464 CORRESPONDENCE AND NOTES

ACKNOWLEDGMENT

The author is thankful to Messrs. Wayne Knight and Fred De’Silva for taking observations and analysing them.

Brewer, A. W., McElroy, C. T. 1973

Dobson, G. M. B. 1958 Galbally, I. E. 1975

Harris, J. E., Moss, D. G. 1974 and Swam, N. R.

Hesstvedt, E. 1974

and Kerr, J. B.

Kulkarni, R. N. 1975

McElroy, M. B., Wefsy, S. C., 1974 Penner, J. E. and McConnell, J. C.

Saltzman, B. E. 1954

Vupputuri, R. K. R. 1975

REFERENCES

Nitrogen dioxide concentration in the atmosphere, Nature,

Annals of ZG Y, Vol. 5, pp. 46-81. Nitrogen oxides (NO, or NO3 in the air at Aspendale and

other places in Victoria, Clean Air, 9, pp. 12-15. H20, 03, NsO and HN03 in the arctic stratosphere, Nature,

250, pp. 475476. Reduction of stratospheric ozone from high flying aircraft

studied in a two dimensional photochemical model with transport, Can. J. Chemistry, 52, pp. 1591-1598.

Measurements of NO2 using Dobson spectrophotometer, J. Atmos. Sci., 32, pp. 1641-1643.

Atmospheric ozone and possible impact of stratospheric aviation, Zbid., 31, pp. 287-303.

246, pp. 129-133.

Colorimetric microdetermination of NO2 in the atmosphere,

Seasonal and latitudinal variation of N20 and NO, in the stratosphere, J. Geophys. Res., 80, pp. 1125-1132.

Analytical Chemistry, 26, pp. 1949-1955.

CSIRO, Division of Atmospheric Physics, P.O. Box 77, Mordialloc, Vic. 3195, Australia. 27 October 1975

551.524.36 : 517.512.2

THE POWER SPECTRUM OF TEMPERATURES IN CENTRAL ENGLAND

By W. C. BAIN

(Communicated by Dr R. Frith)

The very long series of monthly-mean temperatures for central England produced by Manley (1974) merits careful study and among the investigations which should be carried out is the calcula- tion of its power spectrum. This has already been done by Shapiro (1975) but his results show very few points (not more than ten) to cover all frequencies with periods greater than ten years. It therefore seemed desirable to recalculate the spectrum with particular attention to such frequencies, especially as Shapiro concluded that there was no enhancement at periods near eleven years, corresponding to the sunspot cycle.

The first calculation was done with annual mean temperatures for the years 1659-1973 (as in Shapiro Fig. 3). A maximum entropy spectral analysis (MESA) program (Ulrych and Bishop 1975) was used because of its inherent advantages, which make it particularly appropriate for geophysical work (Ulrych 1972), and the results are shown in Fig. 1. There are three obvious peaks in the spectrum, but little power at frequencies near Wmc/yr, corresponding to an 11-year period, in agreement with Shapiro’s finding. However the very marked feature with a period of 23.6 year is near to 22.2 year, twice the period of the ordinary sunspot cycle. Now the double sunspot cycle, obtained by changing the sign of the sunspot numbers in alternate cycles (Bracewell 1953; Bain 1975), may well have greater physical significance than the 11-year cycle. Besides the well-known reversal of magnetic polarity associated with sunspots there appear to be changes in the magnetic field at high solar latitudes with a 22-year periodicity (Howard 1974).

CORRESPONDENCE AND NOTES 465

23 6yr

105 yr i

15 O y r i

Frequency (mc /yr

Figure 1. corresponding to certain peaks are shown.

Power spectra for the annual mean temperatures in central England, 1659-1973. The periods by maximum entropy spectral analysis; x x by

fast Fourier transform.

To show that the peaks in Fig. 1 are detectable not only by a MESA technique, the same data were analysed more conventionally with a fast Fourier transform program. The results are also given in Fig. 1. Although the points are more scattered, especially at low frequencies, the same three peaks are clearly evident in the spectrum.

King et al. (1974) have already drawn attention to the resemblance between some of the Manley data and the double sunspot cycle, using July temperatures from 1750 to 1880. In Fig. 2 the MESA spectrum of July data from 1723 to 1882 is shown, 1723 being chosen because Manley gives this as the start of more reliable data. Again the dominant feature is a peak near a 22-year period, and there is also a smaller peak near 15 year as in the previous case. King (1975) considered that the influence of the double sunspot cycle was less apparent in the July temperatures after 1880 than that of the 1 1-year cycle, and to examine this hypothesis the MESA spectrum was calculated for the period 1882-1973; it, too, is shown in Fig. 2. The relative prominence of the peak near 22 years is indeed reduced. Although i t is still the largest feature, it has moved to 25 years, and this indicates a significant change in the influence of the double sunspot cycle; other methods of analysis will be needed to study it further. The second largest peak occurs at 12-0 years.

A MESA spectrum for January data for 1723-1973 shows only a small effect near the 22-year period, the largest peak being at a period of 16.9 years.

The conclusion is that the Manley temperature data for central England for the whole year and for July each show a strong spectral peak at a period about 23 years, but only the July data

30 r

" 0 50 100 Frequency ( m c / y r )

Figure 2. Power spectra by the MESA technique for July temperatures in central England. The periods corresponding to certain peaks are shown. 1723-1882; . . . . . . . . . 1883-1973.

466 CORRESPONDENCE AND NOTES

for 1882-1973 shows a marked peak near 11 years. The 23-year peak may well be related to the double sunspot cycle.

The author is much indebted to Professor T. J. Ulrych, University of British Columbia, for providing a copy of his efficient computer sub-routine for deriving the filter coefficients in the MESA program in advance of its publication by Ulrych and Bishop (1975). He also acknowledges the assistance of Mr. D. Long and Mrs. A. J. Slater in performing calculations with the fast Fourier transform program. The work described above was carried out at the Science Research Council’s Appleton Laboratory and is published with the permission of the Director.

Bain, W. C. 1975

Bracewell, R. N. Howard, R.

1953 1974

King, J. W., Hurst, E., Slater, A. J. 1974

King, J. W. 1975

Manley, G. 1974

Shapiro, R. 1975

Ulrych, T. J. 1972

Ulrych, T. J. and Bishop, T. N. 1975

Smith, P. A. and Tamkin, B.

Science Research Council, Appleton Laboratory, Ditton Park, Slough. 30 October 1975

REFERENCES

Long-term periodicities in the sunspot cycle, A‘ature, 254,

The sunspot number series, Zbid., 171, pp. 649-650. Studies of solar magnetic fields, I Solar Phys., 38, UP.

p. 362.

- _ _ 283-299.

Agriculture and sunspots, Nature, 252, pp. 2-3.

Sun-weather relationships, Astronautics and Aeronautics, 13, pp. 10-19.

Central England temperatures: monthly means 1659-1973, Quart. J. R. Met. SOC., 100, pp. 389-405.

The variance spectrum of monthly mean central England temperatures, Ibid., 101, pp. 679-681.

Maximum entropy power spectrum of truncated sinusoids, J. Geophys. Res., 77, pp. 1396-1400.

Maximum entropy spectral analysis and autoregressive decomposition, Rev. Geophys. Space Phys., 13, pp. 183-200.

551.521.14 : 551.577.38 : 551.585.53

COMMENT ON THE PAPER ‘DYNAMICS OF DESERTS AND DROUGHT IN THE SAHEL’ BY J. G. CHARNEY

By E. A. RIPLEY

The idea that the recent drought in the Sahelian zone of Africa can be at least partially blamed on overgrazing of the area by sheep and goats (Charney 1975) is an attractive one. If proven, the hypothesis could lead to the inclusion of these animals in man’s arsenal of weather modification weapons. However, before carrying out a general slaughter of these ubiquitous creatures throughout the Sahel and persuading their former owners to lend their hands to the green revolution, perhaps this hypothesis should be examined more carefully.

Charney’s argument seems to be that vegetation absorbs more solar radiation than desert, thus becoming hotter during the day, resulting in enhanced convection, and hence greater rainfall, over the vegetated areas.

If my interpretation is correct, the above hypothesis appears to be inconsistent even with Charney’s own observation that ‘Desert surfaces are hotter than surrounding regions’. Anyone who has walked barefoot on a beach on a sunny day knows that dry sand, in spite of its higher albedo, is much hotter than wet sand.

Those working in the surface boundary layer have found that vegetated areas are indeed cooler