Climate: Present, Past and Future - Vol. 1

By H. H. Lamb | Go to book overview

CHAPTER 6

Cyclic and quasi-periodic phenomena
In earlier chapters diurnal and seasonal rhythms in the behaviour of winds and weather have been noticed. That these are responses to the regular 24-hourly and yearly astronomical variations of the radiation supply needs no elaboration, and the chains of thermal and dynamical causation that produce them can be fairly simply followed. The semi-diurnal pressure wave (see later, pp. 217-18) which is very prominent in low latitudes, where longer-term variations of pressure are slight, is of partly tidal-i.e. gravitational-and partly thermal origin. No other rhythms are known in the atmosphere which could have such straightforward origins and which can be observed to show a similar approach to regularity in operation. Even the semi-diurnal, diurnal and annual (i.e. seasonal) rhythms are liable to fade, and may be lost for a time, especially in those latitudes where their amplitudes are weak-the former two near the poles, the last-named in the tropics. Variations of wind and heat transport, cloudiness, sky haze, etc., due to other causes, may for a time so affect the radiation receipt at the surface as to suppress, or mask and overlay, the expected rhythm. The three very long cyclic variations in the Earth's orbital arrangements first calculated by MILANKOVITCH (see Chapter 2, pp. 30-37 and Appendix II) must also be supposed to have effects upon the radiation regime, and through that upon the wind circulation and other climatic conditions, as inescapable as those that accompany night and day, winter and summer. These are:
(i) the varying ellipticity of the orbit-period circa 96000 years,
(ii) the varying obliquity of the ecliptic (tilt of the Earth's rotation axis)-period circa 40000 years,
(iii) the precession of the solstices and equinoxes-period circa 21000 years.

The effects of these variations on the input of solar radiation at various latitudes in the summer and winter respectively correspond to superposed sine curves of differing wave length, amplitude and phase, producing an at first sight irregular sequence of fluctuations of varying amplitude (cf. figs. 2.5-2.7). Because the time-scale of these variations is so long, their effects upon the atmosphere cannot be demonstrated with certainty nor in detail to compare with those that accompany night and day or the seasonal round of the year.

-212-

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Climate: Present, Past and Future - Vol. 1
Table of contents

Table of contents

  • Title Page iii
  • Contents v
  • Tables vii
  • Figures ix
  • Acknowledgments xxi
  • Foreword xxiii
  • Introduction xxv
  • Part I · - Fundamentals 1
  • Chapter 1 - Concepts and Definitions: the Weather, the Atmosphere and the Sun 3
  • Chapter 2 - Radiation and the Earth's Heat Supply 14
  • Chapter 3 - The Atmosphere in Motion 68
  • Chapter 4 - Seasonal Changes 138
  • Chapter 5 - The Stratosphere 187
  • Chapter 6 - Cyclic and Quasi-Periodic Phenomena 212
  • Chapter 7 - Anomalous Patterns of Atmospheric Circulation, Weather and Climate 254
  • Chapter 8 - The Oceans 307
  • Chapter 9 - The Water Cycle 346
  • Chapter 10 - Some Observed Causes of Climatic Variation 385
  • Appendixes to Part I 465
  • Appendix I 467
  • Appendix II 491
  • Appendix III 492
  • Appendix IV 496
  • Part II · - Climate Now 499
  • Chapter 11 - Observational Reference Data and Classification of Climates 501
  • References 555
  • Geographical Index 597
  • Subject Index 602
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