CHAPTER III CHARACTERS AND CONTRASTS-CLOUDS OF RADIATION Nebula In 26. THE spherules of water which constitute nebula vary greatly in diameter, approximately between the limits of 01 inch and 0001 inch, and are measurable both mechanically and by the chromatic method. perfectly tranquil air these particles descend to the earth's surface with a velocity chiefly dependent on the square root of their diameters and the pressure and temperature of the air. At a pressure of 29.92 inches (760 mm.), a water particle, regarded as a perfect sphere, having a diameter of 0001 inch (025 mm.), will, at the temperature of the meltingpoint of ice, have a terminal velocity of descent of 1 miles per hour (67 m.p.s.). But perfect tranquillity does not exist in nature, and a very slight horizontal movement is sufficient to arrest, according to a well-known mechanical law, the downward movement of water spherules. 27. The ground fog characteristic of land surfaces in nocturnal and winter conditions is most essentially, to adopt a name borrowed in the first place from D Herschel, the cloud of "radiation." Interfusion, or commixture, of the adjacent laminae of the atmosphere cannot play a very important part in its production. For, in the first place, land fog is often formed over surfaces which are very nearly level, and, in the second place, as is well known to any observer in the country, it is only developed, or it is first developed, over a portion of the earth's surface which can be experimentally shown to be, from one cause or another, of lower temperature than adjacent portions. For instance, in a winter night in the higher latitudes nebula frequently appears over a piece of pasture land before it is produced over arable ground in the vicinity, irrespectively of elevation. Herbage not only keeps the surface of the earth beneath it moist, and supplies much moisture by night as well as by day to the contiguous atmosphere, but radiates heat very freely into space. The air which is in actual contact with herbage precipitates part of its vapour in the form of dew or of hoar-frost; but the layers of atmosphere immediately above this, having their temperature lowered to the dew-point, deposit moisture on the dust particles, which are themselves losing heat by radiation. This process may quietly continue under favourable circumstances up to the coldest period of a winter's night, the hour of the occurrence of which principally depends upon latitude. Under the same circumstances fog begins to be reconverted into vapour soon after insolation begins. Hence we have our misty mornings, followed by bright afternoons, characteristic of settled weather in the colder months of the year in such climates as that of the British Isles. Over extensive plateaux ground fogs are less noticeable, because the surface of the earth is drier and irrigation generally less abundant; and over wide plains in the central portions of large continents they are especially rare, simply because the atmosphere here contains but little of the vapour supplied from oceanic surfaces. Unfavourable conditions may be brought about in several ways. The upward extension of nebula may be so great as to check radiation; clouding of the upper sky may have the same effect; a change in the distribution of atmospheric pressures near the earth's surface may occur, producing motion of the air sufficient to disperse the fog at an hour when it would otherwise continue; just as the continuance or increase of nebula at an unusual hour may be effected either by an increase of terrestrial radiation, due to incidental causes, or to results of artificial increase of dust particles through the augmentation of imperfect combustion during the daytime in large As an example of the first process, it may be mentioned that I have seen a fog of great density, several feet in vertical thickness, suddenly produced on a hot afternoon in August by a thunderstorm, which left the surface of the ground covered with hail two inches deep. In this case, of course, the absorption of heat in the partial melting of ice caused vapour in immediate contact with the ice to be condensed. Nebula must not, however, be regarded as in all cases a very local phenomenon, or again, as restricted to periods when the atmosphere near its floor is calm. towns. In the higher latitudes the extent of a fog largely depends upon the extent and intensity of one of those systems of non-periodic atmospheric circulation within which the pressure of the atmosphere near the earth's surface is relatively great. In these there are in winter none of those upward movements which promote the dispersion of fog; and, further, it is obvious that an increase of atmospheric pressure must, to a slight extent, promote the suspension both of the particles of dust, and of the fog which accompanies it. Near the exterior of these systems, where there is a decided but somewhat equable movement of air, which has recently traversed some portion of a water surface, the favourable effects of great atmospheric pressure more than counterbalance the unfavourable effects of disturbing movement, and fog drifts over the earth's surface. Here terrestrial radiation plays a rather less important part, and the interfusion of adjacent portions of air, differing in temperature and humidity, begins to assert itself. 28. This leads us to the consideration of those fogs which are essentially maritime, and are in this respect dissimilar to the fogs already described. In some parts of the globe the contrast is obvious in reference to the annual maxima and minima of the occurrences of nebula. To take a well-known example: Over the English Channel fog is most prevalent in the month of June, which very nearly coincides with the time of the annual minimum of fog over the inland parts of Western Europe. A local observation, in this connection, may help us to interpret the cause of sea fogs in many parts of the globe. Records of sea temperature, taken in the shallow bay of St. Aubin, Jersey, by the late Mr. Vibert, a friend of the writer, show that here, although the sea feels the effects of insolation earlier in the summer than in the deeper parts of the Channel, the mean temperature of the water is at this season lower than the mean temperature of the air above it. It will be indicated hereafter that non-periodic systems of high barometric pressure are characterised by warm weather in summer for the same reasons which tend to make them areas of cold weather in winter. When one of these systems lies over France, the south-westerly and westerly winds of its northern portion pass over the English Channel, having become nearly saturated, quite saturated, or supersaturated in their passage over some portion of the North Atlantic. The air immediately contiguous to the waters of the Channel is kept cool by the relatively low temperature of the latter; and a moist current moving over it, and partially commingled with it, has its vapour condensed into fog. Nebula formed in this way obviously cannot penetrate into inland localities, where (only for short periods in our own fickle climate) there is a concomitant or subsequent reign of brilliant summer weather. 29. It is obvious, then, that sea fogs are liable to occur whenever and wherever the surface water of the sea is of much lower temperature than that of land or of other surface water in the vicinity, supposing climatic conditions to be such that interfusion is capable of producing nebula, and supposing special atmospheric |