Night Temperature on Mt. Etna

Abstract

THE observatory on Mt. Etna is perched high up on a plateau of the volcano known as the Piano del Lago beneath the summit ridge, which rises about 1000 feet higher. It is sometimes noticed by the officials, who only reside a few days in each month, that a curious rise in temperature, amounting to a couple of degrees or so centigrade, occurs during the middle of the night, constituting a well-marked secondary nocturnal maximum in the diurnal variation of temperature. During a visit to the station in August 1920, Prof. Filippo Eredia noticed that the nocturnal inversion in the regular fall of temperature was associated with the arrival of sulphurous fumes from the crater, but notwithstanding the contemporaneous occurrence he does not attribute much causal connexion between the two phenomena. A dozen cases, as shown by thermograph records, are discussed by him in a paper contributed to vol. 31 (1922) of the Rendiconti della Reale Accademia Nazionale dei Lincei. Most of them occurred in the summer, and in conditions botif of calm and of wind of different forces and directions, chiefly N.E. and N.W. The calm cases with clear sky are shown to be analogous to similar nocturnal inversions in other mountain regions, and are attributed partly to the slow descent of air from the summit ridge whereby it is warmed by adiabatic compression, and partly to the latent heat of misty condensation due to the previous general nocturnal chilling of the atmosphere. This, however, is not quite convincing; the effects are too complex to be explained on a purely qualitative basis. The cases with strong wind are found to be associated with a great difference of temperature between the interior of Sicily and the eastern flanks of Etna, giving rise to a circulation which carries warmer air to the high-level station. At Catania on the coast near sea-level there are no corresponding night inversions of the diurnal range of temperature.