Observational evidence of solar cycle asymmetry. Panel (a): scatter plots of peak sunspot number versus rise time (in years). Panel (b): scatter plots of peak sunspot number versus rise rate (in units of sunspot number per year). Panel (c): scatter plots of peak sunspot number versus decay rate near the end of the previous cycle minimum (in the same units as the rise rate). Panel (d): scatter plots of peak sunspot area versus rise time (in years). The corresponding cycle numbers are annotated in blue in each scatter plot.