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scorching heat that surprises the alpine tourist while travelling over fields of snow under a blazing noonday sun. And it is the same cause, the small amount of vapour in the air, that explains the intense heat experienced in the direct rays of the sun in the polar regions, where Captain Scoresby observed it to melt the pitch on the side of his ship exposed to the sun, while ice was rapidly generated at the other side.
344. When observations with black-bulb thermometers, showing the force of solar radiation in different parts of the world, are stated, they appear at first sight absurd, if not contradictory. Thus at Port Louis, Mauritius, in lat. 29° 9' 56" south, the highest reading of the black-bulb in vacuo was 125° in 1864, and 130° in 1865. Now in Scotland, a thermometer placed in these circumstances, particularly in the eastern districts, would register higher temperatures than those, during at least four months of the year; and in the case recorded by Captain Scoresby, the heat of the sun's rays in melting the pitch must have been about 130°, that is, as high as occurred in the Mauritius during two whole years. An interesting collection of facts of solar radiation in different latitudes and at different elevations is given in Daniell's * Essays' (2d ed., p. 208 et seq.) From the facts adduced, the conclusion is drawn that the power of solar radiation in the atmosphere increases from the equator to the poles, and from below upwards,—a result which these remarks on the vapour of the atmosphere explain. It follows that hygrometric observations ought to accompany all observations on solar radiation, in order to render them intelligible.
345. The above considerations explain in part the nervous derangement and general unhealthiness produced by the east wind in spring ; for as the air is then very dry, the part of the person exposed to the sun's rays is greatly heated as compared with the part in shadow, and this strain on the physical frame few constitutions except the most robust can bear without positive discomfort. On the other hand, exposure to the sun's rays in the tropics is, on account of the thick screen of vapour above, not attended with the intense heat which
might have been expected. Nothing is more common than for natives of the West Indies and other warm moist climates to complain of the, to them, intolerable heat of the sun in our British climate in spring and summer. *
* In the above remarks on the diathermancy of the atmosphere in reference to the influence of its moisture on solar and terrestrial radiation, I have used the word “ vapour" to include all states of the moisture which are, in a popular sense, invisible—in other words, as inclusive of every form in which water may be suspended in the air, except those of mist, fog, cloud, and rain-drops. In 1862, Professor Tyndall made experiments on the vapour of water, from which it was concluded, that while the dry air of the atmosphere was diathermanous, the pure vapour was not so. As this result seemed to give a very satisfactory explanation of the observed effects of solar and terrestrial radiation, it was very generally adopted by meteorologists. In the first edition of this work it was assumed throughout.
But in June 1867, Professor Magnus of Berlin published a more extensive series of experiments, by which it was shown that the results obtained differ not according as the air which is forced through the tubes is moist or dry, but according to the condition of the sides of the tube through which the currents of dry air and moist air are forced. Hence, while Professor Tyndall's experiments, so far as they go, are good, they do not warrant the conclusion which was drawn from them, since the results are shown to be due to the condition of the sides of the tubes used, and not to any diathermical difference between moist air and dry air. Others have confirmed Professor Magnus's experiments.
The observations of solar and terrestrial radiation in relation to the moisture of the atmosphere are probably to be accounted for by the presence of moisture in states other than that of pure vapour, which are imperfectly diathermanous,--such as moisture in a vesicular state, and at the same time invisible, and in states intermediate between the vesicular and that of pure vapour. The conditions under which atinospheric moisture may be formed and maintained in these states, so as to account for the varied and often surprising phenomena of radiation, are very imperfectly known, and some of them perhaps not even yet suspected.
MISTS, FOGS, AND CLOUDS.
346. Mists and fogs are visible vapours floating in the air near the surface of the earth. They are produced in various ways,—by the mixing of cold air with air that is warm and moist, or generally by whatever tends to lower the temperature of the air below the dew-point.
347. During a calm clear night, when the air over a level country has been cooled by radiation, and dew begun to be deposited, the portion of the air in contact with the ground is lowered to the dew-point, and thus becomes colder than the air above it. Since in these circumstances there is nothing to disturb the equilibrium and give rise to currents of air, and there being no cause in operation which can reduce the temperature much below the point of saturation, the air within a few feet of the surface remains free from mist or fog. But if the ground slopes, the cold air, being heavier, must necessarily flow down and fill the lower grounds; and since it is colder than the saturated air which it meets with in its course, it will reduce its temperature considerably below the point of saturation, and thus produce mist, or radiation fog, as it is sometimes termed. When a lake, river, or marsh fills up the valley, the air, being thereby more saturated, often gives rise to denser fogs; and, on the other hand, when the low grounds are sandy or dry, mist is less frequently produced.
348. When an oceanic current meets a shoal in its course, the cold water of the lower depths is brought to the surface, 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