VAI'OUE-PLANE. This term, which was probably adopted by Luko Howard from De Luc, denotes a region of the atmosphere hori zontal in its general direction, though subject to elevations and depressions, at or on and above which clouds form by the condensation of aqueous vapour, and on which, therefore, they appear to float. its position is most readily recognised by the eye, by observing the modi fication of cloud called cumulus, the various of which, or distinct clouds, have a common base-lino, or have their inferior surfaces at the same height. The subject has already been noticed iu the articles CLOUD (coL 981) and DEW-POINT; but in the latter tho vs pour plane is erroneously stated to be the superior, instead of the inferior, limit of a certain stratum of the atmosphere, and also that cloud forms below instead of above it, when it is regarded as a mathematical plane. But there are in fact as many vapour-planes in the atmosphere of any locality on the earth's surface, and at any time, as there are strata of clouds at different elevations ; and in each case the plane becomes itself a physical plane, or stratum, throughout which cloud is produced. These successive strata originate, of course, in alternations of tempe rature and in the amount of aqueous vapour present in a given volume of air.
Tho following statement, founded upon the principal results deduced by the late Mr. J. Welsh from experiments made by him and described in his account of meteorological observations in four balloon scents made in 1852 (' PhiL Trans.,' 1853, pp. 311-346), already cited under CLOUD, gives a general viow of the nature of the alteruations in question, which may be compared with the particulars of tho height of the several strata of cloud passed through in those ascents, and stated in that article, col. 983.
The temperature of the air decreases uniformly with the height above the earth's surface, until, at a certain elevation, varying on different days, the decrease is arrested, and for a space of from 2000 to 3000 feet the temperature remains nearly constant, or even increases by a small amount, the regular diminution being afterwards resumed and generally maintained, at a rate slightly less rapid than in the lower part of the atmosphere, and commencing from a higher temperature than would have existed but for the Interruption noticed. This inter ruption in the decrease of temperature is accompanied by a large and abrupt fall in the temperature of the dew-point, or by actual condensa tion of vapour, from which it may be inferred that the disturbance in the progression of temperature arises from a development of heat in the neighbenrbood of the plane of condensation, or rapour-planc. The subeerpi-ut falls in tho temperature of the dew-point are generally of an abrupt character, and productive of as many vapour-planes ; corre• spending interruptions in the decreasing progression of temperature are sometimes distinguishable, but in a less degree, as might indeed be expected from the fact that at greater elevations, and consequently lower temperatures, the variations in the absolute amount of aqueous vapour are necessarily smaller, and their thermic effects consequently diminished.