universal popularity until they were discovered and taken up by See also:French critics in 1896.

The demand then became prodigious, and a new work, See also:

Piccolo Mondo antico (1896), which critics far from friendly to Fogazzaro's religious and philosophical ideas pronounced the best Italian novel since I Promessi Sposi, went through numerous See also:editions. Even greater sensation was caused by his novel Il Santo (The See also:Saint, 1906), on account of its being treated as unorthodox by the Vatican; and Fogazzaro's sympathy with the Liberal See also:Catholic movement-his own Catholicism being well known-made this novel a centre of discussion in the See also:Roman Catholic See also:world. See the See also:biography by Molmenti (1900).

End of Article: FOG

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