Abstract: In this paper a convenient parameter is presented to measure the daily(or from one period to the other) evolution of planetary frontal zone-variation index. This index not only describes the stability of variation within a natural synoptical period(or the elementary period), but also shows well the date of their shift. It is more effective than the empirical methods. The result shows the existence of complex period, each ineluding 2-4 elementary periods. Within a season the length of the elementary period is not quasi-constant. However, the length difference between two neighboring periods in most cases does not exceed one day.