Maxima of potential gradient at Apia

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The average values of potential gradient of the atmosphere between successive hourly periods 0h–1h, 1h–2h, 23h–24h, on complete days free from negative potential during the period 1924–34 have been tabulated and the means of these average values for each year are given in Table 1. The departures of the hourly means from the daily mean have been plotted to give the curve (full line) in Figure 1, the departure for 0h–1h having been plotted as the ordinate at 1h and so on.The harmonic analysis for this period of eleven years, using 24 ordinates and taking the mean average value for the period 23h–24h as the midnight value givesdV/dh = 115.3 + 15.8 sin (280° + 15t) + 33.6 sin (183° + 30t)+ 6.5 sin (30° + 45t) + 18.2 sin (295° + 60t) + 4.2 sin (146° + 75t)+ 9.1 sin (50° + 90t) where local midnight (zone time, that is, 165° west meridian time at Apia) is the epoch. Figure 1 also shows the curve (dotted line) for the above harmonic series omitting the first (constant) term. The fifth and sixth harmonics have been included because it was found that they decidedly improve the shape of the harmonic curve. The phase‐angle of the 24‐hour wave referred to midnight G. M. T. is 115° (280° zone time) and this agrees very well with the mean phase‐angle of 113° G. M. T. obtained at Mount Stromlo, Australia; but, as C. W. Allen1 points out, this represents a considerable retardation if compared with the 24‐hour wave of the world gradient, the phase‐angle of which is about 183° G. M. T. The maximum of the 24‐hour component at Apia is at 11h.3 zone time (22h.3 G. M. T.), whereas the actual maximum of potential gradient occurs between 8h and 9h (19h and 20h G. M. T.) in the morning and a secondary maximum occurs between 20h and 21h (7h and 8h G. M. T.) in the evening. From the analysis it is seen that the times of the maxima are determined mainly by the 12‐hour and 6‐hour terms, both of which have greater amplitudes than the first harmonic and hence the diurnal‐variation curve shows little resemblance to a single sine‐oscillation. In fact, under normal conditions the potential gradient in Samoa shows a strong 12‐hourly variation in the form of these two well defined maxima, the morning maximum usually being of higher potential and more protracted than the evening maximum.