140 CLIMATIC CYCLES AND TREE GROWTH 



The 19-20-Year Cycle — Several centuries of tree-ring records are necessary 

 in order to get a picture of this long cycle and even now it remains uncertain 

 whether it is not a mixture rather than one single cycle. A cycle of about this 

 length has dominated Arizona ring records since 1800. It shows in recent 

 tree growth and in comparative rainfall records, with apparently a lag of some 

 two years in the tree records. Its last maximum was near 1928; its total 

 range of variation has averaged 35 per cent. In the rainfall records it has 

 been about the same. Its number of repetitions has already run very high 

 and its expectation is growing less. An examination of long chronologies 

 shows that some cycle of this length is very persistent through the centuries 

 though its amplitude is far from constant. It would not be surprising if its 

 time of diminution were approaching. It does not seem likely to disappear 

 altogether. In its solar relation its half value, 10 years, was prominent in the 

 sun (and trees) two hundred years ago. The 10-year cycle, sometimes seen 

 in trees and in various phenomena, and 20 years, and possibly a 37 or 38-year 

 cycle of variable amplitude that persisted for many centuries in Arizona tree 

 records, are probably solar though not commonly in the spots. A 10-year 

 cycle showed in the occasional spots during the great dearth near 1700. It 

 has not, as far as known, come out strongly in recent years. There is some 

 suspicion that a cycle of 5 years is influencing southern California rain to an 

 increasing extent since 1900. 



The 14-Year Cycle — This cycle, often double-crested, takes a value near 

 13.7 years in the last century. Its total range of amplitude is near 25 per 

 cent. It must be studied in trees, since meteorological records do not go 

 back far enough. There are slight alternative choices of exact value in the 

 Flagstaff 500-year records through which it has lasted. Its dominance or 

 intensity varies. A maximum perhaps came in the period 1933 to 1935. 

 This cycle has been persistent since 1850 or so; it possibly changed phase near 

 1820. In cyclograms it shows extensively by interference with other cycles, 

 giving the impression that we may be dealing with a submultiple of a longer cycle. 



As to phenomena in the sun, a 14-year cycle was most evident in the 

 smoothed annual numbers between 1788 (maximum) and 1837 (maximum), 

 the last cycle in this interval being 7 years in length. 



The ll\-Year Cycle — This has appeared in a double-crested form in trees 

 since 1850 or before with a total range in amplitude of 20 per cent; its maxima 

 have come in the ring sequences about two years after the maxima and minima 

 in the sunspot numbers. It has had various appearances in California ring 

 chronology that suggest a 270-year recurrence cycle. At each appearance 

 it has lasted a considerable period, of the order of 100 years. Thus at the 

 present time we can cast this Hellmann cycle forward a half or whole 11 years 

 length with the feeling that the length or intensity has a change in the not 

 distant future. 



Probably other cycles both longer and shorter could be included in this 

 list, but the various cycles mentioned, when assembled into a picture, give 

 us at least an idea of the prediction mozaic. 



