and in all cases where its temperature is lower than the dewpoint of the air, fogs are formed over the shoal. For a similar reason icebergs are frequently enveloped in fogs. Analogous to the above is the mist which is sometimes seen to rise from rivers whose temperature is lower than that of the air. Thus the waters of the Swiss rivers which issue from the cold glaciers have a temperature considerably lower than that of the air; consequently they cool the air in contact with them below the point of saturation, and mist is thereby often produced. Similarly such rivers as the Mississippi, which flow directly into warmer latitudes, and are therefore colder than the air above them, are often covered with mist or fogs. 349. When rivers are considerably warmer than the air, they give rise to fogs, because the more rapid evaporation from the warm water pours more vapour into the atmosphere than it can hold suspended in an invisible state, and the surplus vapour is condensed into mist by the colder air through which it rises. Thus deep lakes, and rivers flowing out of them, are in winter generally much warmer than the air, and hence when the air is cold and its humidity great they are covered with fogs. When Sir Humphry Davy descended the Danube in 1818, he observed that mist was always formed during the night when the temperature of the air on shore was from 3 to 6° lower than that of the stream; but when the sun rose, and the temperatures were brought to an equality, the mist rapidly disappeared. 350. The densest fogs occur during the cold months in large towns built on rivers, the causes which produce fogs being then at the maximum. The denseness of the London November fogs is notorious, giving significance to a capital sketch in Punch,' which represented a street-boy springing into the air, exclaiming, "I am monarch of all I survey." Their peculiar denseness is caused by the warmth of the river-bed, and it is increased by the sources of artificial heat which London affords; and from the circumstance that the temperature is falling everywhere, and the humidity being then great, the vapour of the atmosphere is quickly and copiously condensed by the gently-flowing cold easterly winds which generally prevail in November. 351. In all these cases fogs are very locally distributed, being confined to the basin of the river or lake where they are formed, and do not extend far up into the atmosphere. There are, however, other fogs that spread over large districts, which are originated under different meteorological conditions. Fogs often accompany the breaking-up of frosts in winter. For when the humid south-west wind has gained the ascendancy, and is now advancing over the earth's surface as a "light air," it is chilled by contact with the cold ground, and its abundant vapour thereby condensed into a widespread mist. 352. Mountains are frequently covered with mist. Since the pressure and consequently the temperature of the air falls with the height, it follows that as warm air is driven up the slopes of the mountain by the wind, it becomes gradually colder, and its capacity for moisture is diminished until condensation takes place, and the mountain is swathed with mist. Owing to the peculiarity of their temperature (par. 180), forests have a marked effect on the mists as well as on the rainfall of mountainous regions. Mists often appear sooner on the parts of hills covered with trees than elsewhere. This happens especially when the mist begins to form after mid-day, because then the temperature of the trees is lower than that of the grassy slopes. Mists also linger longer over forests, probably on account of the increased cold arising from the large extent of evaporating surface presented by their leaves when drenched with mist. During his residence at the Cape of Good Hope, Sir John Herschel observed a remarkable illustration of the influence of trees in condensing the vapour of the atmosphere. On the side of Table Mountain, from which the wind blew, the clouds spread out and descended very low, frequently without any rain falling, while on the opposite side they covered the mountain in dense masses of vapour. When walking beneath tall fir-trees at the time these clouds were closely overhead, he was subjected to a heavy shower of rain; but on going out from be neath the trees the rain ceased. The explanation he gave of the phenomenon was that the clouds were condensed into rain on the cool tops of the trees. And doubtless the innu merable fine leaves of the fir-trees, adding largely to the surfaces of evaporation, increased the cold, and thus condensed the vapours into a more copious shower. 353. I am informed by the Rev. J. Farquharson, Selkirk, that when the atmosphere is very moist and the south-west wind is blowing strongly, a mist or cloud is sometimes seen to settle over Bowhill, which is situated at the junction of the classic Yarrow and Ettrick; and that the cloud thus formed is subject to great and rapid changes, both as regards its outline and its size. This is a highly instructive observation, considered with reference to the causes which produce the phenomenon. Both valleys lie nearly in the direction of the south-west wind, but the Vale of Ettrick is the more highly wooded of the two; hence the temperature of the two valleys will, from what has been stated above, be generally different the one from the other. Now, when a steady humid southwest wind is blowing, each will acquire the temperature of the valley down which it has flowed, and be at the same time at or near the point of saturation; and at Bowhill, where the two aërial currents meet and mix together, cloud will be formed, in accordance with the well-known law by which two volumes of air, each saturated but of different temperatures, can when mixed no longer hold all the vapour in suspension; consequently part is condensed into cloud or rain. The same phenomenon is stated by Dr James Bryce, in his 'Arran,' to occur occasionally at Brodick, at the point where Glens Rosaand Shiraig meet. The explanation is the same as that of the cloud formed at Bowhill. 354. Extensive fogs also prevail where great differences occur in the temperature of contiguous regions. Thus promontories running out into the sea are frequently enveloped in mist; for since land is generally warmer than the sea in summer and colder in winter, the difference of temperature is generally sufficient to cause mists with the veerings of the wind landward or seaward. The same cause explains the mists and fogs which frequently prevail on the coast. These mists generally occur in the morning and evening, seldom advance far inland, and usually accompany fine weather. There is a weather prognostic current in Elie, Fifeshire, as follows: "When Kellie Law gets on his cap, Largo Law may laugh at that; Kellie Law, the lower of the two hills, lies to the north-east of Elie, and Largo Law to the north-west. Hence the mist, which covers Kellie Law only, is brought by a local, temporary, and superficial sea-breeze from the east, which does not advance so far to the westward as Largo Law; but when Largo Law is involved in clouds while Kellie Law is yet clear, we may be sure that the south-westerly current, laden with moisture, has already reached that hill, and consequently will speedily advance on Kellie Law and pour down its rain over the district. 355. The British Islands, being bounded by the warm waters of the Atlantic on the one side, and separated from the Continent on the other only by narrow belts of sea, are subject to fogs during winter. For the same reason dense thick fogs are prevalent in Norway, Newfoundland, along the coast of Peru, and South Africa, and in the polar regions. The Gulf Stream is notorious for dense and long-continued fogs, which seriously obstruct the navigation of that part of the Atlantic, particularly at its northern limit, where it meets the polar current. The high temperature of the stream, which is often from 16° to 18°, and sometimes 30°, higher than that part of the sea past which it flows, fully explains the denseness and persistency of these fogs. 356. Occasionally the summit of a hill or an isolated peak is wrapped in mist or cloud, while elsewhere the atmosphere is clear; and though a breeze be blowing over the hill, still "Overhead The light cloud smoulders on the summer crag," apparently motionless and unchanged. This phenomenon is instructive, and is easily explained. The temperature at the top is below the dew-point of the atmospheric current. Hence when the air rises to this region its moisture is condensed into cloud, which is borne forward over the top of the hill and down the other side, acquiring heat as it descends till it is again dissolved and disappears. Meanwhile its place is constantly supplied by fresh condensations which take place as the current, rising to the height of the cloud, falls below the temperature of saturation. Thus, though the cloud on the top of the hill appears to remain motionless and unchanged, the watery particles of which it is composed are continually undergoing renewal. 357. There is another sort of fog of occasional occurrence, differing from any of the foregoing in several important particulars, which, from its relation to storms, is of considerable importance in meteorology. It would appear to originate from the juxtaposition of the polar and equatorial currents. When these currents flow side by side, fog frequently fills up the comparatively calm space intervening between them. It results from the mixing together of the two currents, the cold of the polar current condensing the vapours of the southwest wind. It sometimes stretches several hundred miles in the form of a long narrow strip. At other times, and more usually, it is a precursor of storms which succeed fine dry weather, during which the wind has been chiefly from the north-east. The south-west wind is seen to prevail in the upper regions of the atmosphere by the direction in which the thin cirrus cloud is blown, some time before it is felt on the surface of the earth. During this interval the humid equatorial current overlaps the polar current, and the fog which prevails is due to the mixing of the two currents. Hence, in discussing storms, fogs constitute one of the most important elements which require consideration, and they supply valuable help towards the foretelling of storms. |