Resumen
A study was undertaken to establish the spatio-temporal behavior of the extreme daily values of atmospheric pressure at ground level in a zone comprising the central part of the Iberian Peninsula (Spanish Central Plateau). The study involves the time period between 1961 and 2003. From the data series of minimum values of daily pressure, measured at 14 weather observatories in the zone over the time period considered, the series of anomalies in daily pressure (differences with respect to the mean) were constructed for each of the observatories and for the Spanish Central Plateau and for each of the zones into which this latter can be subdivided. The extreme values of these anomalies were established considering as threshold values those of the P05 percentile, for the lowest values, and of the P95 percentile for the highest one. The series of extreme, annual and seasonal anomaly frequencies were constructed and, finally, the trend of those frequencies was analyzed, both for each individual weather observatory and for the regions considered. In all cases, the results proved to be in agreement. Analysis of the trend of the annual series of extreme anomaly frequencies over the Spanish Central Plateau revealed that the lowest values of minimum daily pressure at ground level follow a decreasing trend while for the highest ones the trend is increasing. That is, between 1961 and 2003 the number of days per year with higher of minimum atmospheric pressure values at ground level increased whereas the number of days per year with lower values decreased. Additionally, an analysis was made of the trend of seasonal series; from the results, it is concluded that only the winter is determinant as regards the behaviour of the annual series.