Recognizing periodicity in a time series can be valuable. You may be
able to anticipate several years of drought or of excessive rain. We seem
to be in a period of global warming on a time scale that includes the retreat
of continental glaciers, but man is accelerating the effects by generating
vast amount of carbon dioxide and by releasing chemicals that damage the
ozone layers.
See how moving averages can clarify relationships.
Graph with a moving average.
Fluctuations are changes with no firm pattern. The characteristics to look for are the frequency and the magnitude of the changes. Some events such as firing of nerve cells or earth movements due to earthquakes may appear to be random fluctuations while in fact the intervals and intensities convey information about process conditions that trigger them.