The Sierra de La Laguna is a group of mountains in the south of the Baja California Peninsula that, because of their height, show marked climatic differences. The life forms there are different from the rest of the Peninsula and the area is the site of many ecological studies. The objective of this paper is to construct a regional climatic series that can be useful for detecting annual dry and wet periods. An aridity index is calculated according to Martonne, using precipitation and temperature data from nine meteorological stations in the middle and lower ranges of the Sierra. When an auto-correction and Fourier analysis of the resultant series is made, a tendency to less aridity is observed with a fundamental frequency of 24 years, which approximates conformance to the double cycle of sunspots.