The semidiurnal temperature wave and peculiarities of the diurnal variations of temperature on mountain summits

Abstract

On Mount Washington, the monthly average diurnal temperature variation shows, in many cases, significant deviations from the normal type. Theprimary minimum occurs in theevening; asecondary night maximum, mostly followed by a slightsecondary morning minimum is observed. The frequency of occurrence of this anomalous monthly average diurnal march of temperature is about 50 per cent. The same kind of anomalous diurnal temperature variations were found on Obir-Hannwarte (Austria, 2141m=7024ft), on the other side of the Atlantic; this suggests that we deal here with a phenomenon of a general geophysical nature. — In a recent investigation into the diurnal march of temperature on summits, the present writer arrived at the conclusion that the first two harmonic terms represent the diurnal variation with sufficient accuracy. The second term has, moreover, its own physical meaning and represents a more or less independent meteorological element. — If these conclusions are valid, the anomalous diurnal temperature variation is explained by the superposition of the half-day wave upon the whole-day wave if the ratio of the amplitudes of the two harmonic terms,a2/a1, is greater than 0.4 approximately. Generally, such ratios are found in winter with the sun at low altitudes. In this season,a1 is diminished to its smallest values, whilea2 is relatively quasi-constant in the course of the year, so thata2/a1 reaches its greatest values. The anomalous diurnal temperature variation is, however, not confined to the cold season, but only depends on the magnitude of the ratioa2/a1. This is demonstrated by means of the march of temperature onovercast days of eight July months; they show, on Obir-Hannwarte, a strongly anomalous average diurnal temperature variation with a ratioa2/a1=1.18 — Conversely, the correctness of the fundamental assumption, mentioned above, concerning the half-day wave appears to be well supported by the fact that the assumption leads to a simple explanation of the anomalous diurnal march of temperature on summits.