The variability of the Sun's luminosity is thought to have effect on the world climate and it is partly measured by the sunspot numbers. For the above study a powerful statistical tool i.e. harmonic analysis has been applied for the sunspot numbers for a period of about ten decades (106 years : 1891-1996) i.e. during ten solar cycles 13 to 22. A wave number ten explains 59 percent variance with predominant periodicity at 10.6 years having an amplitude 51 and phase angle 140 degrees.
The years of weak rainfall are classified as greater than or equal to -10% from mean rainfall (mean rainfall is obtained by taking 1871 to 1990 years rainfall). There are 16 weak rainfall years. Those years are marked on the solar cycle curve of wave number ten.
It is very interesting to note that the weak rainfall years are positioned very systematic way in the solar cycle curve. At least one weak year is observed in every half cycle at the negative side. The detail discussion is given in the paper.