MIST. The vapour of water, when mixed with air of the same or a higher temperature, is invisible; but when the temperature of the air is reduced below that of the vapour, the vapour becomes and forma a mist. 1Vatcr, in the state of vapour, is continually rising into the atmosphere at all the usual temperatures. At and even below the freezingpoint water evaporates, and ice and snow, in a dry atmos phere, gradually disappear without becoming sensibly liquid. But as heat is the solo cause of the conversion of water into vapour, the quantity produced other things remaiuing the same, is in proportion to the temperature; so that in very hot weather the air is not easily saturated with vapour, and in cold weather evaporation is slow : thus, there is more vapour in the air in summer than in winter, and in hot countries than in temperate climates, in all cases where similar sur faces of water are exposed to the sun's rays. Indeed, it has been found that the quantity of vapour in the air diminishes nearly uniformly with the temperature from the equator to the poles. But as the quantity of vapour which the air will hold at any given temperature is limited, whenever that quantity is near or at the point of saturation, a very small reduction of temperature renders the air misty, and a further reduction converts the vapour into rain.
, As every reduction of the temperature of the air has a tendency to destroy the transparency of the vapour which it contains, the atmos phere in our variable climate is seldom very clear. Soon after sunrise however, in fair weather, the vapour near the earth having been precipitated by the night-cold in the form of dew, and the sloping rays of the sun having little power to raise more vapour, the air is almost perfectly transparent, and every object has a clearness and sharpness of outline which it never has at any other time of the day.
When the mist is very thick, it is called a fog. The fogs which fre quently occur in London in the winter arise from the large quantity of vapour produced by a great city being condensed by cold : and as it is not carried off by winds, it is mixed with the smoke, and forms a thick mass in and about the town ; while at a short distance the air is often quite clear, and the limits of the fog may be distinctly observed. The comparative cold of the local atmosphere in the valley of the Thames below London, and its extension up the river, are also concerned in the production of London fogs.
The preceding is a very general view of the nature of mist and its production. On our rivers and seas, towards the end of summer, throughout the autumn and the beginning of winter its frequency and amount are remarkable, and are the cause of much delay and risk to the navigator. The immediate and topical dependence of the forma tion of mist, however, on the ordinary terraqueous surface, is upon the temperature of rivers and collections of water. • This was made a special subject of attention by Sir H. Davy, in his second journey on the continent of Europe, in ISIS, and he published the results in the 'Philosophical Transactions' for the following year, pp. 123-131. He thus describes the process by which mist is formed :—"As soon as the sun has disappeared from any part of the globe, the surface begins to lose heat by radiation, and in greater proportions as the sky is clearer ; but the land and water are cooled by this operation in a very different manner : the impression of cooling on the land is limited to the surface, and very slowly transmitted to the interior ; whereas in water above 45° Fahr., as soon es the upper stratum is cooled, whether by radiation or
evaporation, it sinks in the mass of fluid, and its place is supplied by warmer water from below, and till the temperature of the whole mass is reduced nearly to 40° Fahr. the surface cannot be the coolest part. It follows, therefore, that wherever water exists in considerable masses, and has a temperature nearly equal to that of the land, or only a few degrees below it, and above 45° Fahr. at sunset, its surface, during the night, in calm and clear weather, will be warmer than that of the con tiguous land ; and the air above the land will necessarily be colder than that above the water ; and when they both contain their due propor tion of aqueous vapour, and the situation of the ground is such as to permit the cold air from the land to mix with the warmer air above the water, mist or fog will be the result ; which will be so much the greater in quantity, as the land surrounding or inclosing the water is higher, the water deeper, and the temperature of the water, which will coincide with the quantity or strength of vapour in the air above it, greater." He proceeds to detail some observations in proof of the correctness of this view, made on the Danube from Ratisbon to Vienna, and at the junctions with it of the Inn and the Ilz ; on the Rhine, the Raab in Hungary, in Carnlola, on the coast of Istria, and in Italy, including the Po and the Tiber, and the small lakes in the Campagna of Rome.
After mists have been formed above rivers and lakes, their increase seems not only to depend upon the constant operation of the cause which originally produced them ; but likewise, Davy inferred, npon the radiation of heat from the superficial particles of water composing the mist, which produces a descending current of cold air in the very body of the mist, whilst the warm water continually sends up vapour ; it is to these circumstances that the phenomena must be ascribed of mists from a river or lake, sometimes arising considerably above the surrounding hills. When rivers rise from great sources in the interior of rocks or strata, as they have the mean temperature of the climate, mists can rarely form upon them, except in winter, or late in autumn, or early in spring. Great dryness in the air, or a current of dry air passing across a river, will prevent the formation of mist, even when the temperature of the water is much higher than that of the atmosphere.
This topical cause of the deposition of moisture from the atmosphere is of considerable extent and variety in its modifications, and is not with out effect in the economy of nature, " for verdure and fertility, in hot climates, generally follow the courses of rivers, and by the operation of this cause, are extended to the hills, and even to the plains surround ing their banks." When the vapours in the upper regions of the atmosphere are con densed and become visible, they form clouds. [CLOUD.] When those near the surface of the earth arc condensed upon cold objects, they form dew and hear frost. [DEW ; HOAR-FROST.]